Accomodating Human and Small Animal Use of Washington Wildlife Passage Structures

Item

Title

Eng Accomodating Human and Small Animal Use of Washington Wildlife Passage Structures

Date

2015

Creator

Eng Greene, Sean Patrick

Subject

Eng Environmental Studies

extracted text

ACCOMODATING HUMAN AND SMALL ANIMAL USE
OF WASHINGTON WILDLIFE PASSAGE STRUCTURES

by
Sean Patrick Greene

A Thesis
Submitted in partial fulfillment
of the requirements for the degree
Master of Environmental Studies
The Evergreen State College
June 2015

©2015 by Sean Greene. All rights reserved.

This Thesis for the Master of Environmental Studies Degree
by
Sean Patrick Greene

has been approved for
The Evergreen State College
by

________________________
Dina Roberts, Ph.D.
Member of the Faculty

________________________
Date

ABSTRACT
Accommodating Human and Small Animal Use
of Washington Wildlife Passage Structures
Sean Patrick Greene

It is well documented in the road ecology literature that transportation
infrastructure has a negative impact on wildlife populations through habitat and
population loss and fragmentation (Soulé, 2001). The development of wildlife-focused
passage structures like bridge underpasses, culverts, and overpasses has proven effective
at mitigating some of these impacts of roads on ungulates and other large mammals
(Kintsch & Cramer, 2011). However, two populations that are incorporated into the use
community for these structures, namely human pedestrians and small animals, are often
discounted as incidental (Niemi et al., 2012). In partnership with the Washington State
Department of Transportation, this study used data collected from camera traps to
observe the communities using a variety of passage structures across western
Washington. This study explored how human use impacted wildlife use and what passage
elements appeared most preferable to smaller mammalian vertebrates. Ultimately, this
study identified 26 different species that successfully passed at least one individual
through a passage structure over this annual cycle, including 19 smaller vertebrate
species (<50 lbs.). In addition, increased human use rates demonstrated a likely negative
impact on wildlife of any size or behavioral type. The number of individuals and species
richness differed between paired sites suggesting that the presence of permanent running
water without available dry paths is a significant barrier to use, increased cross-sectional
area is preferred by humans and larger animals while the smaller confines of culverts
seem to have higher small animal use rates, and the availability of elevated paths to cross
above ground level may facilitate and encourage small animal movement through
structures. Future research should be considered to better gather a full understanding of
the types of use these structures receive and how they could be designed in the interests
of promoting use by humans, large mammals, and small wildlife equally.

Table of Contents
Chapter 1: Literature Review
Introduction………………………………………………………………………1
Current Road Ecology Research……………………………………………..….2
Wildlife Passage Structures…………………………………………………..….4
Small Animal Concerns……………………………………………………...…10
Small Animal Structure Elements…………………………………………..….13
Effect of Human Use…………………………………………………………….17
Human Structure Elements…………………………………………………….20
Literature Review Summary and Thesis Research Questions………………..23
Chapter 2: Analysis of Human and Small Animal Use of Passage Structures
Study Introduction…………………………………………………………...…25
Materials and Methods…………………………………………………………28
Study Origination……………………………………………………….28
Site Identification and Camera Trap Installation……………………..28
Study Area………………………………………………………………31
Data Collection……………………………………………………….…39
Data Processing & Statistical Analysis……………………………..….40
Results………………………………………………………………………...…42
Passage Structure Styles and Elements……………………………..….42
Species Diversity in Passage Structures…………………………….….46
Human Impact on Wildlife Passage………………………………...….51
Discussion…………………………………………………………………….…53
Passage Structure Styles and Elements……………………………..….54
Species Diversity in Passage Structures…………………………….….59
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Human Impact on Wildlife Passage……………………………………65
Conclusions……………………………………………………………………...68
Chapter 3: Conclusions and Management Implications
Conclusions…………………………………………………………………...…69
Management Implications…………………………………………………...…71
Recommendations for Future Research…………………………………….…73
References…………………………………………………………………………….…99

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List of Figures
Figure 1: I-90 Price/Noble Wildlife Overcrossing………………………………….…79
Figure 2: Deer Carcass Kernel Density Analysis…………………………………..…80
Figure 3: Camera Installation……………………………………………………….…81
Figure 4: Study Areas………………………………………………………………..…82
Figure 5: Study Structures…………………………………………………………..…83
Figure 6: Human and Wildlife Weekly Average Observations by Structure Type...85
Figure 7: Confirmed Crossing Rates by Structure Type………………………….…86
Figure 8: Polynomial Fit Analysis of Wildlife Observations by Cross-sectional Area
of Structure…………………………………………………………………………...…87
Figure 9: Bivariate Fit Analysis of Wildlife Observations by Openness Ratio of
Structure.………………………………………………………………………………..88
Figure 10: Wildlife Use of Tree Branches to Enter Culverts………………………...89
Figure 11: Wildlife Use of Branches That Do Not Enter Culvert…………………....90
Figure 12: Observed Species by Structure……………………………………..………93
Figure 13: Bivariate Fit of Number of Wildlife Individuals per Week by Number of Human
Individuals per Week………………………………………………………..…................95
Figure 14: One-way Analysis of Number of Wildlife Individuals per Week by Number of
Human Individuals per Week Categorical……………………………….…………………..96
Figure 15: Bivariate Fit of Number of Large & Small Mammal Individuals Per Week By
Number of Human Individuals Per Week……………………………………………………..97
Figure 16: One-way Analysis of Number of Wildlife Individuals per Week by Structure
Type…………………………………………………………………………………………...…98

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List of Tables
Table 1: Usage of Study Structures and Confirmed Crossing Rates……………..…84
Table 2: Crossings by Species……………………………………………………….….91

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Acknowledgements
This thesis is the end result of numerous hours devoted to data collection,
analysis, research, and writing over the previous twelve months. Yet, as this final draft
stands completed, it owes more to the support network that surrounded me throughout
this process than to any personal time spent in the execution. I stand eternally grateful for
the help and encouragement that I have received from a number of avenues and can only
hope that the end result of this thesis proves worthy of the people who helped in its
creation.
I must thank the Washington State Department of Transportation’s Environmental
Services Office for allowing me the use of the imagery data collected as part of their
Habitat Connectivity Program that proved so instrumental in reaching the conclusions
presented in this thesis. In particular, thank you to Marion Carey, the Fish and Wildlife
Program Manager, who hired me as an intern in July, 2014, and tasked me with servicing
the program’s many wildlife cameras, laying the groundwork upon which this thesis has
been built. In addition, I owe a great deal of thanks to Kelly McAllister, Wildlife
Biologist, who supervised my work throughout my time with the agency, offering sage
advice, invaluable input, and welcome companionship on countless occasions. His efforts
have made me a better scientist and his revisions and review of this thesis’ earlier drafts
went a long way in making it a more presentable project. I also thank Stacey Plumley,
GIS Analyst, for opening my eyes to computer tools and fields of study that were beyond
my imagination.
The faculty and students of the Master of Environmental Studies program at The
Evergreen State College also played a fundamental role in this success. First and
foremost, thank you to Dina Roberts, my thesis reader and sounding board, was heavily
involved with every step of this project, offering endless support and feedback regardless
of the hurdles that appeared throughout the process. I say this without hyperbole when I
say that without Dina, this thesis would simply not exist. I thank Kevin Francis and Erin
Martin for helping me to think and act in a scientific manner and expanding the horizons
of my knowledge. I thank Carri LeRoy for being the only person ever to prove capable of
making the vagaries of statistics seem straightforward. I also thank Mike Ruth for his
enthusiasm and limitless knowledge of GIS, both traits that I hope to have retained a little
of.
Finally, the appreciation that I hold for the support that I have received throughout
my life, deserved or not, from my family is infinite. My family, especially my parents,
have never stopped believing in my abilities, nor have they ever ceased to push me to
becoming the person and professional that they know be capable of becoming. For that, I
have no words to accurately express my gratitude.

viii

Chapter 1: Literature Review
Introduction

Road ecology is a field of science that began when scientists and road managers
realized that the increasing rate of road construction and use brought people’s needs into
sharp conflict with wild animal populations’ use of habitat near roadways. While this
observation may seem intuitive today, 1996 marked the relatively recent point when the
first major scientific conference focusing on this discipline was held in Orlando, Florida.
Over 4 million miles of public roadways exist across the United States touching
the lives of virtually every one of the 320 million people living in this nation, and yet this
arena of science is still underexplored (Forman et al., 2003). A search the word “ecology”
for the years 1956 (when the Interstate Highway System was formed) to the present in the
Evergreen State College database cataloging peer-reviewed journal articles resulted in
227,884 results; when the search was altered to look for “road ecology” with the same
date range, however, the number of results dropped to 2,212, less than 0.01% of the
previous total. Despite increased attention to this topic from many state and federal
agencies in recent years, this traditional oversight does not seem to be fading away as
when the same search was run for the years 1996 (when the first road ecology conference
was held) to present, the percentage remained unchanged. Most of these interested
agencies have direct ties to roadway management, such as the Washington State
Department of Transportation (WSDOT), and deploy a suite of tools to study the topic. In
the case of WSDOT, the agency uses 40-50 wildlife cameras and a dozen radio collars to
observe animal movements near state roads. They also maintain a carcass removal

1

database that dates back to 1973 tracking the locations of roadkill for selected species,
primarily ungulates and large carnivores, throughout the state. The limited time and tools
available to scientists in this field are devoted largely to Wildlife Vehicle Collisions
(WVCs) and habitat connectivity initiatives.
Current Road Ecology Research
WVCs are a major and continuing issue of interest for those who study road
ecology due to their prevalence and clear impact on human society. As of 2007, an
estimated 1-2 million WVCs involving large animal species occur annually in the United
States, accounting for approximately 26,000 human injuries, 200 human deaths, and $8.4
billion in damages each year (Huijser et al., 2007). The impressive scope of this negative
interaction between people and wildlife has perhaps dominated the traditional discussion
of how to manage the overlapping spheres of wildlife habitat and human development.
Washington State bears a smaller share of these events than many other states as WSDOT
employees remove an average of 3,700 large mammal carcasses from state roads
annually, but it remains an area of interest for the agency (Washington State Department
of Transportation, 2015). In addition, the Washington State Patrol records an annual
average of 1,100 WVCs a year resulting in more than 150 human injuries (Washington
State Patrol, 2015). This suggests that the actual number of annual WVCs involving large
animals in Washington State may approach or exceed 10,000 with damages numbering in
the tens of millions of dollars based on the observed report deficiency rate in other
studies (Huijser et al., 2007). WVCs are a difficult problem to tackle given the wide
variety of factors that can contribute to the problem. Human factors, such as vehicle
speed, traffic volume, and driver awareness require social movement to reduce.
2

Environmental factors, such as seasonality, weather, and time of day are beyond any
powers of control. Finally, wildlife dynamics are animal-centric, like animal abundance,
animal species, and habitat connectivity and can be difficult to directly influence
(Litvaitis & Tash, 2008).
Habitat connectivity became a driving focus of study in part as a result of these
worries over WVCs and other concerns related to fragmentation effects on wildlife in
general. Human vehicle traffic on roadways continues to grow unabated and, as a result,
there has been a sustained, continuous increase in WVCs of 8-20 percent per year
nationwide, though this has not been yet observed in Washington (Gaskill, 2013). There
is only so much that can be done to make drivers more aware of the dangers and to take
preventative actions, so policy makers and scientists have expanded efforts to assess why
wildlife cross roadways despite the obvious repulsive forces like noise, light, pollution,
and the risk of injury or death (Fahrig & Rytwinski, 2009). The massive network of roads
in the US was planned according to human concerns like minimizing cost and travel
distance. As a result of this narrow viewpoint, people built straight, artificial lines of nonpermeability, fragmenting historic animal ranges. Animals of various sizes and species
still needed to move around their habitat to find food, mate, and migrate, leaving
dangerous crossings of roadways as their sole option (Forman et al., 2003). This inclusion
of paved, trafficked roadways into their habitat ranges places animals into a position of
attempting to adapt behaviors trained through generations for a specific terrain to one that
can be wholly unsuited for their survival. Having understood the source of the problem,
namely that roadways impede animal movement through once-contiguous habitat, the
challenge was then how to allow for free and easy movement of animals below, or
3

sometimes above, roadways. Passage structures specifically designed for animal use are
usually presented as the solution to this issue of roadways breaking apart preexisting
habitat connections, but they are often unnatural structures placed in an environment
where they can be unappealing to wildlife.
Wildlife Passage Structures
The goal of maintaining habitat connectivity for wildlife species near roadways in
an effort to reduce WVCs has been addressed by a number of wildlife passage structures.
These structures are engineered to funnel animals to points where they can move across
roadways without having to come into contact with vehicles. Wildlife passage infrastructure
has four primary types of composition, each with its own subtypes, which are used by

transportation agencies to control animal movement: underpasses, overpasses, barriers,
and one-way structures.
By far the most common passage structure put into use by transportation agencies
are underpasses, specifically bridges and culverts. These structures provide an excellent
balance of effectiveness, customization, and convenience for both human and wildlife
use. Bridges have a long historical use in road construction to traverse rivers and valleys,
and have the added benefit of allowing for animal passage beneath roads, though the
open space beneath is more of an engineering decision than an ecological one. Bridge
designs vary, however; a large bridge with mostly open space beneath it can be very
successful in reducing fatalities for megafauna like black-tailed deer (Odocoileus
hemionus), elk (Cervus canadensis), black bear (Ursus americanus), and moose (Alces
alces). One project in which highway construction elected to incorporate such bridge

4

designs into the construction plan in North Carolina observed that white-tailed deer
(Odocoileus virginianus) use of the areas were underpasses were put into place with
accompanying fencing increased 6.7 times when compared to the baseline passage rate
for the areas prior to construction. As a result of the great success in shifting most deer
movement into the relative safety of an open bridge underpass, deer fatalities dropped by
58% (McCollister & van Manen, 2010). These extensive bridges have been shown to be
the preferred passage method for ungulates and carnivores due to the high visibility they
offer (Kintsch & Cramer, 2011). Ungulates likely prefer the clear sightlines as they rely
upon speed and quick reaction time to elude predators. Smaller bridges that present
wildlife with a more enclosed space are no less effective, but are used by a different
target population: one that prefers a low-visibility habitat. Medium sized and small
mammals, reptiles, and amphibians prefer heavy cover, especially at the entrances to the
bridge to break line-of-sight and can find the confinement of a narrow passage more
tolerable (Kintsch & Cramer, 2011). When presented with the decision, wild ungulates
and certain other megafauna will actively avoid the use of smaller, confined passages,
electing to brave passage over the roadways instead (Rodriguez, Crema, & Delibes,
1996).
Carnivores have proven to be more difficult to predict as preferences for passage
structures differ distinctly by species and even by individual, with sex and age playing
significant roles in influencing passage rates (Clevenger et al., 2002). Carnivores with
less tolerance to human-related disturbances, either through noise or physical disturbance,
such as grizzly bears tend to favor open bridges with ready access to covering foliage on
either end. Carnivores that are more resilient to human influence like cougar (Puma
5

concolor) and black bear have instead shown a marked preference for more compact
passage structures with open ground on the ends (Clevenger & Waltho, 2005). In
addition, habitat preferences gleaned from carnivore predation ranges cannot be assumed
to be accurate for passage structures as the evidence that predators use bridges as traps to
hunt prey is “scant, largely anecdotal and tends to indicate infrequent opportunism rather
than the establishment of patterns of recurring predation” (Little, Harcourt, & Clevenger,
2005). Because of all of these caveats attributed to carnivore passage, the consensus
opinion is to ensure that when passage structures are installed, a variety of forms and
sizes are used (Clevenger & Waltho, 2005).
Culverts are similar to bridges in that they come in many forms and sizes that can
be tailored to target a particular species of interest, but can be placed wherever needed
without installing an expensive bridge. They can range in size from small, water-only
pipes on the order of 1 foot in diameter (though these are of limited utility for animals,
they are still used when water movement is the sole concern of the project) to 10 or more
feet in diameter. These spiral corrugated metal or concrete pipes were originally intended
to allow for streams, runoff, and stormwater to pass under roadways in the interests of
driver safety and roadway preservation, but have proven to be widely used by various
animal species (Kintsch & Cramer, 2011). In particular, they are the primary tool used by
transportation agencies to maintain fish passage routes via streams that intersect with
roadways (Anderson et al., 2012). Their primary role as a method of transporting water
can negatively impact the use of these culverts by wildlife though, with smaller
carnivores exhibiting significantly decreased passage rates when culverts carried water
more than 3 cm deep or covering more than 70% of the culvert base (Serronha et al.,
6

2013). The diversity of preferences for carnivores is still evident when dealing with
culverts though, as larger carnivores do not seem to be deterred much by water in culverts
(Craighead Institute, 2010). While these structures are widely-used and offer many
benefits, they can be difficult to design for a multiple-use system in which people and
animals of many sizes can find equal access to safe, convenient passage. The simple
solution, building more culverts and bridges to take into account the requirements of
various species on an individual basis, is stymied by rising costs and shrinking budgets in
transportation agencies.
Overpasses offer much more in the pursuit of multiple-use structures, but are held
in check by a commensurate increase in costs. These “land bridges” are a relatively new
phenomenon in road design as they attempt to create a more natural passage system. An
overpass is a structure placed over a road, creating a tunnel for drivers, that is topped by
soil and vegetation, characteristic to the surrounding environment, reconnecting the
habitat on either side of the roadway into a single ecosystem (Smith, 2011). These
provide an exceptional passage route for birds, mammals, reptiles, and insects, though
fish are largely excluded as moving water features cannot transverse the sloped sides of
the overpass. The shallow ponds, coarse woody debris, and dense vegetation that may be
part of the design of overpasses can also serve as useful habitat for amphibians (Owens et
al., 2008). In addition, evidence suggests that the largest forms of wildlife, such as
moose, distinctly prefer to use overpasses in place of even the most open of underpasses
when given the option (Huijser et al., 2013). This is understandable given that these
structures are open to sunlight, precipitation, and certain elements of the natural system.
These factors, when combined with screening vegetation placed along the edges of the
7

overpass to hide the traffic from animal view, can create an exceedingly comfortable
passage system for most terrestrial animals (Kintsch & Cramer, 2011). It is even possible
to incorporate stationary water features into the design, though running water is currently
impossible without significant, expensive feats of engineering ingenuity. Washington is
currently developing an innovative wildlife overpass along Snoqualmie Pass East on I-90
that aims to be one of the first of its kind in the world (Figure 1). The 15-mile stretch of
road that includes the 800-foot long Price/Noble Creek Overpass is being built at a cost
of approximately $100 million and the overpass is notable for the devoted effort being
made to include every aspect of the surrounding environment so as to make the structure
virtually indistinguishable from the natural habitat for wildlife (Smith, 2011).
Barriers and one-way structures are often used in conjunction with overpasses or
underpasses to dissuade wildlife from crossing roadways and convincing them to use
passage structures instead. Barriers commonly take the form of fencing, but the category
can include elevated walls or cement barriers. Essentially barriers exist to deter any
passage attempts by wildlife and are normally placed along the edge of roadways. In that
position they serve as a vital part of any population movement control as they artificially
boost passage rates at desirable locations where structures have been built to
accommodate this process. In one project in Montana, effective use of fencing,
underpasses, and one-way structures saw the number of deer that crossed the roadway at
a underpass increase 5.2 times from 1,732 a year before the structures were in place to
9,084 afterwards (Huijser et al., 2013). Another project, this one from WSDOT, built
approximately 9 miles of fencing along US97 Alternate Route north of Wenatchee, WA
along with several one-way structures to reduce the number of deer and bighorn sheep
8

WVCs along the roadway. In the 15 years prior to the project, deer and sheep carcasses
removed from the targeted stretch of highway had reached densities of 10.8/mile/year and
1.0/mile/year, respectively. After the project, carcass removal rates fell to a respective
0.3/mile/year and 0.0/mile/year (McAllister et al., 2014). The effectiveness of barrier wall
and culvert combinations is not limited to larger species such as these, though, as a
project in Payne’s Prairie State Preserve, Florida using this method saw a year-to-year
93.5% decrease in road fatalities for amphibians following construction (Dodd,
Barichivich, & Smith, 2003). While fencing in particular has proven highly effective at
enhancing areas of safe passage to wildlife, it is not foolproof. Animals will only travel
parallel to roadways along fencing for so long before trying to force a passage if no
structure presents itself. There is a strong correlation between lower mortality rates and
wildlife fencing along roadways, but at distances too far away from an accompanying
passage structure, mortality rates will begin to rise again as animals attempt to jump the
fence, unaware an alternative exists (McCollister & van Manen, 2010).
One-way structures include jumpouts and wildlife guards. Jumpouts consist of
ramps built at equal level with roadways that allow for animals that have been trapped on
the wrong side of a barrier to escape the roadway and return to the natural environment,
but their vertical height where they intersect fencing does not allow for movement in the
opposite direction. Jumpouts are not included as a means of regular animal traffic as,
ideally, the fencing and passage structure system has been designed effectively enough
that no individual finds itself on the roadside of the fencing. Instead, they exist as a sort
of emergency exit for wildlife. Wildlife guards are a series of spaced metal pipes placed
over a hole dug into an arterial road that connects with a major roadway that is otherwise
9

enclosed by fencing. Wildlife guards are necessary to allow vehicles to enter or exit the
major roadway without undercutting the effectiveness of barriers. Ungulates can observe
the space between the metal pipes and understand that their hooves will fall in between
and so do not attempt passage, but smaller animals or larger animals with padded feet like
carnivores are not as easily dissuaded. Aside from most animal species (though a
minority of gross individuals due to the abundance of ungulates) being able to traverse
wildlife guards with relative ease, there have also been observed instances of ungulates
attempting to cross wildlife guards and risking injury through a trapped leg or a fall.
Alternatives to wildlife guards such as radio-triggered fencing with transmitters delivered
to private individuals on the far side of the barrier have been discussed, but are generally
dismissed due to the increase in complexity, cost, and maintenance.
Small Animal Concerns
The vast majority of road ecology research is centered on reducing WVCs and,
therefore, passage structures are heavily weighted towards megafauna, particularly
ungulates. The negative impact that roads have on the many remaining forms of wildlife,
like small mammals, birds, reptiles, and amphibians, is somewhat uncertain given the
general lack of interest and funding. Surveys of small animal roadkill are few and far
between and it is unclear if any transportation agency in the country maintains a
comprehensive database covering carcass removals for small animals outside of a select
few charismatic species like bobcats (Lynx rufus) and bald eagles (Haliaeetus
leucocephalus). No agency has a freely-available report on small animal roadkill at any
rate and discussions with representatives from multiple state agencies in the Pacific
Northwest have suggested that the impact of WVCs on small animal populations is not a
10

current interest area in internal research. New research in this field is starting to raise
alarms, however, as it appears that the relationship between these species and roadways is
just as perilous as that of the more well-understood species like deer. As an example of
the potential danger of this oversight, a recent study has suggested that as many as 340
million birds are killed on US roads annually as a result of vehicle strikes, 4-6 times
greater than the previous 2005 estimate of 60-80 million (Loss, Will, & Marra, 2014;
Erickson, Johnson, & Young, 2007). Meanwhile, the 0.25-0.5 million birds that die in
wind turbine strikes annually receive heavy media attention and research funding. A
confident estimate of just how many small animals are killed by WVCs every year
doesn’t exist in the current research. Ideally, this could be a scenario where steps can be
taken by scientists to solve a problem whose scope is not yet understood.
One effect that roads have on wildlife that is better covered by the existing
literature is the way in which they alter habitat preferences for small mammals and
reptiles. There exist animals, such as the hedgehog (Erinaceus europaeus), that see
significantly reduced population densities near roadways. In the hedgehog’s case,
population density dropped by a full 30% near roadways due to unfavorable habitat and
vehicular strikes (Huijser & Bergers, 2000). Perhaps counterintuitively, not all small
species populations are negatively impacted by roadways and some, in fact, see a positive
impact. Vultures and other scavengers are attracted to the roadkill produced by WVCs as
a source of food, but are capable of avoiding vehicles, leading to a net population gain.
Other animals actually derive a benefit from the disturbance provided by traffic; small
mammals settle on roadway verges to use the noise and movement of traffic to scare
away predators while avoiding the roads themselves (Fahrig & Rytwinski, 2009).
11

Maintenance efforts to sustain roadways clear of vegetation generally leads to a regular
regime of mowing roadway verges by transportation agencies. This regular mowing
creates a vegetation community composed primarily of grasses that are kept short,
serving as prime habitat for seed-eating small mammals (Oxley, Fenton, & Carmody,
1974). These road verges have become such a preferable habitat for some species like the
wood mouse (Apodemus sylvaticus) that studies have shown that they can prefer habitat
close to roads over habitat distant from roads by up to a 9:2 ratio (Ruiz-Capillas, Mata, &
Male, 2013).
This positive habitat selection preference by some small mammal and reptile
species for road proximity can have significant negative results, though. Not every animal
that elects to live in road verges is capable of avoiding vehicles, leading to greatly
increased collision rates. Medium sized predators such as raccoons (Procyon lotor) and
bobcats with large movement ranges often come into conflict with roads that pass
through their habitat area. These high-movement animals are often put doubly at risk as
they predate on the small mammals that settle on road verges (Fahrig & Rytwinski,
2009). Nesting animals have shown a similar tendency to lay eggs or rear young on road
verges. Higher proportions of juvenile mice have been found closer to roads than further
away in Spain and up to 30% of freshwater turtle species in northwestern Florida build
nests directly adjacent to the highway shoulder where soil and vegetation conditions are
often optimal for nesting (Ruiz-Capillas, Mata, & Male, 2013; Aresco, 2005). In the case
of the turtles in particular this has become an issue as WVCs are resulting in nearly twice
as many female turtles being killed as males due to this nesting behavior (Aresco, 2005).
These behavioral traits present a unique issue in developing a beneficial passage system
12

for smaller animals as some traditional methods like hazing or population culling that
have been used with some success on larger species are of minimal utility (Huijser et al.,
2007). Hazing, the use of negative harassment techniques like odors and noises to
discourage animal presence, and population culling, the killing of a portion of a local
population usually through hunting, are not as effective with smaller animals due to their
small size and large population numbers.
Small Animal Structure Elements
If some animals are actively seeking out habitat adjacent to roadways, efforts to
design effective passage structures for them becomes all the more paramount. As
previously discussed, the needs of small animals versus those of large mammals with
regard to these structures do not necessarily overlap and there is no one-size-fits-all
solution. There exists a variety of research on the topic of small animal passage structure
design, but most studies are very specific in the scope of species studied, mandating that
the lessons learned should be extrapolated to other species with caution.
In Hungary, researchers found that amphibians with migration paths that crossed
roadways seldom used existing culverts with only 0.5% of the observed populations
making use of the structures with the rest passing directly over roads. Those individuals
who did pass through the culverts exhibited a clear preference towards older, larger
culverts (160-170 cm) as compared to newer, smaller culverts (40-60 cm) by
approximately a 5:1 ratio (Puky, Mester, & Mechura, 2013). Woltz, Gibbs, & Ducey
(2008), in an impressive multiple species analysis for passage structure preferences
among amphibians and reptiles, concluded along similar lines, recommending tunnels

13

larger than 500 cm in diameter lined with soil or gravel and accompanied by fencing at
least 0.6 m in height. A community event in Amherst, Massachusetts was put at risk in
1987 when observers noted that the annual spring migration of salamanders was resulting
in an increasing number of salamanders being killed by traffic along a two-lane road that
the animals needed to cross. Activists pushed for amphibian passage structures with
designs similar to those suggested by existing research. Several agencies combined funds
to construct two “salamander tunnels,” small, moist culverts with a slotted top to allow
light to penetrate along the full length, and fencing to block salamanders from crossing
the road. Citizen scientists studying the results found that salamanders, even at extreme
ends of the funneling fence, managed to find the tunnels and more than 75% of those that
reached the tunnels successfully used them to cross (Jackson & Tyning, n.d.). The
Payne’s Prairie Nature Preserve working group that formed in central Florida with the
goal of tackling a stretch of US 441 that paced the state in roadkill reports, particularly
for amphibians and reptiles. This group observed that containers used by zoos and private
pet owners for these species tend to have a lip at the top that prevents reptiles and
amphibians from climbing out. They adopted this lip concept by adding it to the top of a
1.8 mile low wall along the stretch of highway, drastically cutting down roadkill rates as
amphibians and reptiles were effectively funneled to culverts (Southall, n.d.).
Small mammals have particular needs in a passage structure as well. Unlike their
larger relatives, small mammals actively avoid large, open spaces when presented with
the option under bridges and in culverts. Deer and elk rely upon speed and agility to
avoid predation and thus need clear lines of sight to give them ample time to be alerted to
the presence of a predator. Smaller mammals use speed as a defense of last resort and
14

instead need cover and enclosed spaces to allow them to hide from predators. If the
passages get too small, however, some small mammals will eschew them so as not to feel
confined or constricted (Kintsch & Cramer, 2011). In a study in Montana, researchers
took baseline data of the passage rates of several small mammal species through large,
open underpasses of a 7x4 meter area. Cover in the form of dead tree branches and other
plant debris were added to some of the underpasses, resulting in a 42.9% increase in
passage rate versus control culverts (Connolly-Newman et al., 2013). In addition,
mammals with particularly small body sizes are often unwilling use culverts where water
dominates the passage due to their reliance on terrestrial environments for ease of
movement (Wolff & Guthrie, 1985). This presents a problem given that habitat
connectivity concerns are often secondary to water control when electing how and where
to construct underpasses. The solution is often an economic and utility compromise that
draws from observation of natural behavior patterns for the targeted species. In a natural
system, they use logs and branches to pass over areas of water when necessary. Placing
similar debris in culverts would be counterproductive to maintaining clear waterways and
only a stop-gap measure until the wood was washed away or rotted. Instead, wildlife
shelving has been developed. This shelving can be installed on the sides of bridges and
culverts and allows for continued use of culverts even when partially filled with water. In
another case study in Montana, 14 small mammal species were observed to use a series of
culverts when dry but virtually none did when the culverts were wet. After installing
wildlife shelving, all 14 species were observed making use of them while the culverts
were wet, effectively solving one of the issues impeding small animal underpass use,
under their specific set of circumstances (Foresman, 2004). As this shelving is installed

15

approximately halfway up the side of a culvert, water passage and access for maintenance
staff remain unimpeded.
Ultimately, the hurdle inhibiting efficient small animal passage structure
development isn’t a lack of ideas, but a lack of implementation and analysis due to
insufficient funding. While adding cover and shelving were both impressively successful
in the described case studies, the demands of each scenario are defined by the species,
habitat, and peculiarities present. It is for this reason that post-activity observation is
paramount so that more knowledge can be added to the collective scientific
consciousness. A project in Ontario, Canada that put into place a number of mitigations
to aid in habitat connectivity and animal passage for reptiles was observed afterwards to
have no significant change in population abundance (Baxter-Gilbert, Lesbarrѐres, &
Litzgus, 2013). This particular project may have had other benefits that the researchers
were unaware of, such as improved genetic diversity due to more interrelationship
between population segments, or benefits that will only become apparent following a
greater period of observation. In the short-term, the researchers were able to make
recommendations for improving the effectiveness of mitigation measures based on their
methodology. Because of post-project analysis, this project, which the authors stated
failed to meet their objectives, can now contribute to future developments as part of the
rigorous testing of various mitigation measures. This also highlights the need for further
reporting of negative results in academic journals; as experimental passage designs are
put through field testing, there are bound to be failures. The only way to ensure that these
failures are not repeated is to share them and analyze what went wrong.

16

Effect of Human Use
Perhaps no single factor, however, has a greater impact on wildlife usage of
passage structures than human presence (Gagnon et al., 2011). When WVCs occur on
highways in regions distant from residential areas, human pedestrian presence is often
discounted, but city borders continually expand as the human population grows,
suggesting that these isolated stretches of roadway will become less so over time. Much
like the other variables associated with fauna passage rates, human presence has a
different impact dependent on the form of that presence and the species of wildlife being
observed. For most species, the matter is a determination of the degree of the negative
impact that humans have, but for some high-disturbance prey species, human presence
can actually be a benefit (Fahrig & Rytwinski, 2009). This benefit is largely derived from
human presence serving to lower use by predator species. In the interests of maintaining
high passage rates across a number of species, protecting wildlife populations at an
ecosystem level, and mitigating WVCs, human influence must be minimalized. Research
in the field has shown that any current or future underpass designs “will be minimally
successful if human activity is not managed” (Clevenger & Waltho, 2000). While both
carnivores and ungulates have shown some preferential use for structures with little
human activity, the cause of this avoidance is different for the two groups of interest
(Yanes, Velasco, & Suárez, 1995; Macdonald, 1998).
For carnivores, the issue is one of habitat disturbance and influence rather than an
individual-level interaction. Many carnivores exhibit behavior patterns that lend towards
an avoidance of large numbers of human individuals. Large carnivores will make use of
habitat with limited human activity, but shun habitat that has been significantly altered by
17

human presence or where human activity reaches a particularly high level. This occurs
because humans tend to startle game species away and can change the land-cover type
into an unnatural form (Dellinger et al., 2013). Carnivores and humans also differ in
temporal use patterns; specifically, the fact that humans primarily make use of passage
structures during the day while carnivores tend to be nocturnal in nature, combined with
the secretive and solitary natures of many carnivores means that an in-person interaction
with a carnivore for human pedestrians is unlikely (Rodriguez, Crema, & Delibes, 1996).
Carnivores may often remain active during the daytime in natural environments, but
evidence suggests that they prefer nocturnal activity when in proximity to human
settlements (Hemson et al., 2009). Some large carnivores relevant to Washington,
specifically black bear and cougar exhibit this change in use patterns. Research in
western Washington shows that when they interact with residential human regions, they
traverse the area rapidly and primarily at night in an effort to limit their interaction with
an environment that no longer suits their needs (Kertson et al., 2011). Carnivores remain
sensitive to human interference in areas near hunting ranges or alterations to hunting
trails (Yanes, Velasco, & Suárez, 1995). With carnivores, therefore, the issue is not so
much occasional human passage, but the threat that a continued, substantial human
presence can irrevocably alter the habitat through prey exclusion, activity levels, noise, or
physical impact so as it make it unsuitable for carnivores (Gagnon et al., 2011). A study
analyzing the interrelationship between humans, gray wolves (Canis lupus), and elk in
Jasper National Park, Alberta, Canada showed that elk, one of the primary prey species
for wolves, developed a habituation to human activity and suggested that the elk
remained near human populations as a source of refuge from predators (Shepherd &

18

Whittington, 2006). While carnivores, like other animals, can adapt to landscape changes,
human use tends to depreciate an area for carnivores and many prove unable to adapt as
easily to human interactions, driving carnivores to regions with less human intrusion.
Compared to a general avoidance by larger carnivores to human presence,
ungulates have shown a greater ability to adapt to sharing habitat with humans. In point
of fact, non-carnivores in general show an impressive aptitude for shifting their home
ranges based on human influences and can even find human proximity advantageous in
certain circumstances. Ungulates have activity patterns that occur throughout both the
day and the night, with most activity in Pacific Northwest ungulates like elk taking place
during daylight hours (Ensing et al., 2014). Elk are largely crepuscular in their feeding
habits and there is an increased rate of movement during these periods, making them
likely occasions for direct human interaction (Ager et al., 2003). Ungulates do not
therefore necessarily have the benefit of a temporally independent use period for passage
structures, meaning in-person interactions are far more frequent, “causing run backs,
hesitation, and eliciting visual alarm responses” when they come into contact with
humans near passage structures (Pedevillano & Gerald Wright, 1987). Rather than
interact with humans directly, non-carnivores will often adjust their ranges so as to best
cohabitate with human influences and do so with great proficiency. When starlings in
New York City, which find the urban setting ideal for finding food, were pressed to find a
place to nest away from the constant presence of humans managed to identify “an area
with fewer humans afoot than any within miles of the city,” yet still within the heart of
the urban extent, revealing their ability to adapt behaviors to habitat aspects (Leedy,
Franklin, & Hekimian, 1975). In addition, Gagnon et al. (2011) found that white-tailed
19

deer use of passage structures increased over time, even with a constant human presence,
as the population adapted to existing conditions. This suggests that human influence is
not a primary concern for ungulates deciding where to move within their range as they
can habituate themselves to shared environments. When designing multiple use passage
structures with ungulates and other prey species in mind, the focus must be on ensuring
as little direct interaction with people as possible (Mata et al., 2008).
The vast majority of highway passage structures like bridges and culverts are
designed with a primarily human transportation benefit or benefit in mind: to traverse
rivers or gorges, cross over or under other roads or railroad tracks, or to allow for
floodwater to pass beneath busy roadways to protect infrastructure projects. Research
delving into the relationship between wildlife and road passage structures should, as a
result, take into account the human element. Human pedestrian benefits of these
structures are secondary to this prioritized transportation need and are sometimes nothing
more than opportunistic in utility for this purpose. Yet this pedestrian use does occur and
failure to account for how human pedestrians impact efforts to encourage wildlife use of
passage structure will result in a significant decline in positive results.
Human Structure Elements
As understanding of how human presence around and use of passage structures
impacts wildlife expands, scientists and road managers have experimented with methods
to alleviate some of the stress involved with passage by multiple species. These efforts
largely fall under one of two camps: structure placement and structure design.

20

In an ideal world, human and wildlife use of passage structures could be
completely divested from one another due to geographic separation of the populations. In
reality, human development and infrastructure expansion creates an every-expanding
region of overlapping habitation with wildlife, necessitating adaptation on the part of the
infrastructure designers and wildlife biologists. As previously noted, the primary tool that
biologists can use to direct wildlife towards preferable passage points is fencing or barrier
walls, though passage structures must be placed somewhat regularly along the fencing
lest the animals attempt to break through the fence (McCollister & van Manen, 2010).
The solution is not as simple as merely fencing entire highway or placing bridges and
culverts liberally as if resources were unlimited.
When, as is commonly the case, bridges and culverts must be placed in areas of
human presence, they should be located as far from human settlements as is reasonable to
best promote wildlife passing without hassle through underpasses and along overpasses
(Olsson, Widén, & Larkin, 2008). In the common event that faunal populations are too
closely intertwined with human populations to make this a practical possibility, these
structures could be built in areas with a high degree of private land ownership
(Rodriguez, Crema, & Delibes, 1996). Private land has the primary benefit of reducing
the accessibility of passage structures for human pedestrians as human foot traffic is
heavier in public areas. Placing the actual structure on private land can prove problematic
in that the ability of an agency to manage the structure for wildlife is negatively
impacted, but in public areas with a high degree of surrounding private land ownership, it
can help minimize human pedestrian presence. Reducing the number of human
pedestrians who make use of a structure is the most efficient way of making such a
21

structure viable for animal use, but that number need not be reduced to zero. A number of
studies have shown that so long as human passage rates remain relatively low, there is no
measurable impact on wildlife passage rates (Mata et al., 2008; Yanes, Velasco, &
Suárez, 1995). Planners must also account for the fact that as general human disturbance
of a passage lane decreases increasing the utility of the structure for wildlife, the
likelihood of direct physical contact between any single human pedestrian and an animal
increases (Macdonald, 1998).
Developing structures for occasions where wildlife and humans must use the
structures simultaneously presents several unique challenges, but has become an
increasingly familiar issue in construction. As more roads are built and existing roads are
expanded with more lanes, human pedestrians have found themselves equal with wildlife
in their diminished ability to transit by foot (National Public Radio, 2014). Some have
suggested that this co-use is best accommodated by making use of paired structures of a
relatively close geographic proximity. In a number of cases, humans and wildlife have
voluntarily segregated themselves to one of the other structure (Olsson, Widén, & Larkin,
2008). Observational evidence suggests that human walkers elect to make use of the
structure that is most convenient or best fits their needs, while wildlife will choose the
least-disturbed passage available (Macdonald, 1998). Should these populations be forced
to make use of the same structure, the primary method of management that is advised is
screening vegetation and other visual barriers. Placing screening above an underpass,
thus shielding wildlife from the sight and some of the sound of passing vehicular or
pedestrian traffic above, has been showing to reduce visible disturbance by half (Phillips,
Alldredge, & Andree, 2001). Additionally, placement of screening vegetation at the
22

entrances to bridges and culverts can serve to discourage human use and limit their ability
to disturb the terrain (Gagnon et al., 2011). In cases of high passage rates, such as a when
a popular walking trail runs underneath a bridge, planners have suggested designing the
ground to have a vertical separation so that humans and wildlife make use of parallel, but
separate trails (Macdonald, 1998). This method can also be combined with screening
vegetation that runs the length of the underpass, ensuring minimal sight lines between the
human and animal trails.
The full recognition of the interrelation among both human and non-human
populations of interest that make use of passage structures has been recent, but as the
field has expanded, it has become evident that human pedestrians and animals are both
making common use of what were initially viewed as largely vehicle transportation
structures. Even now, when there is a major interest in developing wildlife-centric
elements like ecosystem overpasses to maintain habitat connectivity, human presence is
not discounted. Innovation in this field is focused on ensuring the co-use is incorporated
into design documents and all species’ needs are accounted for (National Public Radio,
2014).
Literature Review Summary and Thesis Research Questions
Human population growth and associated development will continue at an
increasing rate in the foreseeable future in the US and Washington State (United States
Bureau of the Census, 2000). The field of road ecology can then be expected to grow in
kind as scientists further explore the negative impact that road construction has on
wildlife populations and habitat connectivity. Bridges, culverts, underpasses, and
overpasses, originally designed purely with engineering interests in mind, have
23

increasingly become tied to ecological concerns and especially to enhancing connectivity
for the habitat used by wildlife populations. This partnership that allows for habitat
maintenance as an element of human development, rather than as a contrary element,
must be preserved.
In the interests of preserving and further developing a holistic approach to road
construction that takes into account the habitat and population requirements of all
affected species, it is imperative that scientific study of these issues not be limited to
ungulates and large carnivores merely because they are involved with the WVCs that
endanger human lives and property. The hundreds of millions of WVCs that go
unrecorded each year in the US due to the size of the animal struck are having an
unknown, but surely significant, impact on wildlife populations. There are conspicuous
gaps in the existing research regarding important aspects of road ecology that must be
filled. The following research seeks to work towards that goal by:
1. Assessing and analyzing the impact that human use has on animal use of wildlife
passage structures.
2. Summarizing several years’ worth of data on 8 unique structures throughout
Washington State and discuss the biodiversity implications.
3. Highlighting the impact that several specific design elements of structures have
on small vertebrate use rates.
There is no clear “right” answer when discussing passage structure design, nor is
there even a clear hierarchy or priorities. Some researchers claim that location of
construction is preeminent, while others insist on specific structural dimension, and

24

others yet focus on the makeup of the community using these locations (Foster &
Humphrey, 1995). This research will endeavor to suggest that a detailed, site-specific
knowledge is the best tool available to a planner. Upon completion, this research will be
shared with the Washington State Department of Transportation to ideally inform future
construction projects on how best to accommodate non-target species like small
vertebrates and humans when designing or retrofitting passage structures.
Chapter 2: Analysis of Human and Small Animal Use of Passage Structures
Study Introduction
Every year, an estimated 1-2 million vehicles collide with large animals in the
United States, prompting numerous studies investigating how to mitigate this source of
conflict between human and wildlife travel requirement (Huijser et al., 2007). Millions of
dollars are spent by state and federal agencies developing strategies and structures to
maintain habitat connectivity for these ungulates and large carnivore in an effort to
reduce WVCs. Road construction negatively affects wildlife in ways beyond simple road
mortality by limiting genetic diversity among populations by placing impassible barriers
and fragmenting traditional desirable habitat ranges (Forman et al., 2003). Resource and
transportation agencies have recognized this issue as a natural resource management
priority, even going so far as to spend tens of millions of dollars to design, build, and
maintain bridges, culverts, and wildlife-focused overpasses that mimic surrounding
habitat and allow for wildlife to pass below or above busy roadways (Smith, 2011;
O’Malley, 2004). This effort is well-intentioned and has proven effective in numerous
cases in reducing roadway mortality for large mammalian species by offering preferable
alternatives to crossing the road at grade (Hartmann, 2003; Dodd et al., 2004). The use of

25

these structures is not limited to specifically targeted species like deer and elk, however,
as human pedestrian use and small animal use occur at relatively high levels (Hartmann,
2003; Connolly-Newman et al., 2013). Historically, it has made sense to focus wildlife
passage structure research on the large mammalian populations responsible for so much
property damage and risk to human life, but the positives that these structures offer to
small animals in terms of limiting roadkill and promoting habitat connectivity is
deserving of future study.
At the highest estimate, less than 2 million large mammals are killed annually in
the US as a result of WVCs, but as many as 340 million birds are struck and killed by US
drivers every year and untold millions of small mammals, amphibians, and reptiles likely
share that fate (Huijser et al., 2007; Loss, Will, & Mara, 2014). No reasonable estimates
of the total number of smaller animals killed by vehicles exist, this despite an increasing
awareness of the importance that roads play in habitat selection and reproduction
pressures in small animals (Ruiz-Capillas, Mata, & Male, 2013; Aresco, 2005). What is
known, however, is that dry paths under road bridges and through underground culverts
have proven very effective in reducing roadkill numbers for small mammals, amphibians,
and reptiles, especially when paired with fencing or other methods to funnel population
movements (Huijser & Bergers, 2000; Niemi et al., 2012). Existing research shows that
these structures are effective in moving small wildlife safely across roadways, especially
when efforts are made to accommodate their particular needs with elements like elevated
crossing paths and entrance coverage (Jackson & Tyning, n.d.; Connolly-Newman et al.,
2013). The current breadth of study still requires more observations of what particular
elements appeal to which species and how passage rates differ by species groups.
26

Human use of passage structures is also perhaps more overlooked than it should
be. Human pedestrian passage through these wildlife crossing structures has a largely
negative effect on the likelihood that animals of any size will elect to make use of them
(Clevenger & Waltho, 2000). Recognizing this problem, most current research largely
suggests methods to limit human use either by placing structures in locations where
human use is unlikely, specifically distant from known settlements, or by actively
restricting human use (Hartmann, 2003; Rodriguez, Crema, & Delibes, 1996). In the
interest of promoting wildlife use, these strategies may be ideal, but they are not
necessarily realistic in many cases as road construction continues at pace with human
development in previously-natural areas, making human and animal co-use increasingly
likely (Gunson, Mountrakis, & Quackenbush, 2011). Most current studies emphasize that
human presence limits animal use of crossing structures, either through direct physical
interactions, noise pollution, or habitat alteration (Smith, 2003; Pedevillano & Gerald
Wright, 1987; Gagnon et al., 2011). Given the undue influence that humans have on
animal use, then, it is important to analyze just how deleterious this effect is and to find
ways to promote co-use.
This study uses existing camera data gathered as part of the Washington State
Department of Transportation’s Habitat Connectivity program observing existing bridge
underpasses and culverts across Washington State. The goal of this research is to better
understand the use communities of these structures and attempt to observe how the
interactions between human and wildlife populations combined with structural
dissimilarities between sites influences what species make use of these structures and
how often they successfully cross.
27

Materials and Methods
Study Origination
As part of WSDOT’s Habitat Connectivity policy directive affirming the agency’s
focus on protecting environmental systems and working towards the maintenance of
traditional habitat ranges for wildlife, since 2011 WSDOT has deployed cameras on
bridges and culverts across the state to study how wildlife use these structures. These data
were collected primarily with the goal of reducing large mammalian passage across
roadways. However, during processing of the imagery data, it became apparent that these
passage structures are used by a variety of species that far exceeds the scope of the initial
study topic. The research in this project is intended to explore the role that bridges and
culverts play in population movements for those other species, specifically small wildlife
and human pedestrians. The data used in this paper were gathered from a selection of the
dozens of cameras that WSDOT has deployed throughout the state of Washington since
that initial offering in 2011 and were analyzed with the goal of developing structural
elements that will better facilitate co-use. At the least, this information allows for a better
understanding of the true number of species and patterns of use that center around bridges
and culverts.
Site Identification and Camera Trap Installation
One of multiple WSDOT projects studying wildlife mortality on state highways
involved performing statistical hotspot and kernel density analysis of WSDOT’s Carcass
Removal Database to determine where the highest rates of WVC clustering were present
throughout the state. The analysis was performed on data from the years 2009-2013 and
included all deer roadkill collected from state highways by WSDOT maintenance

28

personnel during that period, accounting for 17,588 records (McAllister & Plumley,
2015). Deer were selected as the species of interest because this database only tracks
large animals large enough to pose a risk to humans through WVCs and, of the tracked
species, deer represented 95% of the recorded carcasses. Through statistical and
geographic information system analysis, WSDOT was able to identify portions of the
state highway system with the highest rate of clustering of WVCs for deer (Figure 2).
These locations were then narrowed to places within a half-day’s drive of WSDOT
headquarters in Olympia to guarantee ease of access for servicing of cameras by
eliminating the potential of spending multiple days on each monthly service. A number of
potential sites were scouted with an eye towards identifying regions of likely wildlife use
of passage structures and preference was given to locations with multiple distinct
structures that were separated, yet close enough to justifiably be considered paired. Five
sites were finally selected offering insights into a range of environmental, structural, and
community factors. Site were located in the Puget Sound region west of the Cascades, in
the mountains of the Cascade Mountains, in a valley within the same mountain range, and
the drier region east of the mountains. These sites have a combination of bridges and
culverts of various sizes and placement and exhibit unique use patterns, especially in
terms of percentage use by humans.
Once locations appropriate for the proposed study had been designated, motiontriggered, infrared wildlife cameras were installed in positons where they offered a clear
view of each structure of interest. Four models of camera were used throughout the
duration of this study. Initial cameras were Reconyx PC85 Professional Color IR models,
but as the project expanded, new installations primarily made use of modern Bushnell
29

Model 119476 and Reconyx HC600 HyperFire High Output Covert IR models. A grant
attained in the beginning of 2015 allowed for the purchase of a large number of Reconyx
PC900 HyperFire Professional IR cameras and the older, less reliable models, are
gradually being replaced with the PC900 models in the field as the older models begin to
fail due to age. As this project moves on past the timeframe of this specific study, the
cameras deployed in the field will continue to be swapped out with newer models as
technology progresses.
Three methods of installation were used depending on the structure being
observed and the local terrain: utility box installation, tree mounting, and Telespar
mounting (Figure 3). For utility box placement, cameras were disguised in steel utility
boxes and set in a concrete foundation of about 18-27 kg with a sheathed bike cable and
padlock combination attaching the camera to the cement base and a bolt and nut holding
the camera in place within the utility box. The front face of the utility box was screwed
into place and locked externally with a second padlock. For tree mounting, cameras were
encased in metal housing frames and bolted to a tree through the use of a thick metal
bracket. The front face of the casing was secured with a padlock. Telespar mounting was
functionally similar to tree mounting, but the frame was bolted to a metal Telespar post
so that the camera could be closer to the ground (Sullivan, 2014). All cameras were
programmed with electronic code locks so that unauthorized access to the programming
or data was made more difficult. Camera installations were all camouflaged, either as an
electric utility in the case of the boxes or through the use of paint in the case of the
frames, in an effort to reduce their visibility to human pedestrians to avoid vandalism or
theft.
30

Study Area
For the sites selected for this study, data has been collected from 21 cameras
which observed 9 structures at 5 sites in various parts of Washington State. In the
interests of protecting ongoing studies from theft or other disturbance, only general
locational information is provided here. The first, “Western Forest Trail”, was located
west of the Cascade Mountain range in a forested area of the Puget Sound climate zone.
The second and third sites, “Cascades Wet Culverts” and “Cascades Dry Culvert” were
located in the Cascades Mountain Range, in high-elevation forests. The fourth site,
named “Cascades River Valley,” was situated in one of the Cascade Mountains’ many
river valleys, providing for a unique ecosystem segment to analyze. The final location,
“East Dry Forest,” could be found on the eastern slopes of the Cascades, in a climate with
far less precipitation than the other sites. These sites were chosen for their roles as WVC
hotspots as defined by WSDOT data, locations of continuing concern for habitat
connectivity, their possession of multiple close, but distinct paired passage structures, and
to offer perspective on a variety of ecoregions (Figure 4).
Western Forest Trail
Western Forest Trail is a popular running and bicycling trail near a town of less
than 10,000 people. The trail runs north-south between the verges of the highway and a
sizeable river. This area is heavily forested and, while it stands only a few hundred feet
above sea level in elevation, has ample mountainous terrain in the surrounding region.
This location lies to the west of the Cascade Mountain Range, meaning it shares the
climate of much of the area around the Puget Sound, namely mild, wet winters and warm,
dry summers. These environmental factors combine to offer excellent habitat to
31

ungulates. High ungulate populations near Western Forest Trail combined with the high
traffic volumes on nearby roadways have made this stretch of highway a hotspot for
WVCs.
A 1976 steel and concrete bridge with approximately 10 feet of vertical clearance
underneath is the structure under observation for this study area (Figure 5). This structure
was built by WSDOT to allow for a highway crossing over the local river. A six-foothigh (1.8 m) fence has been in place for some time to prevent wildlife from moving onto
the highway, but the fencing is in disrepair at some spots, so wildlife of all sizes can
occasionally be found on both sides. Columbia black-tailed deer (Odocoileus hemionus
columbianus) and elk in particular make common use of this area due to the easy
accessibility of the river and the riparian vegetation (Sullivan, 2014). Dikes, in place on
both sides of the river, offer relatively level, unobstructed paths of travel that are
attractive for animal passage.
East Trail
Two Reconyx HC600 HyperFire cameras were installed on trees along the east
side of the river on 8/1/2012 observing the pedestrian trail and the riverbank. The
cameras were placed at the top of an embankment less than 100 m from the river and had
separate fields of view. This side of the river is a popular recreation area for the nearby
residential population, with joggers, cyclists, fishers, and bathers all making regular use.
These cameras and brackets were recovered on 8/13/2014 after 743 days of service due to
the completion of the desired length of observation.
West Trail
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Two Reconyx HC600 HyperFire cameras were installed on trees along the
western bank of the river on 12/26/2012, observing the bridge’s abutment and pier. The
cameras lay within 100 m of the river and have some overlap in field of view. These
cameras were installed when it was recognized that despite the east trail offering an ideal
passage opportunity for the recorded ungulate population in the area, very few detections
were being made due to high human use. This side of the bridge had very little human use
and contained open area surrounded by screening vegetation. The cameras and
accompanying equipment was removed on 6/19/2014 after 541 days of service due to the
completion of the desired length of observation.
Cascades Wet Culvert
Cascades Wet Culvert allows for the passage of a small creek to pass beneath a
highway in the Cascades from south to north. This creek persists year-round but the rate
of flow is highly seasonal. The structure under observation at this site is a cement doublebox culvert with each opening being approximately 6 ft. x 4 ft. and 60 ft. long (Figure 5).
The culvert is found in a forested area with rocky terrain on the banks of the stream. The
eastern of the paired structures is consistently filled with several inches of water but the
western of the paired structures remains clear of water during the dry summers and
occasionally carries a small amount running water during the wet seasons. The substrate
of the passage is covered with stones of various sizes. While there is heavy vegetation on
either side of the structure, there is no screening foliage between the creek and the
highway.

33

A single Telespar-mounted Reconyx HC600 camera was installed here on
4/22/2014 observing the south side of both box culverts. This camera was placed
approximately 3 feet above the ground adjacent to the creek bank. The most recent
recorded data for this camera were taken on 4/13/2015, representing 357 days of service,
and this camera remains in place as of this writing.
Cascades Dry Culverts
This study site is located in an evergreen forest within the Cascade Mountain
Range. This site offers a unique necessity for habitat connectivity as the surrounding
region is actually located between the westbound and eastbound portions of a large
highway. This site is mostly undeveloped, but the few residences in the area are closely
located to the structures of note.
The structures being observed at this site are corrugated steel culverts that span
the westbound half of a highway (Figure 5). There is a significant embankment between
the entrances to the culverts and the highway, meaning that traffic sight and sound is
mitigated. The vegetation at the entrances of the culverts is largely salmonberry apart
from smaller brush, so the area is well screened during the warmer growing season, but
sparser during the winter. The wildlife in the area are almost universally smaller due to
the compacted habitable area between the separated highway segments.
West Culvert
Two Reconyx PC900 HyperFire cameras are located on either end of this culvert
with the southern, tree-mounted one being installed on 6/19/2014 and the northern,
Telespar-mounted one being installed on 3/9/2015. This culvert is approximately 6 feet
34

wide, 8 feet high, and 200 feet long, moving moderately uphill from south to north. A
small stream is present on the southern end of this culvert, but no water runs through the
actual pipe. This site is also relatively commonly used by human climbers who park on a
forest road on the southern side and traverse to a climbing wall on the northern side. The
most recent recorded data for these cameras were taken on 4/13/2015, jointly
representing 299 days of service, and these cameras remain in place as of this writing.
East Culvert
Three Reconyx PC900 HyperFire cameras are located on either end of this culvert
with the first southern, tree-mounted one being installed on 6/19/2014, the northern, treemounted one being installed on 8/13/14, and the second southern, Telespar-mounted one
being installed on 9/18/2014. The number of cameras in place gradually increased as
resources became available with the goal of capturing as much data as possible. This
culvert is approximately 5 feet wide, 5 feet high, and 200 feet long, moving moderately
uphill from south to north. No water is found on either end of this culvert and it remains
dry year-round. There is more vegetation coverage at this site and small wildlife make
regular use of woody debris to approach the culvert before crossing. The most recent
recorded data for these cameras were taken on 4/13/2015, jointly representing 299 days
of service, and these cameras remain in place as of this writing.
Cascades River Valley
The river valley within the Cascades selected that includes this site covers a
portion of the east-west oriented US Route 12. The studied bridges are located within a
valley at an elevation of approximately 1,000 ft.
35

This river valley is a prominent area for agriculture and ranching as the flat,
riparian terrain is well suited to the industry. The surrounding environment to these
bridges is largely grassland and pasture with riparian zones that flood during wetter
seasons. The relatively sparse human population combined with floodplain soils that
makes domestic farm animals so prosperous also encourages a large population of wild
ungulates. These ungulates travel across the grasslands from one fragmented forest patch
to another. There are two bridges being observed at this site, one that spans a river and a
second built over a common path of seasonal flooding, but is otherwise covering dry
ground (Figure 5).
Main Bridge
Three Reconyx PC85 Professional cameras are located near this bridge with two
utility box cameras being installed on 12/29/2011 on either side of the bridge and a
further utility box camera being installed on the eastern bank on 4/24/2013. This structure
is a large bridge with more than 15 feet of clearance and ample open space under the
roadway. A major river runs constantly beneath the bridge with embankments on either
side, blocking passage from east to west. The western bank is covered with heavy
vegetation, especially blackberry, while the eastern bank is more defined by tall grasses.
The eastern bank borders a fenced-in cattle ranch that regularly impacts wildlife passage
in a negative manner. The most recent recorded data for these cameras were taken on
3/24/2015, jointly representing 1182 days of service, and these camera remain in place as
of this writing.
Overflow Bridge

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Two Bushnell Model 119476 cameras are located near this bridge with the first
Telespar-mounted one being installed on 9/3/2013 observing the north face of the bridge
and a second Telespar-mounted camera being installed beneath the bridge on 5/22/2014.
This structure is a smaller bridge with approximately 8 feet of clearance. This bridge was
designed to accommodate floodwaters that would otherwise cover the roadway, but
remains dry outside of severe flood events. The area around this bridge is covered with
tall grasses and is bordered by ranchland on either side. The most recent recorded data for
these cameras were taken on 3/24/2015, jointly representing 568 days of service, and
these camera remains in place as of this writing.
East Dry Forest
The East Dry Forest camera site is found along a north-south oriented highway
east of the Cascade Mountains. The climate and environment of this area is complex. As
the Cascades border this region to the west, the area surrounding this camera location
falls under the rain shadow effect. The land is arid and mostly defined by Ponderosa Pina,
brush, and grass, with extensive forested areas to the north as the elevation climbs.
The stretch of road encompassed by this study area is a likely place for WVCs
due to a large population of black-tailed deer that move through the dry forests and
grasslands and interact with the moderately-trafficked 2-lane highway (McAllister, &
Plumley, 2015). Much of the traffic that travels along this highway is compositionally
dominated by industry. Many of the vehicles are semi-trailer trucks moving goods
between distant cities and there are relatively fewer smaller vehicles due to the limited

37

human settlement nearby. A paired bridge and culvert are being observed at this site
(Figure 5).
Bridge
Four Reconyx PC85 Professional cameras are located around this bridge with two
tree-mounted cameras being installed on 12/3/2012 on either side of the bridge along
with two accompanying utility box cameras installed on 4/2/2013. This structure is a
large bridge with more than 15 feet of clearance, but has narrow, steep rip-rap abutment
armoring that border closely to the creek, impacting passage during high-water marks.
US97 Creek, which this bridge spans, varies considerably by season, as it rises high
enough to negatively impact north-south passage during wetter seasons but dries to
mostly sub-surface flow during the rainless summer. The paths along the creek are almost
exclusively rounded stones rubbed smooth by water action. With regards to vegetation,
the openings to the bridge are largely clear apart from sparse trees. The most recent
recorded data for these cameras were taken on 4/20/2015, jointly representing 869 days
of service, and these camera remains in place as of this writing.
Culvert
Two Bushnell Model 119476 cameras are located on either end of this culvert that
were installed on 6/10/2013, but were moved to a closer, Telespar-mount on 9/23/2014 in
order to get a better perspective on wildlife use of the culvert. This culvert is
approximately 5 feet wide, 5 feet high, and 40 feet long. No water is found on either end
of this culvert and it remains dry year-round. As there is no likely route for water to pass
through this structure, the original purpose of its installation is uncertain though it was
38

likely installed so that livestock could pass safely under the highway. There is no major
vegetation coverage at the entrances of this culvert. The most recent recorded data for
these cameras were taken on 4/20/2015, jointly representing 680 days of service, and
these camera remains in place as of this writing.
Data Collection
Deployed cameras were visited every four weeks for servicing and maintenance.
Service included replacing all batteries in each camera with a fresh set of 12 AA batteries
or 6 C batteries, depending on the model of camera, and exchanging empty memory cards
for the ones holding data within the cameras. During servicing, cameras were checked to
ensure that trigger settings remained accurate, a step that was especially important with
the Bushnell cameras as the date and time on the camera had a tendency to reset to 0:00,
1/1/2012 when the memory cards were changed. In the event that the number of images
taken by a camera was clearly influenced by an environmental factor (like waving
vegetation), usually intuited by the number of recorded images exceeding 1,000 over the
previous month, steps were taken to clear the field of view. This was usually
accomplished by cutting down nearby vegetation with a machete, though in some
instances the camera’s location or angle needed to be shifted. Cameras were also
maintained during these visits with meticulous records being kept of the dates and types
of malfunctions. Cameras that malfunctioned multiple times were replaced.
There was also an issue with members of the general public “interacting” with the
cameras. On several occasions, pedestrians who noticed the cameras apparently
attempted to tamper with the installations. In all occasions save one, the metal housing of
the cameras proved sufficient to protect the cameras from damage or theft. On October
39

10, 2014 one individual committed an act of vandalism and stole 9 cameras from a study
site near North Bend, WA. This resulted in a removal of the remaining cameras in that
region and a redesign of the protective housing used for the cameras. The results of this
process included thicker metal housing for the cameras, welding of brackets to the
Telespar mounting posts, and locating cameras in places where they would not be as
easily visible to passersby.
When cards with data were returned to the Olympia WSDOT office, they were
processed by visual interpretation of detections. Relevant information was recorded for
each detection including: date, temperature, start time of the detection (in Pacific
Standard Time), end time of the detection (in Pacific Standard Time), species, age,
gender (if identifiable), total individual count, and a determination as to whether the
observed animal passed through the relevant structure (Sullivan, 2014). In the event of
multiple individuals of any species in a single detection, a single record was made, but
the counts for species, age, etc. included all individuals within that record. Species
determinations were carried to the species level whenever possible though due to poor
image quality or camera angle, some observations proved unidentifiable, especially
smaller mammals. Data were organized onto a series of Excel spreadsheets that were
updated monthly.
Data Processing & Statistical Analysis
Running spreadsheets were maintained for each camera that were updated each
month as new images were collected on the data cards. These spreadsheets were then
merged into a single running spreadsheet for each structure. In most cases, multiple
cameras were in place on a structure, to help confirm passage or to cover the larger
40

spaces beneath bridges. Where there were multiple cameras on a single structure,
observations were combined from all relevant sources with duplicate observations being
excluded. A notation was also made in the structure-level spreadsheet whether the
observed individual successfully crossed the structure based on the multiple angles and
sides under camera coverage.
From the deployment of the first camera selected for this study until the end of
data collection for this study, a total of 36,896 individuals were recorded in 11,839
detections over a period of 5,538 concurrent (1,209 sequential) days of observation. For
the purposes of data analysis, the observations from each structure were limited to the
most current complete annual cycle. This retained 18,702 individuals observed during
5,671 detections. It should be noted that the majority of these, 15,038, were humans and
related species. As some camera have been decommissioned and others had period of
malfunction where a month of data was lost, the dates of these annual cycles are not
necessarily the same. In the cases of the Cascades Wet and Dry Culverts, a full annual
cycle was not available with 357 days’ worth of data collected for the former and 299
days’ worth for the latter. To correct for the temporal disparity, observation and passage
rates were calculated as weekly rates.
Once observations were identified down to the lowest taxonomic level (typically
to species), a total of 33 different animal types had been recorded, from Wild Turkey
(Meleagris gallopavo) to cougars to pika (Ochotona princeps). As this number was seen
as unwieldy for comparison of passage communities across structures, all observed
species were summarized into one of 6 animal type groups with presumed similar distinct
behavioral traits and habitat requirements: Human (including domesticated canines and
41

horses), Ungulate (hooved mammals), Large Carnivore Mammals, Small Carnivore
Mammals, Small Prey Mammals, and Birds. Once summarized thusly, pie charts were
constructed for each studied structure to visually interpret the differing compositions of
use communities.
As much of this study is observational in nature, statistical analysis is not present
omnipresent. When discussing how animal use of paired structures differed or how
movement of small mammals changes with the introduction of elevated paths, descriptive
text and visual observations are noted rather than statistical methodology. For the
remaining sections, where statistical analyses were necessary, a combination of JMP, R,
and Resampling Statistics for Excel were used. As seen in the following results,
regression, analysis of variance (ANOVA), and Pearson’s chi-square tests were
performed when relevant to the variables.
Results
Passage Structure Styles and Elements
The data collected from the suite of deployed wildlife cameras over a single
calendar year revealed several distinct patterns of human and wildlife presence that can
be partially attributed to the dimensions, type, and form of the structures studied (Table
1).
Bridges vs Culverts
When performing an analysis of structure use by structure type, two of the nine
sites were excluded: Cascades Wet Culvert and Western Forest Trail 1. Cascades Wet
Culvert was excluded due to a number of confounding variables present at that site but
42

not others, most importantly the sizeable flow of water through the structure without any
dry passage possible. This factor resulted in extremely little use of the structure by either
human or animal populations. Western Forest Trail 1 was excluded as a major dataset
outlier. As Western Forest Trail 1 covers a popular running trail, mean human use stood
at 135.5 individuals per week; the remaining structures all operated within a range of 0.5
– 4.1 individuals per week.
In an analysis comparing mean weekly observation rates for wildlife by structure,
no significant difference was found, though there may be a slightly higher rate near
bridges where wildlife were observed at a rate of 11.29 individuals per week per structure
as contrasted to the 8.19 individuals per week seen on average at culverts (p = 0.5067).
When the same analysis was performed for human observations by structure, there was
again no significant difference found with both structure types exhibiting an observation
rate of approximately 1.8 individuals per week per structure, though the number of
observations of researchers (sometimes at a rate as high as 1 per week) combined with
the relatively low totals overall likely diluted any potential preference (p = 0.9535)
(Figure 6).
When the confirmed crossing rates were instead analyzed, however, more telling
information was apparent. A One-way ANOVA of percentages of confirmed crossings by
all individuals out of the total observations bordered on statistical significance with
human and wildlife apparently crossing at a higher percentage of observations through
bridges rather than through culverts (p = 0.0527). When the data was limited to only
wildlife observations for the analysis, there was an even stronger pattern showing
significantly higher confirmed crossing rates for wildlife through open bridge
43

underpasses, at 67.2% of total observations, rather than narrow culverts, at 26.9% of total
observations (p = 0.0216) (Figure 7). It should be noted that bird observations were
included in the observation analysis, but not the crossing rate analysis due to the
difficulty inherent in judging movement routes of birds via still-frame images.
Dimensions and Environment
Neither structure cross-sectional area nor length proved significant predictors of
usage patterns. A bivariate fit of wildlife individuals per week and confirmed passage
percentage by passage length resulted in insignificant relationships (p = 0.7234 & p =
0.3345, respectively). The fit does seem to indicate the possibility that increased passage
length decreased confirmed crossing rates, however.
A bivariate fit of successful passage percentage by cross-sectional area also
showed no significant relationship, though the line of best fit did show a positive slope
indicating some likelihood that passage rates may improve with larger cross-sectional
areas (p = 0.4918). A binomial fit of wildlife individuals per week by cross-sectional area
demonstrated the strongest relationship, but it was again not statistically significant (p =
0.1590). This binomial fit showed an increase in wildlife observations as cross-sectional
area approached approximately 200 m² before decreasing with areas greater than that
(Figure 8).
Many studies suggest that an “openness ratio,” defined as cross-sectional area
divided by length, is an important determinant of use by mammals, especially larger and
medium-sized ones (Cain et al., 2003; Jacobson, 2002). Using this metric in place of
cross-section alone to predict wildlife activity offered mixed results in this study. The
44

binomial fit of number of individuals observed per week versus openness ratio offered a
marginally worse fit than cross-section (p = 0.1618). The bivariate linear fit of successful
crossing rate for wildlife showed a marginally better fit than cross-section (p = 0.4886)
(Figure 9).
One structure from this study, specifically the double box Cascades Wet Culvert,
is notable for the presence of a continuous stream running through. No other observed
culvert has more than an occasional trickle of rainwater or snow melt and the bridges
observed either were dry as well or were wide enough to accommodate dry passages on
either side of the spanned river. It is notable, therefore, that the Cascades Wet Culvert
structure represented the lowest passage totals and rates of any structure. In the 357 days
that this culvert was under the view of a motion-triggered camera, every one of the 27
human observations was attributed to researchers servicing the camera on a monthly
basis. The remaining 17 wildlife sightings resulted in only 2 confirmed crossings, both
numbers representing a use profile far lower than any other structure (Table 1). Of the
paired square cement culverts, both confirmed crossings occurred through the western
option, which carries significantly less water than the eastern tunnel.
Structure Elements
As previously discussed, smaller wildlife have a tendency to pass along elevated
structures when the option is available. While none of the structures from this study were
equipped with wildlife shelving (whether such a structure even exists in Washington
State is unknown), anecdotal evidence from these cameras would seem to support this
supposition. Upon installation of the first cameras at the Cascades Dry Culvert East on

45

June 19, 2014, it was observed that many of the animals that approached the structure
made use of a fallen tree branch on the southern end that ran towards the entrance. In the
case of species like Douglas squirrels and deer mice, this proportion was particularly
large. During the 11/10/2015 and 3/9/2015 services, further tree branches were moved
towards the entrances of the structure on both the northern and southern ends of the
structure to see how wildlife would respond. The reaction was nearly instantaneous as
rodents passing through the culvert began to make virtually exclusive use of the branches
to transit to and from the entrances (Figure 10). The branch placed on the southern
entrance, however, had a unique use pattern as animals approaching the culvert from the
south made use of the branch, but stopped at the end as, unlike the northern branch, the
southern branch stopped just shy of the actual tunnel. In some cases animals spent full
minutes perched on the end of the branch and moving back and forth in apparent
indecision or confusion before electing to enter the structure without using the branch
(Figure 11).
Species Diversity in Passage Structures
Of the 16,744 individuals recorded as having passed through one of the observed
structures during the one-year periods that the data were parsed to, 16,698 were identified
down the species level, revealing a complex and interrelated passage community that
perhaps outstrips the general perception in terms of diversity. The vast majority of these
individual crossings, 14,920, were identified as human, canine, and horses. In total, 33
species were identified as having approached one of the 9 passage structures observed,
and 26 species could be confirmed as having had at least one individual successfully

46

cross through. The combined passage rate for all observed individuals was 89.53%,
though that rate dropped to 49.78% when the human subsection was excluded (Table 2).
Confirmed Passage Percentages
A number of details became apparent as data were collected about speciesspecific crossings, especially a number of striking disparities in confirmation rates. First,
the highest passage rate by summarized species type was a perfect 100% by large
carnivore mammals, but this was the result of only 5 observations between all cameras,
all of a single cougar individual on multiple occasions at the Eastern Dry Forest location.
After this population came humans and related species like domestic canines and horses
at a 99.22% success rate, though it should be noted that when canines were observed near
structures absent humans, their success rate fell to 57.14% (Table 2).
Ungulates passed through available structures at a 56.38% rate, but this does not
fully reveal the disparity in this collection as black-tailed deer successfully crossed at a
73.18% rate, far higher than the 47.54% rate for the larger elk species. This divergence in
confirmed passage rates was analyzed using a chi-square test and found to be of a high
statistical significance (p < 0.005). No successful passages were recorded for cattle, but
the only cattle observed were caught by one of the Cascades River Valley cameras that
could see into a neighboring fenced-in cattle field that cattle could not enter from (Table
2). Cattle accounted for less than 2% of the total ungulate observations, but when they
were excluded, successful passage rate rose to 57.74%.
Small carnivorous mammals achieved a 68.75% success rate as 88 of the 128
observed individuals made use of these structures for transit. Bobcats were the most

47

commonly observed species of carnivore, accounting for almost 40% of the total and
crossed structures at a higher-than-average rate of 93.75%. Coyotes represent something
of a trend in that they are again a larger species within a subset that show a much smaller
chance of successful transit, with only 13.64% of the observations being confirmed
successes (Table 2). A chi-square test comparing the passage rates between bobcats and
coyotes, again similar to the deer/elk relationship, showed a strong statistical difference
(p < 0.005).
Small prey mammals proved difficult to identify at times due to their small size,
speed, and generally nocturnal active periods, which combined to produce blurred
pictures on many occasions. For these reasons, a relatively high number of small prey
mammals remained unidentified in terms of species, 234 of the 799 whole. Of those that
could be identified, Douglas squirrels, at 144, bushy-tailed woodrats, at 136, and deer
mice, at 213, comprised most of the population counts. Contrary to the pattern established
with ungulates and small carnivore mammals, with small prey animals the larger species
passed at a higher rate with snowshoe hares, Douglas squirrels, and bushy-tailed
woodrats succeeding 68.42%, 64.58%, and 42.65% of the time, respectively. Conversely,
smaller species like deer mice and Townsend’s chipmunks crossed at lower rates, 18.31%
and 18.18%, respectively (Table 2).
Birds were not a group specifically targeted by this study, but their relative
abundance near passage structures merited their inclusion in this data analysis. In fact,
nearly twice as many birds were observed by these mostly low-angled, close-view
cameras as small carnivore mammals. Ninety-five of the 203 observed birds were
identified as American Robins, 37 were Varied Thrushes, 20 were Steller’s Jays, and 14
48

were Wild Turkeys; these represented the major populations observed. As the movement
patterns of birds, namely long periods of being stationary before a sudden burst of quick
motion resulting in the vacating of an area, is unsuited for the motion-triggered cameras
used, very few confirmed passages could be accounted for, resulting in a success rate of
7.39%. The actual number may, in fact, be higher, but this study was not designed to
account for bird movements and thus can only confidently discuss bird observations, not
crossings other than to note that all confirmed passages took place beneath bridges and
the only species that could routinely be confirmed as crossing beneath were the larger,
slower, ground-dwelling Wild Turkeys (Table 2).
Paired Structure Analysis
Except for the Cascades Wet Culvert, where the structures were close enough that
they cannot reasonably be considered independent from one another, every structure in
this study had a paired structure in a similar, nearby location, but with each matching
structure possessing one targeted difference. A number of differences, including
dimensions, vegetation coverage, and camera coverage existed between these paired
structures, but each pairing also had one of the several large disparities here described
that could influence use communities.
As noted before, the Cascades Wet Culver was mostly filled by water throughout
the year. This had a major effect on the crossing community in that it effectively ensured
that none existed. In fact, the presence of researchers servicing the camera accounted for
nearly 2/3 of the total observations. Of the perceived animals, all were small and
generally uninterested in passage (Figure 12).

49

The paired Cascades Wet Culverts differed in size and human pedestrian use. The
western culvert had a cross section of 3.53 m² and saw 174 humans make use of it
throughout the study period in contrast to the eastern culvert, which had a cross section of
1.13 m² and only recorded 23 human uses, most of which were attributed to the camera
operators. These differences had very little impact on small carnivore use, but the number
of small prey animals was more than four times higher in the smaller, more secluded
eastern culvert where they made up more than 3/4 of the entire population (Figure 12).
The Western Forest Trail structures were, in fact, the same bridge, but on either
side of a major river, dike embankments, and fencing. The communities could thus
reasonably be considered separated, with the eastern trail having much higher human use.
With nearly 15,000 humans passing through the eastern structure over the study’s span,
wildlife presence was excluded almost entirely (46 observations), but on the western side
of the river, ungulates moved with relative ease as they represented the vast majority of
observations at 377 of the total of 421 (Figure 12).
The Cascades River Valley Main Bridge and Overflow Bridge differed in that
Main Bridge spanned a river while the overflow bridge had a dry underpass. This resulted
in higher human presence (202 individuals) beneath Main Bridge, mostly fishers and
recreational swimmers, and much lower ungulate use. Despite only being located only a
few hundred feet down road from Main Bridge, Overflow Bridge had more than three
times more ungulate observations, 1,015 versus 324, in part likely due to the significant
decrease in human use, down to 55 individuals (Figure 12).

50

A bridge and culvert were selected on for the East Dry Forest site due to their
close proximity, but clear structural dissimilarity. This pairing resulted in much more
biodiversity near the culvert as small carnivore mammals, small prey mammals, and birds
accounted for 42.5% of the observations where they were only identified in 3.7% of the
bridge observations. It must be noted that the cameras installed observing the bridge were
not installed with a design intended for observing small animals, but with four
overlapping cameras that have shown a capability to capture animals as small as bobcats,
canines, and California ground squirrels on occasions, the disparity remains suggestive.
In addition, the larger cross-sectional area East Dry Forest Bridge contributed to the
counting of nearly five times as many ungulates as the smaller culvert (628 versus 131
individuals) (Figure 12).
Human Impact on Wildlife Passage
Human and Wildlife Use Rates
A bivariate linear fit of mean weekly human observations by mean weekly
wildlife observations resulted in a highly suggestive negative relationship that is
significant at the p = 0.1 level, but not at the p = 0.05 level (p = 0.092) (Figure 13). For
this correlation analysis, the Cascades Wet Culvert site was excluded due to its
environmental confounding variables and the Western Forest Trail 1 site was excluded
due to its extraordinarily high human use rate overwhelming the dataset.
When observing the dispersion of the data, there appeared to be a natural division
in the dataset once human observation rates reached approximately 3 individuals per
week. Performing a One-way ANOVA by dividing human observation rates into

51

categorical variables of >3 or <3 individuals per week resulted in a significant decrease in
wildlife observation rates in the >3 per week categorical (p = 0.016) (Figure 14). This
categorical analysis style also allowed for the reintroduction of the Western Forest Trail 1
site to the dataset as the high human use did not shift the entire graph. This evidence
would support previous studies that argued that a low level of human use of underpasses
has little impact on wildlife use, but once human use reaches a certain level, there is a
significant direct impact.
Human Impact on Wildlife of Different Sizes
A question arose during this analysis as to whether human activity had a different
impact on larger animals when compared to their impact on smaller animals. A bivariate
fit analysis of number of large wildlife individuals per week (including the Ungulate and
Large Carnivore Mammals subsets) by the number of observed human individuals per
week showed no confirmed statistical correlation, but did suggest a slope of -0.98 large
wildlife individuals/human individual (p = 0.6516) (Figure 15). A bivariate fit analysis of
number of small wildlife individuals per week (including Small Carnivore Mammals,
Small Prey Mammals, and Birds) by the number of observed human individuals per week
also showed no confirmed statistical correlation, but had a slope of -0.83 small wildlife
individuals/human individual (p = 0.5527) (Figure 15).
While this analysis would seem to indicate no real difference in human impact on
wildlife species of different sizes, a number of confounding variables limit the utility of
the results. Specifically, though this analysis again excluded Cascades Wet Culvert and
Western Forest Trail 1 as outliers, human observation rates were much higher around

52

bridges, where larger mammals operated almost exclusively. Also approximately 0.5
human observations per week were due to camera servicings by researchers, unduly
influencing sites with low human use.
Human and Wildlife Co-use of Structures
An ANOVA of wildlife observations by structure type showed that, in summary,
wildlife approach bridges and culverts at equal rates, with virtually no difference in
individuals per week between the types (p = 0.968). An ANOVA analyzing the same
distinction among humans again showed no significant difference as well, though a slight
preference for bridges appears largely as a result of the inclusion of the Eastern Forest
Trail 1 site (p = 0.394). This analysis shows that both types of structure are important for
wildlife passage and human passage, though bridges may be slightly more important for
humans.
Discussion
Overall, the findings from this study show that transportation structures like
culverts and bridge underpasses provide passage potential for different use communities
based on a number of factors like structure type, cross-sectional area, water presence, and
human use rate. In particular, this research provides novel information of the number and
variety of small animals that commonly make use of culverts and bridge underpasses in
western Washington and suggests how environmental and architectural factors can
influence these use communities. Based on patterns observed by camera traps placed
around multiple structures located near to one another, inferences can be drawn about
what known differences account for these differences. The observational nature of this

53

study precludes the ability to categorically state what structural or environmental
elements definitively affect wildlife crossing rates either positively or negatively, but
differences in observed populations have proven highly suggestive.
This research presents the determination that the small (<50 lb.) wildlife species
that make use of passage structure, especially smaller culverts, do so at a high rate and
with a great deal of species diversity. In addition, wildlife presence and confirmed
crossing rates generally increase with the increased cross-sectional area presented by
bridge underpasses. Low levels of human pedestrian use seem to have a minor effect on
wildlife use, but increased use results in an apparent, though not statistically significant at
the p <0.05 level, decrease in wildlife use. Finally, small wildlife use of downed tree
branches to enter and exit passage structures, especially ones that actually enter said
structure, would suggest that they prefer to move through culverts above ground level
when the option is available.
Passage Structure Styles and Elements
One site, more than any of the others, seemed to have its use community heavily
defined by certain environmental and structural factors in place. Specifically, Cascades
Wet Culvert was the only site of the 9 selected that did not have a dry passage route
available for most of the year as well as being the only double culvert and the only
culvert constructed out of concrete instead of corrugated metal. Consequently, when
observations of researchers servicing the cameras are excluded, use of the Cascades Wet
Culvert was minimal, less than 10% of the total of the next-least-used structure. This
aversion for small wildlife to make use of a culvert with permanent flowing water fits
with the expectations of the research given existing literature on the subject (Serronha et
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al., 2013). Even among the culverts studied, which generally exhibited lower use rates
than the bridge underpasses, the Cascades Wet Culvert proved a major statistical outlier.
Given the apparent unsuitability of this structure for wildlife use, it is interesting that the
confirmed crossing rate (14.3%), while still less than the remaining culverts, is not
terribly out-of-line with their rates, which were calculated to be as low as 18.9%. How
much of this is due to the small sample size at this structure, numbering a mere 14 nonbird wildlife individuals, and how much is due to willingness for small wildlife to use
typically-unsuitable passage structures when no other option is readily available is up to
interpretation. The bridge cameras, specifically those on the Western Forest Trail and
East Dry Forest bridges that spanned streams did occasionally observe ungulates moving
fairly easily through significant water features, but the small body size of the typical
animals that make use of culverts does not lend those individuals to movement along
anything but dry paths (Wolff & Guthrie, 1985).
The culverts in general recorded fewer observations when camera operator
detections are excluded than the bridge underpasses, with an average of 320.25
individuals seen per culvert and an average of 3,420.8 individuals seen per bridge
underpass. Each structure type included a major dataset outlier, however. As noted,
Cascades Wet Culvert proved to have nominal use compared to other culverts, but
Western Forest Trail 1 exhibited the opposite extreme, recording almost 14 times as
many individuals as any other bridge. When those low and high extremities are excluded
from the dataset, the numbers of individuals recorded per structure type prove to be more
similar, with 418.33 detected per culvert and 675.5 detected per bridge. Restricted
entirely to wildlife, thus excluding the human and related species observations, the
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difference in observations indicate a similar relationship, with 492.8 individuals seen per
bridge and 278.5 per culvert. The difference remains sizeable, but no longer seems so
overwhelming. The values for each individual structure varied widely within type groups;
however, different bridges saw between 46 and 1,029 wildlife individuals over the annual
cycle while different culverts had observations that ranged from 17 to 636 individual
animals. This lack of any semblance of homogeneity within structural design datasets
suggests that a number of factors play a role in determining animal use beyond simply the
type of structure.
One such factor could be the cross-sectional area or openness ratio (crosssectional area/length) of the passage. This study showed that the number of observations
generally increased as cross-sectional area increased before levelling off around 100-200
m2. Although the data also showed an apparent decline in observations per structure past
that apex, the limited camera coverage for larger structures with the camera resources
available is likely the primary cause. It would not be expected for wildlife to show a
negative selection pressure against structures that offer more open passage space. Instead,
the fact that larger structures dictate that cameras be placed further back, combined with
the limited range on the triggering mechanisms for the cameras used for this study, meant
that an unknown number of animals likely made use of these larger structures without
any record being made available.
In addition to cameras located near bridge underpasses identifying higher
numbers of individuals than those on culverts, the bridge cameras also showed a much
higher confirmed crossing rate for humans and wildlife as many individuals that
approached culverts elected not to pass through them. When both populations were
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combined, the passage rate for bridges was approximately 75% compared to
approximately 35% for culverts. The disparity is even more apparent when wildlife
(excluding birds) were analyzed independent of the human population, with a confirmed
crossing rate of about 75% for bridges and about 20% for culverts. Birds were excluded
from confirmed crossing rate analysis due to the ineffectiveness of motion-triggered stillframe cameras in determining bird movement paths. These findings, that large, open
spaces beneath bridges offer a variety of habitats and movement paths that are most
conducive to animal use, support other research findings (Connolly-Newman et al., 2013;
McCollister & van Manen, 2010).
Current scientific knowledge states that small mammal populations will
voluntarily move along branches or other physical elements rather than on the ground
when the option is available and anecdotal evidence from this study supports this
perception (Foresman, 2004). The Cascades Dry Culvert East is surrounded by a fair bit
of debris from fallen tree branches due to the thick mountain forest in which it is located.
After initial deployment of cameras on this structure, it was observed that the majority of
the small wildlife observed, especially Douglas squirrels and deer mice consistently
moved back and forth across one of these downed branches that happened to be in front
of one of the cameras. As this particular branch was not contiguous with the culvert, but
merely in the vicinity, researchers grew curious about whether this behavioral pattern
would change when the elements were combined. Starting with the north end of the
culvert, where smaller mammals tended to pass closely along the western wall of the
culvert and medium sized mammals (mostly bobcats) generally moved down the center, a
nearby branch was placed along the eastern wall of the culvert so as to minimally impact
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traditional movement paths. After a few initial days that the individuals in this
community required to acclimate to the new element of the culvert entry, the apparent
preference for small mammals to use branches to move across the ground replicated with
the new branch was completely replicated. In fact, wildlife grew so accustomed to the
branch leading into the culvert that individuals from a number of other small mammal
species began to make nearly exclusive use of the branch to enter and exit the structure,
shifting their movement path from the western wall to the eastern wall so as to take
advantage of the elevated path. Though this represents a sample size of one and is absent
a control and the other mechanisms to ensure that no false pattern is perceived through
imperfect design, the near-immediate overwhelming reaction of these species to switch
which side of the culvert to move on so as to instead walk along the branch is certainly
interesting and suggests further study is warranted.
Following this relatively successful exercise in ad hoc habitat alteration, the
process was repeated with the southern entrance to this culvert. Again a nearby downed
tree branch was pressed into service to serve as a natural elevated pathway leading into
the Cascades Dry Culvert East. The branch was again placed along the eastern wall of the
culvert both to keep the pathway clear for easy passage and to observe whether small
mammals would voluntarily abandon their normal movement route along the western
wall to instead use the branch. The difference in researcher action in this scenario was
that the branch was placed so that it ended just before the entrance of the culvert, whereas
the branch on the northern end extended approximately two feet into the mouth of the
structure. This again provided for interesting observational data about small mammal
movement. While these animals again, once acclimated, grew to make use of the branch
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almost exclusively, the majority hesitated upon reaching the end of the branch before
turning around, leaving the branch, and entering the structure along the traditional
western wall. Wildlife leaving the culvert often made use of the branch, but not to the
same extent as the northern of the pair. These exercises suggest that elevated pathways
are particularly preferable for small animal movement regimes, but offer the impression
that it is important that any potential elevated pathway should extend at least some
distance into the passage structure or, ideally, provide an elevated pathway through the
entire length of the structure to best maximize usage.
Species Diversity in Passage Structures
A total of 26 independent species were confirmed as having crossed completely
through one of the observed passage structures as part of this study. These species were
divided up into one of 6 different population groups that could be assumed as having
roughly similar habitat needs and behavior traits: human & related, ungulate, large
carnivore mammal, small carnivore mammal, small non-carnivore (or prey) mammal, and
bird.
The first category, human & related, was primarily composed of humans, but
included two species that, in most cases, only made use of passage structure while
accompanied by humans: horses and domestic canines. Seven of the 9 structures
exhibited human use beyond that attributed to the camera operators; only two of the three
smallest structures, Cascades Wet Culvert and Cascades Dry Culvert East had absolute
wildlife use. That humans made use of so many of these structures to cross roadways
despite access limitations due to environmental factors like thick forest or locations
relatively isolated from human settlement speaks to the idea that excluding human use in
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the interests of promoting wildlife use is often not a viable option. Proportionally, human
use of these structures seems to impact some species types more than others. For the
Cascades Dry Culverts, for example, humans accounted for 3.5% of the detections for the
eastern culvert with small prey mammals representing 76.3% of the detections. In the
western culvert, the percentage increase of human use to 46.3% came largely at the
expense of small prey mammals, which dropped proportionally to 31.1%. The
percentages of the use communities composed of small carnivore mammals and birds
remained approximately the same for the two culverts at about 6% and 15%, respectively.
This is likely the result of a temporal divergence in use patterns. Whereas birds were
largely observed near dawn and small carnivore mammals were primarily seen late at
night, human hikers were mostly recorded from the late morning until dusk. While this
temporal human pattern likely did not affect nocturnal prey species like bushy-tailed
woodrats, it seems likely that the other major small prey mammals located in this region
were affected. Townsend’s chipmunks, Douglas squirrels, and deer mice, which are
active during the day or near dusk, were possibly discouraged from approaching the west
culvert due to increased human presence, shifting the proportions of the use community.
Ungulates, as should be expected, primarily made use of the bridges studied here
rather than the culverts due the size constraints of the latter. There seemed to be a fairly
clear inverse relationship between human observations and ungulate observations at
paired structures where an increase in the former would result in a decrease of the latter.
Ungulates appear to have very particular requirements when the decision arises as to
whether to actually make use of a passage structure once approaching it. The Cascade
River Valley Main Bridge and Overflow Bridge offer a perfect example of this behavior.
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Ungulates here followed the pattern of inverse observations with ungulates composing
59.1% of the use community on the Main Bridge and 93.6% of the use community
beneath the Overflow Bridge where humans composed 36.9% and 5.1% of the same
respective structures. However, the passage rates did not mirror this relationship.
Ungulates (excluding cattle that were precluded from passage by nearby pasture fencing)
crossed the Main Bridge successfully in 71.9% cases, but only did so in 18.1% of cases
for the Overflow Bridge. This reinforces the fact that a number of factors play into an
ungulate’s decision on whether to use a passage structure. In this case, the Main Bridge
was significantly larger than the Overflow Bridge (420 m2 cross-section vs 168 m2 crosssection) and the vegetation around and beneath the Main Bridge was preferable for
ungulate feeding. The blackberries and tree shoots that were allowed to freely grow were
more attractive to ungulates in this region than the grasses around the Overflow Bridge
that were routinely cut as a part of regular maintenance. Ungulates in this area found easy
grazing pasture near the Main Bridge and likely crossed it in search of more, but
eschewed the Overflow Bridge as a route of travel due to these limitations.
Large carnivore mammals made up a very small portion of the overall samples,
with a single individual cougar being sighted on five occasions at the East Dry Forest
Bridge. Four of these 5 sightings occurred during daytime, seemingly belying the notion
that large carnivore mammals shift their use patterns to be more nocturnal when a
structure has a substantial human presence (Rodriguez, Crema, & Delibes, 1996). It
should be noted, however, that this cougar has been observed by other WSDOT cameras
in the region moving throughout the area at night on more occasions that don’t fit in with
this dataset. The fact that this cougar elected to make use of the East Dry Forest Bridge
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on all 5 of these occasions, but never approached the nearby East Dry Forest Culvert
despite existing research suggesting that cougars prefer more confined structures is also
somewhat unexpected (Clevenger & Waltho, 2005). This could be because one end of
that culvert is located in fairly dense forest land, while research indicates cougars prefer
clear and open entrances, which is provided by the bridge here (Clevenger & Waltho,
2005). Large carnivores have been seen at some of these sites prior to the window of time
that the data was limited to here for analysis and at other WSDOT sites under observation
as part of the Habitat Connectivity program, especially black bears. The black bears in
these sightings have passed through structures of various sizes and at all times of day,
suggesting that large carnivore mammals may be less restrictive in their use requirements
than expected. It was unanticipated, given prior knowledge of how black bears use these
structures in Washington, that no individuals were observed at any of these sites within
the annual cycle despite all being located in potential black bear habitat.
Bobcats accounted for more than half of the small carnivore mammals that this
study identified and could be confirmed as having successfully crossed through structures
at a very high rate of 93.75%. This was a behavior that was exhibited by all small
carnivore mammal species except coyotes, which crossed in 13.64% of observations, the
only group within this subset to do so less than 54% of the time. This difference is
probably best explained by the size and behavior differences. Coyotes stand several
inches taller at the shoulder and weigh about 10 pounds more on average and a number of
observations of coyotes approaching smaller culverts, such as that at Cascades Dry
Culvert East, showed coyotes approaching the structure before turning and walking away.
While these culverts could easily pass a smaller bobcat through comfortably, a coyote
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would have found a somewhat more constricted route. It is possible that feeding habits
may play a role in this as well. While both species can hunt live prey and scavenge the
kills of other animals, bobcats are primarily hunters while coyotes are the more likely pf
the two species to feed on carrion (Whitaker & Hamilton, 1998). On at least three
occasions, bobcats could be identified in the collected images as having passed one way
through a passage structure before returning later from the opposite site carrying freshlykilled prey. It may be that bobcats recognize the importance that culverts and underpasses
play in maintaining connected hunting ranges and have thus see passage as a necessity
while coyotes, absent the same reliance on hunting practices to satiate hunger, lack this
incentive. Any number of other possible reasons for this disparity may exist, such as
proximity to dens or coyotes may be more resistant to the noise, light, and movement
from traffic and thus be more willing to cross at grade.
It is difficult to interpret structure type preference for some of the smaller animals
appearing in this study such as the small prey mammal group. Because of limited
resources, bridges in this study had between 2 and 4 cameras each and, because they had
to cover a relatively large area, were necessarily placed further back from the structures
and higher above the ground than the cameras placed on culvert. This methodological
requirement did not prove conducive to gathering the best understanding of total use rates
by small prey mammals for bridge underpasses. Of the bridges observed, the most likely
to capture small prey mammal movements would be the Eastern Dry Forest Bridge,
which had 4 separate cameras in place, or the Cascades River Valley Main and Overflow
Bridges, which each had a camera placed near ground level directly beneath the overpass.
Small prey mammals were observed, however, making use of all 4 culverts and 1 of the 5
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bridges in this study, with that one being the Eastern Dry Forest Bridge. Though it is
difficult to confidently describe small prey mammal use of bridges, it was expected that
they would contribute more to the biodiversity in use communities for culverts and the
available data does support that supposition. Small prey mammals compromise at least
29.5% of the observed individuals in each of the culverts studied where the close
confines, darkness, and ready access to nearby screening vegetation at the entrances and
exits would suit their habitat needs. The wide open, mostly rocky bridge underpasses
would leave these animals vulnerable to predation. The relatively high number of
individuals that remain listed as unknown in this study’s data tables is primarily the result
of the initial cameras deployed at the Cascades Dry Culverts being older model Bushnells
incapable of providing a clear image of rapid small mammalian movement at night;
identification rates increased when these sites were resupplied with Reconyx cameras
instead, but that was a relatively recent development in the terms of the timescale of this
project.
Birds had a higher-than-expected presence in these sites. A wide variety of birds
were identified, from perching birds like the Varied Thrush and American Robin to
ground-dwelling birds like Wild Turkey and Ruffed Grouse to a ground foraging
woodpecker, the Northern Flicker. As the cameras used for this study take still images,
they are not adequate for determining whether birds successfully crossed through
structures in most cases, so the low subset crossing rate of 7.39% is likely not
representative of actual behavior. All of the birds that could be confirmed as having
crossed, however, did so through bridge underpasses as, despite a high number of
observed individuals resting near and even in culvert entrances, no confirmed passings
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proved recordable. The primary species of bird that did cross at a relatively high rate of
42.86% was the Wild Turkey, which is understandable given that the species spends most
of its time on the ground. Recent research in Montana suggests that regular passage of
birds along the ground beneath underpasses may not necessarily be reserved for grounddwelling birds, however. Hundreds of sage grouse, a species known as strong flyers, have
been recorded as walking beneath underpasses each year in that state (Peterson, 2014).
Human Impact on Wildlife Passage
Human use was observed in 7 of the 9 structures observed outside of camera
operator contacts. The results showed a negative relationship between increased human
presence and the number of wildlife individuals observed. This supports existing
knowledge on the relationship between these two populations (Pedevillano & Gerald
Wright, 1987). This data was not equally distributed or completely linear, however, with
a clear break in results once the number of human observations per week reached
between 2-3. The structures with human presences below this threshold averaged about
14 wildlife individuals per week while those structures with human presences above that
number averaged less than half as many wildlife observations per week. This result,
suggesting that the negative impact of human use on these structures does not increase
linearly, but has a flat effect at low and high levels, with an exponential growth in impact
in between, is also supported by current literature (Mata et al., 2008).
Western Forest Trail 1 served as an extreme example of how human activity can
make the habitat unsuitable for wildlife passage. With 14,383 humans and related species
seen at this site during the annual cycle of data collected, wildlife were largely excluded
from use, with only 46 individuals being identified. On the opposite end of the spectrum
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of human impact, Cascades Wet Culvert and Cascades Dry Culvert East both saw only
researcher use of the structures by humans. It is notable that statistical analysis of the
results showed that humans appear equally willing to make use of bridges or culverts, but
did not do so for these two structures because they were the only structures included in
this study with a clearance height below 5 ft. (1.5 m). Combined, these results suggest
that so long as a structure is large enough to comfortably allow for human movement,
there is a high likelihood that pedestrians will take advantage of the opportunity. This
challenges the most commonly suggested method of dealing with the combined use
between these disparate populations, namely the idea of merely limiting human use as
much as possible. This data indicates that humans have a similar need to wildlife to pass
beneath roadways and see culverts and bridge underpasses as an ideal tool for achieving
this goal.
If human use is unlikely to be curtailed, but has an apparent negative bearing on
wildlife use, the question remains as to what steps can best be taken to resolve the issue.
This study suggests that the use of paired structures may be the best method to achieve
the desired effect of uninhibited passage by both populations. As discussed in Olsson,
Widén, & Larkin (2008), when multiple similar passage structures are found in relatively
close proximity, human and wildlife populations will voluntarily segregate between the
options. This notion bore out through the results of this study. In all of the paired
structures fitting these requirements, namely relatively close proximity and same
structure type (bridge vs culvert), all exhibited such a division of species. As visualized in
Figure 12, human individuals heavily favored use of Western Forest Trail 1, Cascades
Dry Culvert West, and Cascades River Valley Overflow Bridge while wildlife individuals
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similarly favored their opposite number. It would be inaccurate to state that each
population made use of the structure that best suited their requirements. In reality, the
human-dominated structures in this subset were all larger, more open, and easier to
access, indicating that human individuals naturally made use of the structures best suited
to their needs. Wildlife, in contrast, made use of smaller, less desirable structures in what
may be a reaction to the high human presence at the preferable passages.
It proved interesting that increasing human use had approximately equal negative
effects on large and small mammal use of passage structures. The analysis performed in
this study showed roughly equivalent relationships between humans and animals of either
size group, but in both cases the analysis was not statistically significant. This despite the
expectation that small mammals would be more influenced by increased human use both
as a result of their self-preservation instincts, more solitary nature, and the fact that larger
mammals, especially ungulates, have shown an ability to acclimate to human influence
(Gagnon et al., 2011). What information better fits this supposition is the fact that the
baseline use rate when compared to human individuals is higher in larger mammals. The
y-intercept, and thus the assumed number of observed individuals per week in the
absence of any human presence, in Figure 15 shows a value more than double in large
mammals versus small mammals. As a requirement of servicing cameras, researchers
needed to approach these sites monthly so no site in this study could be entirely devoid of
human presence. It is not unreasonable to imagine that the disparity in these y-intercept
values may be even greater than that observed here if it had been possible to measure
passage rates without observer influence.

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Conclusions
The research performed here has reconfirmed the important role that wildlife
passage structures hold in the movement regimes for animals of all sizes and suggests
that a greater degree of use by small mammalian vertebrates exists than may have
previously been suspected or understood. This information also supports existing
knowledge of the negative influence of human presence in passage structures while
recognizing that elimination of human passage is not always a viable management
strategy.
In total, 33 separate species were recorded as having made use of the culverts and
bridges in this study, with 26 species providing at least one instance of an individual
being confirmed as having crossed through the passage. These species covered a wide
variety of sizes and behavioral groups, from small and large carnivores to small and large
non-carnivorous prey species. As a whole, wildlife showed a possible, though statistically
insignificant, preference for bridges as a movement vector over culverts. Confirmed
crossing percentage, calculated as the number of individuals who could be confirmed as
having passed through the structure divided by the total number of individuals, for
mammalian wildlife was much higher through bridges than culverts, indicating a likely
preference for greater cross-sectional areas in passages. Observational analysis of small
wildlife interaction with the elevated pathways offered by downed tree branches both
near and within culverts suggests that these animals may preferentially use these paths to
the exclusion of natural regimes that have been developed.
When multiple spatial close structures were analyzed together, it became apparent
that human pedestrians largely confined their use to only one of the paired structures,
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while wildlife would mostly make use of the remaining option. Increased human use of
passage structures was at least partly responsible for a consequent decrease in wildlife
use of those same structures. This relationship was especially detrimental once human
pedestrian observations surpassed 2-3 individuals per week, at which point wildlife use
dropped precipitously. Human pedestrians made use of bridges and culverts both
frequently, though appear to prefer bridge underpasses due to the increase in open area.
Human influence had a similar negative linear relationship for both large and small
mammalian wildlife, but larger species may be more capable of adapting to human
presence than smaller species.
This study and future research can provide useful information to determine how
transportation bridges and culverts, structures originally designed to traverse waterways
and protect drivers from floods, are increasingly becoming important to populations
beyond motorists. It is already well-established that these structures can reduce WVCs
with large mammals, reducing motorist risk of injury and property damage (Clevenger &
Waltho, 2005). However, as research continues into how to improve and expand the field
of wildlife passage structures, it is important that unrecognized, yet sizeable, segments of
the use community are acknowledged. Small animal and human pedestrian use of bridge
underpasses and culverts is higher than may be predicted as these structures provide
crucial pathways for safe movement for a diversity of species.
Chapter 3: Conclusions and Management Implications
Conclusions
Wildlife passage structures offer an important resource to all wildlife, namely
freedom of daily movement and for migration between seasons or habitats. This is
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especially important with species of limited mobility like small animals where their
smaller habitats suffer proportionally more when fragmented by roadways. This was
observed as small and large carnivore mammals made regular use of underpasses and
culverts in search of prey while hunting and, in the case of one bobcat individual, actually
catching prey on one side of a structure before carrying the prey back to its den on the
opposite end. Instead of having these animals cross roadways at grade where they are at
risk of vehicle strikes as they seek to maintain their movement through a traditional
range, bridges and culverts offer a valuable service in preserving habitat connectivity,
population totals, and genetic diversity due to the presence of more individuals in each
population and perhaps linkages between metapopulations.
Variety of structure type, size, and location is important for maintaining
biodiversity in use communities. Smaller animals may prefer structures with smaller
cross-sectional areas, though they were observed to pass readily through open bridge
underpasses as well. Ungulates in particular seem to require very large passage structures
with clear entryways and available forage vegetation. Human pedestrians also tend to
favor larger structures, passing beneath bridges with some regularity while eschewing
culverts with a height under 5 feet entirely among this sample set. Installation of multiple
structures of varying sizes with different environmental elements seems the best method
for ensuring equal access to humans and wildlife as the populations voluntarily separate
and use different structures when the option is available.
Environmental factors play a large role in determining the size and composition of
use communities. Permeant running water deters use by any species on a reliable basis
unless dry pathways are available adjacent to the waterway. In such cases where a dry
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path exists, waterways may prove incidental to movement patterns or even encourage
species like small prey mammals and ungulates that derive a benefit from the resultant
increase in vegetation. Vegetation type also plays a role in crossing rates as in one pair of
structures, ungulates crossed at a high rate though an underpass with heavy blackberry
presence but crossed at a very low rate through a nearby (albeit smaller) underpass
surrounded by short grasses. Elevated pathways, such as those offered by downed tree
branches or installed wildlife shelving along passage walls would likely be of benefit to
small prey and carnivore mammals given their apparent preference to move above the
ground rather than upon it.
Management Implications
The management implications of this study are few at this junction. Much of what
has been observed over an annual cycle is suggestive of practices that could be altered or
improved to better account for human pedestrian and small animal use of existing or
future structures, but little is conclusive. In truth, WSDOT has proven very proactive
about maintaining habitat connectivity across roadways through the use of passage
structures. Every year WSDOT allocates millions of dollars and employs dozens of
people for the purpose of maintaining and studying the effectiveness of these structures
as platforms for wildlife movement. As much of this funding is tied to reducing WVCs
that endanger human motorist lives, many WSDOT projects are understandably focused
on ungulate and other large mammal needs as those are the species that put motorists at
the most risk. As a result, the benefits derived from small animals and human pedestrians
are sometimes incidental when projects designed to pass water or large mammals beneath
roadways are completed.

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One potential for increased management would be the process of further
segregating human and wildlife passage between paired structures. While human
pedestrians commonly selected which of two possible structures offers the most
preferable passage elements and gravitate towards that one, as evidenced by the data
collected in this study, there can be some overspill that may decrease the utility of the
second paired structure for wildlife. The solution to this may be as simple as signage.
Several months after construction of paired passage structures is completed, so as to
allow for movement regimes to acclimate to the new structures, camera traps could be set
up to observe which of the structures humans tend to prefer. Then small signposts with
arrows directing towards the structure already used by most pedestrians could potentially
redirect a higher proportion of human use through a single structure, leaving the other
free for wildlife use. Placement of barriers or signage designating the second structure as
wildlife habitat may prove counterproductive if pedestrians voluntarily disregard such
elements in search of wildlife interaction, so simple arrows may prove most effective.
The primary implication of this study is to ideally progress down an avenue of
research that receives less attention than large mammal use of wildlife passage structures.
The cameras deployed by WSDOT across western Washington have revealed a complex
community of small vertebrates that make regular use of passage structures and a
population of human pedestrians that cross through even the most isolated of locations.
More needs to be known about these populations, what they look for in passage
structures, what drives them away, and how underpasses, overpasses, and culverts can be
designed to appeal to the parts of the use community equally. Specifically,

72

implementation of several experimental research designs to structure evaluations such as
those suggested by Rytwinski et al. (2015) could provide invaluable data.
Recommendations for Future Research
1. Continue and expand monitoring efforts and improve coverage for bridges
a. Dataset is still too limited spatially and temporally to effectively promote
management strategies
b. Small wildlife use of bridges likely underrepresented due to difficulty in
observing large underpasses so additional detection methods could be
deployed
c. Include more sites to better account for anomalous environmental factors
This study collected tens of thousands of records of humans and wildlife across 9
structures at 5 sites across western Washington, but the differences among the sites were
sometimes so significant as to make categorical comparisons difficult to endorse. In
addition, the cameras stationed at some sites providing the most interesting information,
specifically those located near the Cascades Wet and Dry Culverts, had not yet been in
service for a full annual cycle, meaning that the populations recorded at those locations
were not a complete representation. The time spans were considered near enough to a full
annual cycle so as not to make the results irrelevant for the purposes of this study, but as
these installations remain in place for longer, the likelihood that an accurate
representation of reality is being recorded increases. Cameras should be replaced with
more modern, reliable cameras as time and funding allow (a policy already in place at
WSDOT) as some cameras proved difficult to work with due to triggering failures or

73

poor image quality. Finally, the deployment of more equipment around bridge
underpasses, including more camera frames, track plates, and/or pitfall traps, would help
in achieving a more complete understanding of small animal use of these larger
structures.
2. Limit human presence (specifically camera operators) to eliminate that variable
a. With current methodology, knowledge of structure use completely absent
human presence is not possible
b. Technological advancements offer several (expensive) options to
eliminate servicing need or allow for placement further away from
structures
The influence that camera operators had on sites with high human use was likely
inconsequential, but at most of the sites, there was relatively little human activity. The
level of impact that the passage of 1-3 individuals every 4 weeks to swap out cards and
batteries on these cameras is likely not much, but remains uncertain. On at least two
occasions, researchers came into direct contact with wildlife at these sites, alarming the
animals and forcing them to flee. In some cases it may be possible to hard wire camera
installations in place when power lines are nearby, eliminating the need to replace
batteries. Some camera models also have the capability to send imagery data to
computers via satellite, which would no longer require researchers to visit sites to change
data cards. Alternatively, the use of larger data cards in conjunction with more electric
hard wiring or more efficient battery packs could minimize the number of service trips,
rather than eliminating them outright. Long distance thermal imagery cameras that would
allow for cameras to be set up further back from passage structures, minimizing
74

researcher alteration and interaction with the targeted habitat are also an option. The issue
with implementation for all of these options, however, is a cost point far higher than the
existing system, one that likely doesn’t justify the marginal improvement in the data.
3. Designed studies with controls
a. Observational studies such as this one are important for creating a baseline
understanding, but designed studies can provide a better understanding of
causal relationships and guide better management
b. Allow for targeted testing of the role of culvert shelving, shielding
vegetation, or water presence
Rytwinski et al. (2015) describe some of the many potential designed studies that
can be performed with passage structures, covering a wide breadth of topics from
fencing, vegetation coverage, and structure types, sizes, and numbers. Mostly these
projects are designed to be applied to ungulate and large carnivore mammal concerns.
With modification, any number of them could be used to observe the effects of various
passage elements on small animals and human pedestrians. In addition, wildlife shelving
(which is not mentioned) and water presence (which is only discussed briefly) could be
major modifiers to use communities. Water flow rate, seasonality, and percent coverage
seems to have a definite impact on structure preference and the observation of multiple
similar sites with varying water regimes, perhaps even ones that could be modified,
would provide interesting results. Foliage coverage for structure elements and wildlife
shelving would prove fairly straightforward to design studies around, either by finding
(or building) twin structures in the same region with one of each pair possessing shielding
or shelving and the other serving as a control. Alternatively, pre-and-post installation
75

analysis could be performed where a single structure is observed for a time, much like the
ones in this study, before vegetation shielding or wildlife shelving is installed and
comparing the resultant community to the baseline one.
4. Individual-level analysis for small animals
a. Better representation of the habitat connectivity role these structures play
b. Observe whether small animals make use of multiple structures in
succession as part of normal range movement or seasonal migration
c. Account for individuals that pass through structures on a regular basis,
eliminating replication in the dataset
At many of the sites where small animals were prevalent, it seemed highly likely
that a few individuals were passing through the structures multiple times. Unfortunately,
without any way to define individuals through visual characteristics, there was no way to
be certain of this supposition. The question also arose during research as to whether some
individuals may be using multiple structure, either in series or in parallel. For instance, if
a bobcat shifted its range further north, could it be tracked moving through multiple
structures from south to north? Or, in a month with particularly low human presence in
one of two paired structures, could an individual normally only found in the lessdisturbed structure be found changing its preference? This could be accomplished
through the use of pit traps to capture small animals and then using an ear or leg band or
a passive integrated responder (PIT) tag. PIT tags require no sizeable power source, are
easily distinguishable by one another, and can be triggered remotely by stationary
installations automatically, perhaps making them ideal to this potential field of study
(Gibbons & Andrews, 2004).
76

5. Explore the role of birds in passage structure communities
a. Unexpectedly high numbers of birds were observed near culvert and
bridge entrances
b. Use of motion video in place of still imagery would play a major role in
determining crossing rates
Hundreds of birds were observed by cameras placed on bridge underpasses and
culverts, representing a relatively large use population that was discounted at the start of
this study as likely to be minimal in size. Because of the technological limitations of the
cameras currently in place, it was nearly impossible to say for certain where most of these
individuals moved once they took flight: through the passage or away from it. The only
confirmed crossings happened beneath bridge underpasses and were almost entirely Wild
Turkeys. It would be interesting to see whether the smaller sparrows, thrushes,
woodpeckers, and grouses that commonly searched for food on the ground in the mouths
of culverts eventually passed through or whether these structures play a role in seasonal
migrations as could be intuited from the movement of birds going predominantly north to
south in the fall and south to north in the spring. The best method for achieving this in
future research would be the use of motion video recording from deployed cameras,
though this does increase the data size requirements and greatly extends the time required
to process collected data. This idea of trying to record bird movement by video is, in fact,
something that WSDOT biologists have agreed to explore with one of the cameras at the
Cascades Dry Culvert East site soon to be changed out for one capable of recording
video. Ideally, this will answer some of the lingering questions as to whether small birds
will actually make use of these long, narrow culverts for flight. This could potentially add
77

another population of interest in the ever-growing use community for Washington’s
wildlife passage structures.

78

Figure 1: I-90 Price/Noble Wildlife Overcrossing

Note: Final design intends to include foliage screening along edges of overpass.
Image courtesy of WSDOT

79

Figure 2: Deer Carcass Kernel Density Analysis

Note: 2009-2013 deer carcass kernel density analysis for Washington State. This
map approximates those areas of Washington state roadways with the highest rate
of collisions between deer and vehicles (McAllister & Plumley, 2015).

80

Figure 3: Camera Installation

(1)

(2)

(3)

(4)

Note: Forms of camera installation: (1) Telespar; (2) Tree Mount; (3) Utility Box (Exterior);
(4) Utility Box (Interior)
Image 2 courtesy of Kelly McAllister
81

Figure 4: Study Areas

Western
Forest Trail

Cascades Wet
Culverts

Cascades Wet
Culvert

Cascades
River Valley
East Dry
Forest

Note: Specific sites were selected from larger study for environmental and spatial
variation, habitat connectivity and wildlife vehicle collision concerns, and because
each site contained paired passage structures.

82

Figure 5: Study Structures

(1)

(4)

(3)

(2)

(5)

(6)

(9)

(8)

(7)

Note: (1) Western Forest Trail 1; (2) Western Forest Trail 2;
(3) Cascades Wet Culvert; (4) Cascades Dry Culvert West;
(5) Cascades Dry Culvert East; (6) East Dry Forest Culvert;
(7) East Dry Forest Bridge; (8) Cascades River Valley Main
Bridge; (9) Cascades River Valley Overflow Bridge.
Images 1-7 courtesy of Kelly McAllister

83

Table 1: Usage of Study Structures and Confirmed Crossing Rates
Attributes

Tunnels, Culverts, and Bridge Underpasses
1

2

3

4

5

6

7

8

9

Crossing type

∩

∩

□

○

○

○

∩

∩

∩

Dimensions
Length (m)
Width (m)
Height (m)
Cross-sect. area (m²)
Openness Ratio

12.2
4.6
3.0
13.8
1.13

12.2
12.2
3.0
36.6
3.0

18.3
1.8
1.2
2.16
0.12

62
1.8
2.5
3.53
0.06

62
1.2
1.2
1.13
0.02

12.2
1.5
1.5
1.77
0.15

12.2
43.9
6.1
268
22.0

12.2
91.4
4.6
420
34.4

12.2
67.1
2.5
168
13.8

Observations
Human & Related*
Ungulate
Large Carnivore Mam.
Small Carnivore Mam.
Small Prey Mam.
Bird
Total

14383
44
0
0
0
2
14429

39
377
0
3
0
2
421

27
0
0
1
13
3
44

174
0
0
26
117
59
376

23
0
0
35
503
98
659

42
131
0
32
84
12
301

96
628
5
9
5
14
757

202
324
0
10
0
12
548

55
1015
0
14
0
0
1084

Confirmed crossing %
All Observations
100
99.5 27.3 57.4 35.2 22.9 81.4 74.5 23.6
Wildlife Only
100
99.5 11.8 28.2 33.6 18.9 83.2 65.3 20.8
Note: * = This category includes humans and related domesticated species, specifically
horses and canines. Domestic felines have been categorized as small carnivore mammals.
∩ = bridge underpass; □ = drainage box culvert; ○ = dry cylindrical metal culvert. Bridge
widths exclude portion of underpass occupied by rivers.

84

Figure 6: Human and Wildlife Weekly Average Observations by Structure Type

p = 0.9535
p = 0.5067

Note: Brackets constructed using 1 standard error from the mean. Seven sites
included. Cascades Wet Culvert site excluded as irrelevant for this analysis due to
environmental confounding variables. East Forest Trail 1 excluded as a major
outlier in the dataset in that the value for human individuals per week at that site
was approximately 33.5 times greater than the next highest value.

85

Figure 7: Confirmed Crossing Rates by Structure Type

p = 0.0527
p = 0.0286

Note: As cameras were not set up to accurately assess bird passage rates, birds have
been excluded from this analysis. Brackets constructed using 1 standard error from
the mean.

86

Figure 8: Polynomial Fit Analysis of Wildlife Observations by Cross-sectional Area
of Structure

Note: Number of Wildlife Individuals per Week = 7.1458654 + 0.0539685*Crosssect. area - 0.0002336*(Cross-sect. area - 101.666) ². This analysis is not significant
at the p < 0.05 level, but seems to indicate a pattern of increasing observations as
cross-section increases before reaching a point of diminishing returns. It is probable
that the limited camera coverage of larger structures played a major role in the
leveling-off of wildlife observations and likely explains the eventual decrease as well.

87

Figure 9: Bivariate Fit Analysis of Wildlife Observations by Openness Ratio of
Structure

p = 0.4886

Note: Successful Wildlife Passage Rate = 44.915572 + 0.7645356*Openness Ratio.
This analysis is not significant at the p < 0.05 level, but seems to indicate a pattern of
increasingly successful crossings as openness ratio (height*width/length) increases.

88

Figure 10: Wildlife Use of Tree Branches to Enter Culverts

Note: Bushy-tailed woodrat use of placed woody debris at Cascades Dry Culvert East. Bottom
two rows show assumed use for exit as the camera was not triggered until the animal was past
the branch, but the animal being positioned on far side of camera (in contrast to row 1) is
suggestive. Image dates by row from top: 11/11/2015, 11/13/2015, 11/18/2015.
89

Figure 11: Wildlife Use of Branches That Do Not Enter Culvert

Note: Deer mouse and Douglas squirrel hesitation at end of placed woody debris that does not
enter structure mouth at Cascades Dry Culvert East. Image dates by row from top: 3/12/2015,
3/17/2015, 4/9/2015. All animals eventually crossed without using the available branch.
90

Table 2 Part 1/2: Crossings by Species
Species

Crossings
Yes

Unk.

No

Total

Confirmed
crossings as a % of
total observations

Human

3255
4
109
3368
96.64
(Homo sapiens)
Human & Canine
11356
0
2
11358
99.98
Canine
4
3
0
7
57.14
(Canis familiaris)
Human & Horse
83
0
0
83
100
(Equus ferus caballus)
Human, Horse, & Canine
222
0
0
222
100
All Humans & Related
14920
7
111
15038
99.22
Elk
705
26
752
1483
47.54
(Cervus canadensis)
Black-Tailed Deer
674
36
211
921
73.18
(Odocoileus hemionus)
Cattle
0
0
42
42
0
(Bos taurus)
All Ungulates
1379
62
1005
2446
56.38
Cougar
5
0
0
5
100
(Puma concolor)
All Large Carnivore Mammals 5
0
0
5
100
Bobcat
45
3
0
48
93.75
(Lynx rufus)
Raccoon
7
3
0
10
70
(Procyon lotor)
Coyote
3
5
14
22
13.64
(Canis latrans)
Domestic Cat
6
2
2
10
60
(Felis catus)
Long-Tailed Weasel
13
4
7
24
54.17
(Mustela frenata)
Short-Tailed Weasel
1
0
0
1
100
(Mustela erminea)
Common Opossum
2
0
0
2
100
(Didelphimorphia)
Striped Skunk
11
0
0
11
100
(Mephitis mephitis)
All Small Carnivore Mammals 88
17
23
128
68.75
Note: Use of culverts and bridge underpasses differentiated by species, including
calculation of confirmed successful passages as a percentage of total observations.

91

Table 2 Part 2/2: Crossings by Species
Species

Crossings
Yes

Unk.

No

Total

Confirmed
crossings as a % of
total observations

Mountain Beaver
3
1
0
4
75
(Aplodontia rufa)
Townsend’s Chipmunk
4
6
12
22
18.18
(Neotamias townsendii)
Douglas Squirrel
93
11
40
144
64.58
(Tamiasciurus douglasii)
Bushy-Tailed Woodrat
58
33
45
136
42.65
(Neotoma cinerea)
Deer Mouse
39
6
168
213
18.31
(Peromyscus maniculatus)
Pika
0
1
3
4
0
(Ochotona princeps)
Snowshoe Hare
13
6
0
19
68.42
(Lepus americanus)
Long-Tailed Vole
1
5
1
7
14.29
(Microtus longicaudus)
California Ground Squirrel
3
4
8
15
20
(Otospermophilus beecheyi)
Northern Flying Squirrel
0
0
1
1
0
(Glaucomys sabrinus)
Unknown Species
40
67
127
234
17.09
All Small Prey Mammals
254
140
405
799
31.79
Varied Thrush
0
1
36
37
0
(Ixoreus naevius)
American Robin
2
4
89
95
2.11
(Turdus migratorius)
Dark-Eyed Junco
1
0
6
7
14.29
(Junco hyemalis)
Wild Turkey
6
2
6
14
42.86
(Meleagris gallopavo)
Steller’s Jay
0
0
20
20
0
(Cyanocitta stelleri)
Northern Flicker
0
0
1
1
0
(Colaptes auratus)
Ruffed Grouse
0
0
1
1
0
(Bonasa umbellus)
Unknown Species
6
2
20
28
21.43
All Birds
15
9
179
203
7.39
All Records
16744
235
1723
18702
89.53
Note: Use of culverts and bridge underpasses differentiated by species, including
calculation of confirmed successful passages as a percentage of total observations.
92

Figure 12 Part 1/2: Observed Species by Structure

Observed Species
Cascades Wet Culvert

3
6.8%

Humans

13
29.5%

Small Carnivore
Mammals
Small Prey Mammals

27
61.4%

1
2.3%

59
15.7%

Birds

Observed Species
Cascades Dry Culvert
W
Humans &
Canines

174
46.3%

98
14.9%

Observed Species
Cascades Dry Culvert
35
5.3% E
Humans

Small Carnivore
Mammals

Small Carnivore
Mammals

117
31.1%

Small Prey
Mammals

Small Prey
Mammals

503
76.3%

Birds

26
6.9%

44
0.3%

23
3.5%

Observed Species
Western Forest Trail 1
2
0.0%

14383
99.7%

Humans,
Canines, &
Horses
Ungulates
Birds

Birds

2
0.5%

Observed Species
Western Forest Trail 2
39
9.3%

3
0.7%

Humans &
Canines
Ungulates

377
89.5%

Small Carnivore
Mammals
Birds

Note: Numbers indicate total observed individuals over a one-year period.
Percentages indicate percentage of total observations over a one-year period.

93

Figure 12 Part 2/2: Observed Species by Structure

Observed Species
Cascades River Valley
Main Bridge

12
2.2%
10
1.8%

202
36.9%

Humans &
Canines

14
1.3%

Ungulates
1015
93.6%

Birds

12
4.0%

Observed Species
Eastern Dry Forest
Culvert
42
14.0%

84
27.9%
131
43.5%
32
10.6%

Humans

Ungulates
Small Carnivore
Mammals

324
59.1%

Observed Species
Cascades River Valley
Overflow Bridge

55
5.1%

Humans &
Canines
Ungulates
Small Carnivore
Mammals
Small Prey
Mammals
Birds

9
1.2%

5
0.7%

5
0.7%

Small Mammal
Carnivores

Observed Species
Eastern Dry Forest
14
Bridge
1.8%
Humans &
96
12.7%

628
83.0%

Canines
Ungulates
Large Carnivore
Mammals
Small Carnivore
Mammals
Small Prey
Mammals
Birds

Note: Numbers indicate total observed individuals over a one-year period.
Percentages indicate percentage of total observations over a one-year period.

94

Figure 13: Bivariate Fit of Number of Wildlife Individuals per Week by Number of
Human Individuals per Week

p = 0.092

Note: Number of Wildlife Individuals per Week = 11.585422 - 1.7173899*Number of
Human Individuals per Week. Western Forest Trail 1 and Cascades Wet Culvert
are excluded as major outliers in the dataset.

95

Figure 14: One-way Analysis of Number of Wildlife Individuals per Week by Number of
Human Individuals per Week Categorical

p = 0.016

Note: One-way ANOVA showing a significant divergence in wildlife individual
weekly use rates for passage structures when compared to whether the human
individual weekly use rate for the same structures was above or below 3 individuals
per week.

96

Figure 15: Bivariate Fit of Number of Large & Small Mammal Individuals Per Week By
Number of Human Individuals Per Week

p = 0.6516

Note: Number of Large Mammal Individuals Per Week = 8.5888586 0.9845716*Number of Human Individuals Per Week. Western Forest Trail 1 and
Cascades Wet Culvert are excluded as major outliers in the dataset.

p = 0.5527

Note: Number of Small Mammal Individuals Per Week = 4.2276624 0.826101*Number of Human Individuals Per Week. Western Forest Trail 1 and
Cascades Wet Culvert are excluded as major outliers in the dataset.
97

Figure 16: One-way Analysis of Number of Wildlife Individuals per Week by Structure
Type

One-way Analysis of Number of Human Individuals per Week by Structure Type

Note: One-way ANOVA of wildlife and human weekly passage rates contrasted by
the type of structure being observed. The first graph shows that across the 5 bridges
and 4 culverts observed, wildlife showed no preference for either structure with the
current human use patterns in place. The second graph that there may be a
preference for humans to use bridge underpasses rather than culverts, but the
analysis is statistically insignificant.

98

References
Ager, A. A., Johnson, B. K., Kern, J. W., & Kie, J. G. (2003). Daily and seasonal
movements and habitat use by female rocky mountain elk and mule deer. Journal of
Mammalogy, 84(3), 1076-1088.
Anderson, G., Freeman, M., Freeman, B., Straight, C., Hagler, M., & Peterson, J. (2012).
Dealing With Uncertainty When Assessing Fish Passage Through Culvert Road
Crossings. Environmental Management, 50(3), 462–477.
Aresco, M. (2005). The effect of sex-specific terrestrial movements and roads on the sex
ratio of freshwater turtles. Biological Conservation,123, 37–44. Retrieved from
http://www.sciencedirect.com.bay.evergreen.edu/science/article/pii/S000632070400
4264
Baxter-Gilbert, J., Lesbarrѐres, D., & Litzgus, J. (2013). On the Road Again: Measuring
the Effectiveness of Mitigation Structures for Reducing Reptile Road Mortality and
Maintaining Population Connectivity (p. 16). Presented at the 2013 International
Conference on Ecology and Transportation, Scottsdale, Arizona, USA. Retrieved
from
http://www.icoet.net/ICOET_2013/documents/papers/ICOET2013_Paper104D_Baxt
erGilbert%20_et_al.pdf
Cain, A. T., Tuovila, V. R., Hewitt, D. G., & Tewes, M. E. (2003). Effects of a highway
and mitigation projects on bobcats in Southern Texas. Biological Conservation,
114, 189-197.

99

Clevenger, A. P., Chruszcz, B., Gunson, K., Wierzchowski, J. (2002). Roads and wildlife
in the Canadian Rocky Mountain Parks – movements, mortality and mitigation.
Final report. Prepared for Parks Canada, Banff, Alberta, Canada.
Clevenger, A. P., & Waltho, N. (2000). Factors Influencing the Effectiveness of Wildlife
Underpasses in Banff National Park, Alberta, Canada. Conservation Biology, 14(1),
47–56.
Clevenger, A. P., & Waltho, N. (2005). Performance indices to identify attributes of
highway crossing structures facilitating movement of large mammals. Biological
Conservation, 121(3), 453-464.
Connolly-Newman, H., Huijser, M., Broberg, L., Nelson, C., & Camel-Means, W.
(2013). Effect of Cover on Small Mammal Movements Through Wildlife
Underpasses Along US Highway 93 North, Montana, USA. (p. 12). Presented at the
2013 International Conference on Ecology and Transportation, Scottsdale, Arizona,
USA. Retrieved from
http://www.icoet.net/icoet_2013/documents/papers/ICOET2013_Paper401C_Connol
lyNewman_et_al.pdf
Craighead Institute. (2010, June 24). black bear using culvert [Video file]. Retrieved
from https://www.youtube.com/watch?v=l95GmKHAOg0.
Dellinger, J. A., Proctor, C., Steury, T. D., Kelly, M. J., & Vaughan, M. R. (2013).
Habitat selection of a large carnivore, the red wolf, in a human-altered landscape.
Biological Conservation, 157, 324-330.

100

Dodd, C. K., Jr., Barichivich, W. J., & Smith, L. L. (2004). Effectiveness of a barrier wall
and culverts in reducing wildlife mortality on a heavily traveled highway in Florida.
Biological Conservation, 118(5), 619-631.
Ensing, E. P., Ciuti, S., de Wijs, F. A. L. M., Lentferink, D. H., ten Hoedt, A., Boyce, M.
S., & Hut, R. A. (2014). GPS Based Daily Activity Patterns in European Red Deer
and North American Elk (Cervus elaphus): Indication for a Weak Circadian Clock in
Ungulates. PLoS ONE, 9(9), 1-11.
Erickson, W. P., Johnson, G. D., & Young Jr., D. P. (2005). A Summary and Comparison
of Bird Mortality from Anthropogenic Causes with an Emphasis on
Collisions (General Technical Report No. PSW-GTR-191) (pp. 1029–1042). USDA
Forest Service. Retrieved from
http://www.fs.fed.us/psw/publications/documents/psw_gtr191/Asilomar/pdfs/10291042.pdf
Fahrig, L., & Rytwinski, T. (2009). Effects Of Roads On Animal Abundance: An
Empirical Review And Synthesis. Ecology and Society, 14(1), 21.
Foresman, K. R. (2004). The effects of highways on fragmentation of small mammal
populations and modifications of crossing structures to mitigate such impacts.
Helena, MT: Montana Dept. of Transportation, Research Program ; Springfield, Va.
Forman, R. T. T., Sperling, D., Bissonette, J. A., Clevenger, A. P., Cutshall, C. D., Dale,
V. H., … Winter, T. C. (2003). Road Ecology: Science and Solutions. Washington
D.C.: Island Press.

101

Foster, M. L., & Humphrey, S. R. (1995). Use of Highway Underpasses by Florida
Panthers and Other Wildlife. Wildlife Society Bulletin, 23(1), 95-100.
Gagnon, J. W., Dodd, N. L., Ogren, K. S., & Schweinsburg, R. E. (2011). Factors
Associated With Use of Wildlife Underpasses and Importance of Long- Term
Monitoring. The Journal of Wildlife Management, 75(6), 1477-1487.
Gibbons, J. W., & Andrews, K. M. (2004). PIT tagging: Simple technology at its best.
BioScience 54, 447-454.
Gunson, K. E., Mountrakis, G., & Quackenbush, L. J. (2011). Spatial wildlife-vehicle
collision models: A review of current work and its application to transportation
mitigation projects. Journal of Environmental Management, 92(4), 1074-1085.
Hartmann, M. (2003). Evaluation of Wildlife Crossing Structures: Their Use and
Effectiveness. Wildlands CPR website.
Hemson, G., Maclennan, S., Mills, G., Johnson, P., & Macdonald, D. (2009).
Community, lions, livestock and money: A spatial and social analysis of attitudes to
wildlife and the conservation value of tourism in a human-carnivore conflict in
Botswana. Biological Conservation, 142(11), 2718-2725.
Huijser, M., Fairbank, E., Camel-Means, W., & Purdum, J. (2013). US 93 North PostConstruction Wildlife-Vehicle Collision and Wildlife Crossing Monitoring and
Research on the Flathead Indian Reservation between Evaro and Polson, Montana
Annual Report 2013 (p. 23). Helena, Montana: Western Transportation Institute,
Montana State University. Retrieved from
102

http://www.mdt.mt.gov/other/research/external/docs/research_proj/wildlife_crossing
/phaseii/annual_2013.pdf
Huijser, M. P., & Bergers, P. J. M. (2000). The effect of roads and traffic on hedgehog
(Erinaceus europaeus) populations. Biological Conservation, 95(1), 111–116.
doi:10.1016/S0006-3207(00)00006-9
Huijser, M. P., McGowen, P., Fuller, J., Hardy, A., Kociolek, A., Clevenger, A. P., …
Ament, R. (08-07). Wildlife-vehicle collision reduction study. Report to congress. (p.
260). U.S. Department of Transportation, Federal Highway Administration.
Jackson, S., & Tyning, T. (n.d.). Salamander Tunnels [Government Website]. Retrieved
from http://www.fhwa.dot.gov/environment/critter_crossings/salamand.cfm
Jacobson, S. (2002) Using Wildlife Behavioral Traits to Design Effective Crossing
Structures. Wildlife Crossings Toolkit, U.S. Department of Agriculture’s Forest
Service.
Kertson, B. N., Spencer, R. D., Marzluff, J. M., Hepinstall-Cymerman, J., & Grue, C. E.
(2011). Cougar space use and movements in the wildland-urban landscape of
western Washington. Ecological Applications, 21(8), 2866-2881.
Kintsch, J., & Cramer, P. (2011). Permeability of Existing Structures for Wildlife:
Developing a Passage Assessment System (No. WA-RD 777.1) (p. 188). Olympia,
WA: Washington State Department of Transportation.

103

Leedy, D. L., Franklin, T. M., & Hekimian, E. G. (1975). Highway-wildlife relationships:
Vol. 2. (Report No. FHWA RD 76 5). Final report. Prepared for Federal Highway
Administration, Washington, D.C., USA.
Little, S. J., Harcourt, R. G., & Clevenger, A. P. (2005). Do wildlife passages act as preytraps? Biological Conservation, 107(2), 135-145.
Litvaitis, J., & Tash, J. (2008). An Approach Toward Understanding Wildlife-Vehicle
Collisions. Environmental Management, 42(4), 688–697.
Loss, S. R., Will, T., & Marra, P. P. (2014). Estimation of bird‐vehicle collision mortality
on U.S. roads. The Journal of Wildlife Management,78(5), 763–771.
Mata, C., Hervás, I., Herranz, J., Suárez, F., & Malo, J. E. (2008). Are motorway wildlife
passages worth building? Vertebrate use of road- crossing structures on a Spanish
motorway. Journal of Environmental Management 88. 407-415.
Macdonald, S. (1998). Planning Trails with Wildlife in Mind: A Handbook for Trail
Planners (Handbook) (p. 56). Denver, Colorado, USA: Colorado State Parks.
Retrieved from http://atfiles.org/files/pdf/Trails-for-Wildlife-Handbk.pdf
McAllister, K., & Plumley, S. (2015). Deer Collision Hotspot and Kernel Density
Analysis for Washington State Highways. Final report. Prepared for Washington
State Department of Transportation, Olympia, Washington, USA.
McAllister, K., Reister, M., Bruno, R., Dillin, L., Volsen, D., & Wisen, M. (2014). A
Wildlife Barrier Fence North of Wenatchee, WA – Learning Experiences Involving

104

Rugged Country and Custom Designed Wildlife Guards and Jumpouts. Final report.
Prepared for Washington State Department of Transportation, Olympia, Washington,
USA.
McCollister, M. F., & van Manen, F. T. (2010). Effectiveness of Wildlife Underpasses
and Fencing to Reduce Wildlife–Vehicle Collisions. Journal of Wildlife
Management, 74(8), 1722–1731. Retrieved from
http://www.bioone.org.bay.evergreen.edu/doi/abs/10.2193/2009-535
National Public Radio Staff. (2014). Why Did The Mountain Lion Cross The Freeway?
To Breed. Retrieved from http://www.npr.org/2014/10/19/357404731/why-did-themountain-lion-cross-the-freeway-to-breed
Niemi, M., Jääskeläinen, N. C., Nummi, P., Mäkelä, T., & Norrdahl, K. (2014). Dry paths
effectively reduce road mortality of small and medium-sized terrestrial vertibrates.
Journal of Environmental Management, 144, 51-57.
Olsson, M. P. O., Widén P., & Larkin, J. L. (2008). Effectiveness of a highway overpass
to promote landscape connectivity and movement of moose and roe deer in Sweden.
Landscape and Urban Planning, 85(2), 133-139.
O’Malley, P. G. (2004). The Leading Edge: Managers discover there’s more to a right of
way than vegetation. Erosion Control. Retrieved from
http://www.erosioncontrol.com/EC/Articles/The_Leading_Edge_4301.aspx
Owens, A. K., Moselely, K. R., McCay, T. S., Castleberry, S. B., Kilgo, J. C., & Ford, W.
M. (2008). Amphibian and reptile community response to coarse woody debris
105

manipulations in upland loblolly pine (Pinus taeda) forests. Forest Ecology and
Management, 256(12), 2078-2083.
Oxley, D. J., Fenton, M. B., & Carmody, G. R. (1974). The Effects of Roads on
Population of Small Mammals. Journal of Applied Ecology, 11(1), 51-59.
Pedevillano, C., & Gerald Wright, R. (1987). The influence of visitors on mountain goat
activities in Glacier National Park, Montana. Biological Conservation, 39(1), 1–11.
doi:10.1016/0006-3207(87)90002-4
Peterson, C. (2014, October 13). Sage Grouse Uprising? Rare pictuers catch Wyoming
sage grouse using wildlife underpass. Casper Star-Tribune Communications.
Retrieved from http://trib.com/lifestyles/recreation/rare-pictures-catch-wyomingsage-grouse-using-wildlife-underpass/article_e7beecbb-f2c8-58a3-815631bd27b9b8d3.html
Phillips, G. E., Alldredge, W., & Andree, W. W. (2001). MITIGATING
DISTURBANCE OF MIGRATING MULE DEER CAUSED BY CYCLISTS AND
PEDESTRIANS AT A HIGHWAY UNDERPASS NEAR VAIL, COLORADO.
In 2001 International Conference on Ecology and Transportation. Keystone,
Colorado. Retrieved from http://www.icoet.net/downloads/Posters.pdf#page=24
Puky, M., Mester, B., & Mechura, T. (2013). How Much Does Size Matter? Tunnel Size
Significantly Influence Amphibian Crossings at Parassapuszta, Hungary According
to Mid-Term Monitoring used to Delineate Mitigation Measure Improvement Plans.
Presented at the 2013 International Conference on Ecology and Transportation,

106

Scottsdale, Arizona, USA. Retrieved from
http://www.icoet.net/icoet_2013/documents/posters/ICOET2013_PosterImageP29_P
uky_et_al.pdf
Rodriguez, A., Crema, G., & Delibes, M. (1996). Use of Non-Wildlife Passages Across a
High Speed Railway by Terrestrial Vertebrates. Journal of Applied Ecology, 33(6),
1527–1540. Retrieved from
http://www.jstor.org.bay.evergreen.edu/stable/pdfplus/2404791.pdf?acceptTC=true
&jpdConfirm=true
Ruiz-Capillas, P., Mata, C., & Male, J. E. (2013). Road verges are refuges for small
mammal populations in extensively managed Mediterranean landscapes. Biological
Conservation, 158, 223–229.
Rytwinski, T., van der Ree, R., Cunnington, G. M., Fahrig, L., Findlay, C. S., Houlahan,
J., Jaeger, J. A. G., Soanes, K., & van der Grift, E. (2015). Experimental study
designs to improve the evaluation of road mitigation measures for wildlife. Journal
of Environmental Management, 154 (1), 48-64.
Serronha, A. M., Mateus, A. R., Eaton, F., Santos-Reis, M., & Grilo, C. (2013). Towards
effective culvert design: monitoring seasonal use and behavior by Mediterranean
mesocarnivores. Envionmental Monitoring and Assessment, 185(8), 6235-6246.
Shepherd, B., & Whittington, J. (2006). Response of Wolves to Corridor Restoration and
Human Use Management. Ecology and Society, 11(2), 1. Retrieved from
http://www.ecologyandsociety.org/vol11/iss2/art1/

107

Smith, D. (2003). Monitoring Wildlife Use and Determining Standards for Culvert
Design. Final report presented to the Florida Department of Transportation for
Contract BC354-34, Department of Wildlife Ecology and Conservation, University
of Florida, Gainesville, Florida.
Smith, J. (2011). I-90: Integrating Stewardship into the Highway Design. Yakima, WA:
Washington State Department of Transportation. Retrieved
from http://www.wsdot.wa.gov/NR/rdonlyres/EC7FEC4D-27BE-4601-B7ADF8FC1BB063FE/0/I90ENVIROFolioweb.pdf
Soulé, M. E. (2000). Is connectivity necessary? Presented at the Missing Linkages:
Restoring Connectivity to the California Landscape Conference, San Diego,
California, USA.
Southall, P. (n.d.). Amphibian-Reptile Wall and Culverts [Government Website].
Retrieved from
http://www.fhwa.dot.gov/environment/critter_crossings/amphibin.cfm
Sullivan, M. (2014). A Temporal Analysis of Elk Movement in Relation to Washington’s
Transportation Infrastructure (Master’s Thesis). The Evergreen State College,
Olympia, Washington, USA.
United States Bureau of the Census. (2000). The Census Bureau on Prospects for US
Population Growth in the Twenty-First Century. Population & Development Review,
26(1), 197-200.

108

Washington State Department of Transportation. (2015). Wildlife Carcass Removal
Database [Data file].
Washington State Patrol. (2015). Vehicle Collision Database [Data file].
Whitaker, J. O., & Hamilton, W. J. (1998). Mammals of the Eastern United States. New
York: Cornell University Press.
Wolff, J. O., & Guthrie, R. D. (1985). Why Are Aquatic Small Mammals So Large?
Oikos, 45(3), 365-373
Woltz, H. W., Gibbs, J. P., & Ducey, P. K. (2008). Road crossing structures for
amphibians and reptiles: Informing design through behavioral analysis. Biological
Conservation, 141(11), 2745-2750.
Yanes, M., Velasco, J. M., & Suárez, F. (1995). Permeability of roads and railways to
vertebrates: The importance of culverts. Biological Conservation, 71(3), 217-222.

109

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