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Title (dcterms:title)
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Eng
Shorebird Use of Smooth Cordgrass (Spartina alterniflora) Meadows in Willapa Bay, Washington
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Date (dcterms:date)
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2006
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Creator (dcterms:creator)
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Eng
Parks, Jared
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Subject (dcterms:subject)
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Eng
Environmental Studies
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extracted text (extracttext:extracted_text)
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Shorebird Use of Smooth Cordgrass (Spartina alterniflora) Meadows in
Willapa Bay, Washington
by
Jared R. Parks
A thesis submitted in partial fulfillment
of the requirements for the degree of
Master of Environmental Studies
The Evergreen State College
July
2006
i
This Thesis for the Master of Environmental Studies Degree
by
Jared R. Parks
has been approved for
The Evergreen State College
by
________________________
Steven G. Herman
Member of the Faculty
________________________
John Perkins
Member of the Faculty
________________________
Peter Blank
ii
Abstract
During the late 1800’s, Spartina alterniflora, a marsh grass native to the eastern
United States began to colonize Willapa Bay in southwestern Washington. By the
mid-1980’s, the grass had spread to such an extent that land managers and
aquaculturists initiated steps meant to eradicate it and several congeners from Willapa
Bay and other Washington intertidal areas. Among the reasons cited for the
eradication efforts were the assumed impacts the grass would have on migrating
shorebird populations that use the historically less vegetated intertidal mudflats
during the fall, winter, and spring.
The purpose of the work presented here was to examine the validity of this
contention. Five survey locations were chosen throughout Willapa Bay to determine
whether or not shorebirds are using areas colonized by Spartina during spring
migration.
The literature was consulted to put the Willapa Bay circumstance in perspective
relative to invertebrate communities within Spartina marshes elsewhere, shorebird
feeding ecology, and other studies designed to investigate the possible impacts of the
grass on shorebirds.
Over the four weeks of surveys, shorebirds were regularly and consistently
observed foraging within the colonized areas. There was a significant tidal effect
observed, with birds feeding more frequently in the grass on the ebbing tide than on
the flooding tide. Based on the literature and this research, it is clear that Spartina
does not exclude feeding shorebirds during the spring migration at Willapa Bay.
In presenting this thesis as partial fulfillment of the requirements for a Master’s of
Environmental Studies at the Evergreen State College, I agree that the library will
make copies available. I further allow that copying of this thesis is acceptable only for
scholarly purposes, consistent with “fair use” as prescribed in the United States
Copyright Law. Any other reproduction for any purposes or by any means will not be
allowed without the written permission of the author.
Signature______________________
Jared R. Parks
Date______________________
iii
Table of Contents
Page
Title Page………………………………………………………………………. i
Approval Page…………………………………………………………………. ii
Abstract………………………………………………………………………...
Copyright Page………………………………………………………………… iii
Table of Contents……………………………………………………………… iv
List of Figures………………………………………………………………….
v
Acknowledgements……………………………………………………………. vi
Chapter
1.
INTRODUCTION……………………………………………..
1
Willapa Bay…………………………………………….
Spartina alterniflora……………………………………
Spartina in Washington………………………………...
Spartina Control in Washington……………………….
Shorebirds of Willapa Bay……………………………..
4
6
9
10
12
2.
METHODS……………………………………………………..
15
3.
RESULTS……………………………………………………….
20
4.
DISCUSSION…………………………………………………..
27
Spartina and Shorebirds………………………………...
Spartina and Invertebrate Communities………………..
Shorebird Feeding Ecology……………………………..
27
30
32
5.
CONCLUSION…………………………………………………
36
6.
WORKS CITED………………………………………………...
40
7.
APPENDIX A…………………………………………………..
46
iv
Figures
Figure
1.
2.
3.
4.
5.
6.
7.
Page
Map of Willapa Bay in southwestern Washington………………………...
Map of Pacific County and its major population centers………………….
Photographs depicting Spartina in two stages……………………………..
Author making observations at ‘S-curves’ site…………………………….
Locations of the five survey sites………………………………………….
Differing stem densities of Spartina……………………………………….
Shorebirds at North Site……………………………………………………
5
6
8
15
17
19
25
Tables
Table
1.
2.
3.
4.
5.
6.
Total birds in Spartina by species…………………………………………
Total birds in Spartina by week and species………………………………
Total birds in Spartina by site and week…………………………………..
Total birds in Spartina by site and species………………………………...
Total birds in Spartina by site and tide……………………………………
Total birds in Spartina by species and tide………………………………..
Page
21
22
22
23
24
24
v
Acknowledgements
The field research presented in this thesis would not have been possible without
the keen eyes and patience of Lucas Limbach. His help with surveys kept me sane. In
beautiful and ugly weather alike he was a joy to be with in the field, and I will always
remember the rainbows, sand dollars, and pop-up thunderstorms of Willapa Bay and
our “short” walk around Leadbetter Point.
As well, without the help of my thesis advisor, Steven Herman, I would not have
been unable to produce such a paper. The library at the Evergreen State College and
the public library in Kent County, Maryland also deserve kudos for their great staffs
that helped me find all of the articles that I needed, no matter how obscure. I also
must thank Kim Patten for help selecting sites.
My second and third readers John Perkins and Peter Blank were a great help in
providing me with editorial advice, though I also received a lot of editing help from
my family and my good friends. I would also be remiss if I were not to thank Marty
Acker for all of his assistance while writing this thesis.
Without the help and support of these and many others, I would have been
lost. My greatest thanks and love for all.
vi
INTRODUCTION
Migrating flocks of shorebirds along the west coast of the United States can be
quite impressive, most notably in the spring when 10’s to 100’s of thousands of birds can
be seen in major bays and estuaries. These large flocks of birds tend to concentrate in
large, shallow bodies of water that are characterized by expansive mudflats at low tide.
The most common shorebirds using these sites are Western Sandpiper (Calidris mauri),
Dunlin (Calidris alpina), Sanderling (Calidris alba), dowitcher spp. (Limnodromus spp.),
Greater Yellowlegs (Tringa melanoleuca), Black-bellied Plover (Pluvialis squatarola), and
Semipalmated Plover (Charadrius semipalmatus). In addition, large numbers of godwits
(Limnosa spp.), turnstones (Arenaria spp.), phalaropes (Phalaropus spp.), other Calidris
sandpipers, and members of the genus Numenius can be found in special locales or in
certain years along the same flyway. This paper focuses on seven shorebirds that are most
abundant in the shallow bays and estuaries of the West Coast: Western Sandpiper, Dunlin,
Short- and Long-billed Dowitchers, Greater Yellowlegs, and Black-bellied and
Semipalmated Plovers. Because these species rely on western bays and estuaries as
migratory stopover sites, they are at greatest risk from changes to such areas.
Spartina, a genus of grass most commonly associated with wetland habitats, was
introduced into the western coastal waterways in the late 1800’s. The colonizing species
are wetland obligates of saline waters from many parts of the world. The most common are
Spartina alterniflora, S. anglica, S. patens S., townsendii, and S. densiflora. This paper
focuses on Spartina alterniflora and its colonization of bays of the Pacific Northwest, most
specifically Willapa Bay, Washington.
1
Spartina alterniflora (Smooth Cordgrass) was first introduced to Willapa Bay in the
1890’s (Sayce 1988), but little concern over the introduction was voiced until the early to
mid-1980’s, almost 100 years after its initial colonization. In the beginning the cordgrass
was not seen as a threat because it spread at an almost imperceptible rate. Recently,
however, the pace and scope of the spread has increased to such an extent that public land
managers, aquiculturists, and wildlife enthusiasts are concerned about the effects that the
situation might have on livelihoods and the ecological stability of the bay.
Willapa Bay is a classic example of a shallow, western, coastal estuary. At low tide
large expanses of mudflat dominate the basin. These flats are characteristically almost
devoid of any native emergent vegetation, though Triglochin maritimum and Plantago
maritima are found along a narrow band seaward of the upper marsh, and eel grass
(Zostera spp.), an aquatic grass, is common at lower intertidal levels. The upper marsh is
dominated by Distichlis spicata and Salicornia virginica. Historically, these expanses of
mud have provided excellent oyster and clam beds as well as ideal feeding areas for
migrating shorebirds. As Spartina has continued to spread, the concern has grown over
how much of these mudflats will be lost to the expanding meadows of cordgrass and how
this change will affect the aquatic and avian life in the region.
Since the 1980’s, when the issue of Spartina invasion first began to draw the
attention of the Washington state legislature, control of the exotic has become more and
more of a priority in coastal watershed management. Concern over possible crippling
losses to one of the area’s principal industries, aquiculture interests have been the driving
force behind this effort, though wildlife managers and enthusiasts and those involved in the
fishing industry have also been influential in encouraging attempts to control Spartina. It is
2
contended that the ecological stability of the Washington coastal waterways is threatened
by the spread of Spartina, and that this in turn will have grave impacts on the native (and
perceived-beneficial exotic) flora and fauna of these areas (Revised Code of Washington
Chapter 17.26, Washington State Aquatic Nuisance Species Plan 2001).
Though individual species of Spartina have been introduced to many coastal areas
around the world, the scientific literature is anemic with regard to the direct impacts of the
plant on the native systems into which they are introduced. It is not unusual for ecological
management decisions to be inadequately backed by scientific study, but in all
circumstances it is preferable to find as much backing as possible to justify such decisions.
The assertion has been made that migrant shorebirds will not forage (or do so less
frequently) within the meadows that cordgrass forms as it spreads over native mudflats and
eelgrass beds (Davis and Moss 1984, Millard and Evans 1984, Evans 1986, Nairn 1986,
Aberle 1993, Daehler and Strong 1996, Jaques 2002, WSDA report 2002). Theoretically,
then, shorebirds would have less space available for feeding and less time during the tidal
cycle to feed. If these restrictions limited their access to food to the extent that they failed
to prepare themselves appropriately for the long journey that they face and the conditions
that they may encounter along the way, a negative population impact could result.
The research reported here was designed to examine whether these migrating
sandpipers use or are excluded by the cordgrass. Both literature review and original field
research were used to answer the question: do shorebirds utilize Spartina meadows in
Willapa Bay, Washington and, if so, what species and how?
3
Willapa Bay
Willapa Bay is located on the southwestern coast of Washington state in Pacific
County (Figures 1 & 2). The bay functions as the drainage for the Willapa Hills, receiving
the water from nearly 2400 km2 of land (Andrews 1965). Forest plantations and the
logging industry dominate this area of southwestern Washington. The Willapa Hills border
the eastern and southern shores of the bay while the western shore is formed by the Long or
North Beach Peninsula. As of the 2000 census, 20,844 people lived in the 933 square miles
that comprise Pacific County. Major population centers in Pacific County include
Raymond, South Bend, and Tokeland in the north bay area, and Naselle, Ilwaco, Long
Beach, and Ocean Park in the south and west bay areas. Compared to the rest of western
Washington, Pacific County has a very low population density, with only about 22.5
persons per square mile (state average 88.6), and only about 14,000 housing units
(Washington State web page). The economy of Pacific County relies heavily on logging,
fishing. and aquiculture, as well as tourism and outdoor recreation.
Willapa Bay itself is a long estuary with a barrier beach (the Long Beach Peninsula)
forming its western border. The barrier beach stretches from the mouth of the Columbia
River approximately 25 miles north to Leadbetter Point. At Mean Higher High Water,
MHHW, the bay is covered by about 350 km2 of water, and at Mean Lower Low Water,
MLLW, about 190 km2 is exposed intertidal area, 55% of the bay’s area (Andrews 1965).
Because the bay is a long estuary, there is a significant difference between tidal ranges at
its north (7.5 feet or 2.3 m) and south (11 feet or 3.4 m) reaches (Sayce 1988).
4
The modern economic history of Pacific County began in the late 1700’s with the
emergence of a fur trade in sea otter pelts ). This quickly gave way to fishing as the
predominant industry in the area
Figure 1. Map of Willapa Bay in southwestern Washington State.
when sea otter populations were decimated. Salmon were the primary piscatorial prize for
the shoremen, but oysters, clams, mussels, and crabs were also taken. By the 1890’s the
lumber and logging industries had taken on the preeminent role in the area’s economic
sector, but fishing and aquiculture were still firmly established trades. The economic
picture today is still very similar to that of 1900, but with two large exceptions—recreation
and tourism. Because of the area’s natural beauty and bounty, tourists and outdoor
enthusiasts flock to Pacific County. Boating, recreational fishing and shellfishing, hunting,
birdwatching, beachcombing, and shopping are now major players in the area’s economy.
5
Figure 2. Map of Pacific County and its major population centers.
Spartina alterniflora
Spartina alterniflora Loisel. (Smooth Cordgrass) is the dominant marsh grass of
low intertidal habitats along the East Coast of North America south through the West
Indies and western South America (Mobberley 1956, Chapman 1960, Aberle 1993). In the
northern part of its range Smooth Cordgrass comprises the lowest bands in the typical
zonation within intertidal marshes. Two growth forms exist, though they are genetically
identical (Valiela et al. 1978). The tall form inhabits the lowest intertidal band of
6
vegetation, while the short form occupies the band just landward (Valiela et al. 1978,
Bertness 1988). The phenotypic difference appears to be caused by differing nutrient
availability in the two low marsh bands (Valiela et al. 1978). In the high marsh Spartina
patens forms the seaward band, and Juncus gerardi forms the landward band (Bertness &
Ellison 1987). This distinct pattern in the marsh community arises, in part, from S.
alterniflora’s ability to oxygenate its roots and rhizosphere through aerenchyma when
submerged in saline waters, thus making it more suited than other marsh plants to the more
frequently flooded lower marsh (Teal and Kanwisher 1966, Bertness 1991). Conversely, S.
alterniflora is excluded from the upper marsh by S. patens and J. gerardi, due to the
negative impacts increased peat accumulation and thick turf mass have on S. alterniflora
(Bertness 1987, Bertness 1991).
The typical growth pattern of Spartina alterniflora begins when a seed or a viable
root mass becomes established within the intertidal zone. These new plants form circular
‘clones’ which, in time, expand and coalesce with similar clones to form a solid,
monospecific meadow (Figure 3). Both the clones and the meadows expand by trapping
sediment. This in turn raises the level of the marsh above surrounding areas devoid of
Spartina. Over time V-shape drainage channels become established, creating a mosaic of
waterways and grass within the Spartina meadows.
Smooth cordgrass has been introduced intentionally in a number of locales
worldwide, including England (Mobberley 1956, Ranwell 1967), France (Mobberley 1956,
Ranwell 1967), China (Chung 1989) and New Zealand (Ranwell 1967, Partridge 1987) in
order to take advantage of its ability to trap sediment in the intertidal area stabilizing
coastlines and navigation channels. It has also become established unintentionally in many
7
Figure 3. Photographs depicting Spartina in two stages. The top is of an area in north Willapa Bay (‘North
Site’) where the clones are beginning to coalesce into a meadow, the bottom of a meadow in east Willapa Bay
(‘Bruceport’). [Top picture by Lucas Limbach. Bottom picture by author.]
areas worldwide, including estuaries along the Pacific Coast of North America in
Washington, Oregon, and California (Spicher and Josselyn 1985, Frenkel 1987, Aberle
8
1993). For a more detailed account of the worldwide distribution of S. alterniflora see
Aberle (1993).
Spartina in Washington
In Washington State four species of invasive grasses of the genus Spartina have
been found. Spartina patens, a native of the East Coast of North America, is known to
exist in only one locale, Dosewalips State Park in Jefferson County. It is a small colony
that was first discovered in the early 1990’s, though its origin is not known (WSDA report
1999). Spartina densiflora, a South American native, was discovered in 2002 at two
locations, one in Puget Sound and the other in Grays Harbor (WSDA report 2002).
Spartina anglica, of English origin, has been found in seven Washington counties, all
within the Puget Sound, and, as of 2002, covered over 700 acres (WSDA report 2002).
Spartina alterniflora, however, is the species that has colonized the most intertidal acreage
in Washington. It has been found in five counties, though only about 20 solid acres are
known to exist (or have existed) in four of those counties (WSDA report 1999). The
remaining county, Pacific (where Willapa Bay is located), had over 6,800 solid acres of
Spartina alterniflora in 2002 (WSDA report 2002).
In Willapa Bay Spartina meadows are most prevalent along and at the mouth of
almost every stream entering the bay. The largest meadows in southern portions of the bay
can be found around Porter Point where the Bear River enters the bay and along the eastern
and northern shores of Long Island near the mouth of the Naselle River. In northern
portions of the bay, large meadows have colonized the mouths of the Palix, Bone, and
Willapa Rivers. Smaller meadows are found along the eastern shore of the North Beach
9
Peninsula and in northern areas of the bay at the mouths of the Cedar and North Rivers and
in protected waters surrounding Toke Point. As stated above, S. alterniflora covers 6,800
solid acres in the bay. However, ‘solid acres’ refers to the total area of coverage if all
clones and meadows were contiguous. In actuality more than 6,800 acres are affected, and,
before treatment efforts began, the grass had colonized nearly the entire shoreline ringing
the bay.
Spartina Control in Washington
In 1989 Spartina alterniflora was added to the Washington State noxious weed list,
and control efforts were begun immediately by the Washington State Department of
Natural Resources (WDNR) and the United States Fish and Wildlife Service (USFWS). In
1995 the Washington State legislature mandated that the Washington State Department of
Agriculture (WSDA) assume sole responsibility for the control of all Spartina species
statewide in an attempt to facilitate an efficient and appropriate response to the newly
declared ecological emergency (RCW 17.26.005 and 006). The legislature also loosened
restrictions on control activities in aquatic environments to aid in the effort. Since the
legislative action in 1995, the WSDA has been provided with an ever increasing budget for
the control efforts. In fiscal year 2002 over $1.1million was spent statewide- of which over
$766,000 was directed towards control efforts in Willapa Bay, and an estimated $1.9
million was earmarked for control efforts for fiscal year 2003 of which over $1.1million
was to be spent in Willapa Bay (WSDA report 2002).
Though the WSDA is the lead agency in the Spartina control effort, a multitude of
agencies and local groups play an integral role in management. The Washington State
10
Department of Fish and Wildlife (WDFW) and WDNR along with the USFWS, county
noxious weed boards, tribal entities, private landowners, state universities, and WSDA treat
Spartina and explore the impacts of the exotic and the treatment methods used to eradicate
it. As lead agency, WSDA also allocates funds to the other entities, provides technical
assistance, and reports to the state legislature on the progress of control efforts.
An integrated pest management plan is used in an effort to maximize control
efficacy. The cordgrass is mowed, crushed, disked, rototilled, dug out, covered with black
plastic, and sprayed with herbicide. The University of Washington- Olympic Natural
Resource Center is also conducting a biological control project in Willapa Bay using
planthoppers (Prokelisia marginata). If the control efforts are continued with adequate
funding, the WSDA estimates that Spartina will be eradicated from Willapa Bay before
2010 (WSDA report 2002).
Though local support for control efforts is high, it would be a gross misstatement to
say that everyone backs the control efforts or views Spartina in the same way. Many
people are adamantly against the spraying of herbicide in any aquatic system, and others do
not share the opinion that Spartina poses an imminent ecological threat. The managers
mostly dismiss the concerns of the dissenting minority, though little scientific study is
present to back their stance. Little is known about the possible implications of spraying
pesticides in these aquatic systems, and very few studies have been conducted to quantify
the effects Spartina has on estuarine communities where it is not native.
11
Shorebirds of Willapa Bay
Willapa Bay is a very important estuary for west coast migratory and wintering
shorebirds, and meets the criteria to be considered a Site of International Significance for
spring migratory shorebirds (Harrington and Perry 1995). Annually, hundreds of thousands
of shorebirds use the bay during spring migration, and up to 90,000 use the bay as a
wintering ground (Buchanan and Evenson 1997).
The first in depth study of shorebirds in Willapa Bay was conducted between June
1978 and June 1979 by Ralph Widrig (1979). The study, 61 survey days over twelve
months, concentrated on the birds utilizing both sides of the Long Beach Peninsula.
However, many of the birds that feed on the bay’s northern, eastern and southern reaches
roost along the outer beaches of the peninsula during high tide, so it is possible to get an
idea of the numbers and species make up within the bay during the late 1970’s. Since
Widrig’s census, there have been studies of the shorebirds in Willapa Bay at all seasons,
but non have been as thorough.
Presently the Willapa National Wildlife Refuge (WNWR) is studying the possible
effects that Spartina is having on shorebirds in the bay. This study includes many survey
techniques to determine where the birds are feeding and roosting in the bay, and if and/or
how they are using areas being occupied by Spartina and those that are being treated
(Jaques 2002). Kim Patten, a biologist with Washington State University, has also
developed a protocol for “watching” the birds remotely with cameras placed above areas in
south bay in hopes of learning how or if the birds are using Spartina meadows and clone
fields.
12
Common wintering shorebirds include Black-bellied Plovers (Pluvialis squatarola),
Western Sandpipers (C. mauri), and Marbled Godwits (Limosa fedoa), but Dunlins
(Calidris alpina) are by far the most common with up to 70,000 individuals present during
the winter months (Widrig 1979, Buchanan and Evenson 1997). Many other species are
found in lesser numbers during the same period. The birds can be found feeding all over
the bay during rising and falling tides (Widrig 1979, Buchanan and Evenson 1997, Jaques
2002). It has been suggested that winter is the time of year that birds could be most
affected by the continued spread of cordgrass, because, if it is true that the birds will not
forage in the Spartina meadows, the loss of higher intertidal areas to Spartina spread
coupled with shorter days could seriously impact the length of available feeding time for
the birds (Goss-Custard and Moser 1988, Jaques 2002).
During spring migration, Dunlins, Western Sandpipers, and Short-billed Dowitchers
(Limnodromus griseus) are the most common shorebirds found using Willapa Bay as a
northbound, migratory staging area. From February to mid-May, groups numbering in the
thousands can be found throughout the bay. Dunlins are the earliest migrants to move
through the area. They slowly begin to increase from their wintering numbers by late
January and reach their peak in late February, though their migration lasts into May
(Widrig 1979). Western Sandpipers are the most abundant spring migrant, moving through
between early-April and mid-May, though the bulk of the movement happens usually
within a few days between mid-April and the first week of May (Widrig 1979, Jaques
2002). During this push, over 80,000 Western Sandpipers can be found in the bay at one
time, though this is but a fraction of the number that can be found in Grays Harbor, just to
the north, at the same time (Herman & Bulger 1981, Buchanan and Evenson 1997, Jaques
13
2002). Dowitchers, numbering in the thousands, can be found using the bay between earlyApril and early-May with a peak migration very similar in time to that of the Western
Sandpipers (Widrig 1979).
The preferred feeding habitat for these wintering and migrating shorebirds (as well
as that of Greater Yellowlegs, Tringa melanoleuca, and Semipalmated Plover, Charadrius
semipalmatus: other fairly common shorebirds found in Willapa Bay) is estuarine mud flats
(Hayman et al. 1986, Washington Dept. Fish and Wildlife [WDFW] 2002). The Western
Sandpipers, Dunlins, both plovers, and Marbled Godwits can also be found on sandy
coastal beaches (WDFW 2002). Pastureland and flooded fields are also suitable feeding
and resting areas for many of these species, especially during high tide when the more
preferred intertidal areas are unusable (Herman & Bulger 1981, Colwell & Dodd 1995,
Colwell & Dodd 1997, WDFW 2002, personal observations). When not actively feeding,
these birds tend to aggregate in large groups to roost on nearby outer beaches, sand islands,
and upper marsh and pasture areas (Widrig 1979, Herman & Bulger 1981, Buchanan and
Evenson 1997, Jaques 2002, personal observations). Most census techniques used to date
in Willapa Bay employ methods for counting birds in these aggregations to estimate overall
numbers of shorebirds in the bay (Widrig 1979, Buchanan and Evenson 1997, Jaques
2002).
14
METHODS
The field work for this study was done with one very precise purpose in mind—to
determine if shorebirds utilizing Willapa Bay as a spring migratory staging area are
employing the Spartina meadows and clone fields for foraging and/or roosting areas. All
surveys were conducted during the peak migration period during the spring of 2003. The
peak migration for the greatest variety of shorebird species was predicted using data
collected by Widrig (1979), Buchanan and Evenson (1997), and Jaques (2002). Thus, it
was decided that four surveys would be conducted at each site over the period April 11,
2003 to May 4, 2003 (Appendix A).
The surveys were conducted from the shore by two researchers at each site
recording direct observations of birds within those sites (Figure 4). The researchers were
Figure 4. Author making observations at ‘S-Curves’ site. [Picture by Lucas Limbach]
15
each equipped with binoculars and 15-60x spotting scopes mounted on tripods. All
observations including, but not limited to, shorebird activity, species and numbers; tidal
movement; weather; and raptor presence were recorded on micro cassette and in field
notebooks. One site was surveyed on each half of a tidal cycle, and only one full tidal
cycle reliably fell within daylight hours each day. Thus, the maximum number of surveys
conducted in one day was two. Each site was surveyed once each weekend over the four
weekend period with surveys being conducted for each site twice during the falling tide and
twice during the rising tide (See Appendix A for a summary of tide magnitude and timing
during the survey dates for each site).
Survey times varied between 1 hour and 4 hours 10 minutes (Appendix A). The
difference in survey time was necessary because each site had differing degrees of Spartina
coverage and the magnitude of the tide varied by site and day. The actual survey length
was determined by the distance Spartina had spread into the intertidal and the magnitude of
the tidal movement for each site and each survey day. Since most shorebirds tend to feed
at the edge of the ebbing or flowing tide line, the goal was to be present at each site as the
tide line was passing through the band of cordgrass in order to determine if birds were
feeding in the grass. Surveys were conducted between 2 hours after high tide to 2 ½ hours
before low tide on falling tides and 1 ½ hours after low tide to 2 ½ hours before high tide
on rising tides, though there was some variation due to availability of researcher time and
differences in tidal magnitude.
Site selection began two weeks prior to the first survey in an attempt to ensure that
Spartina cover was the same when the sites were chosen and when they were surveyed.
Kim Patton was a great help in suggesting areas that fit the basic criteria needed for the
16
study. The most important aspect for the site was its Spartina cover. Sites were chosen
that had between 20 and 80% grass cover when the surveys began. Sites were also chosen
based on their accessibility. The sites had to be within easy walking distance of public
access areas and close enough to a public road to make travel between sites relatively fast.
Finally, because bird use is not uniform throughout the bay, an attempt was made to choose
sites that were distributed evenly around it.
Figure 5. Locations of the five survey sites: S-Curves (SC), 113TH (113), North Nemah (NN),
Stony Point (SP), and North Site (NS).
17
Five sites were chosen, two on the eastern shore of the peninsula, two on the eastern
shore of the bay south of the Willapa River, and one on the northern shore of the bay just
northwest of the mouth of the Willapa River (Figure 5). Site size varied due to shoreline
shape and observation site lines, though an attempt was made to concentrate on bird
activity within an approximate 1000 m semicircle surrounding the observation spot. All
visible bird activity was noted when possible.
All the sites had Spartina clones and meadows to some degree, though cover was
not identical (Figure 6). Three sites had been treated to some extent in 2002, but only one
had been treated in 2001 (WSDA report 2001 and 2002). Though some of the sites had
been treated in the previous two years, all but one site (S-curve site) had uninterrupted,
solid expanses of Spartina present. The S-curve site was treated in 2002, and was chosen
because it had several large clones in the intertidal and a band of mixed Spartina and
arrowgrass along the lower intertidal upper marsh boundary. The goal was to survey the
sites as the tide passed through the outer ‘clone fields’ first and then through the meadows
(or visa versa) in order to see how the birds reacted to the grass. The amount of grass
present and the proximity in time of its last treatment are surely factors in overall bird use,
but this study was not designed to account for these variables. The study was only meant
to document whether birds were using areas covered by varying amounts of Spartina.
All numbers of birds reported in this paper are count-estimates (Herman 1980).
Whenever possible, actual counts of birds were made. However, many instances arose
during the surveys that required quick estimation of total numbers. In these cases two
methods of estimation were used. The first method resulted in concrete numbers. The
18
Figure 6. Differing stem densities of Spartina. These two pictures show differing stem densities of
Spartina plants at the same site (Stony Point). Not only was overall Spartina coverage of the sites
different, but stem density was also different within and between sites.
second resulted in ranges. When compiling the data, the estimates were added to the
concrete numbers resulting in total numbers for each site. If the results of the tabulations
ended up being ranges, then the mean of the ranges were used to calculate and compare site
numbers.
19
RESULTS
Shorebirds were documented utilizing areas of Spartina cover in all five sites
surveyed. In all 10,825 individuals were observed in Spartina during the four weekends of
study. Birds were seen feeding, roosting, and flying in and out of these areas. Often it was
difficult to determine exactly what the birds were doing while in the cordgrass because the
grass blocked the researchers’ view. This became an increasing obstacle as the grass grew
taller over the duration of the study. All instances of birds using Spartina were included in
the general category of “birds in Spartina,” even when instances of feeding, roosting, or
other uses were determined by the observers. Seven species were documented utilizing
cordgrass in the study sites, and one additional species was documented in cordgrass
outside of the study areas (Table 1).
Dunlin (DUNL) was the species occurring in the greatest number in the grass with
4389 individuals recorded, and dowitcher (Long and Short-billed were lumped together,
though Short-billed comprised >99% of the positively identified dowitchers [DOW spp])
was the second most numerous species positively identified with 2495 individuals
recorded. Unidentified shorebirds (UID) were the second most numerous overall (3290).
Western Sandpiper (WESA), Greater Yellowlegs (GRYL), Black-bellied Plover (BBPL),
and Semipalmated Plover (SEPL) were also present in varying numbers. Four Least
Sandpipers were identified in the grass on a trip to Leadbetter Point on April 26. It is a
good possibility that Leasts were present in the study areas as well but were not separated
from the other small sandpipers.
20
Table 1
Total birds in Spartina by species
Dunlin
4389
Unidentified *
3290
Dowitcher spp. **
2495
Western Sandpiper ***
325
Greater Yellowlegs
250
Black-bellied Plover
51
Semipalmated Plover
25
* Unidentified consisted of unidentified "peeps" and mixed flocks
with no % species make-up noted.
** Both Short-billed (99%) and Long-billed (1%) Dowitchers were
identified in Spartina during the study.
*** 4 Least Sandpipers were observed in Spartina on Leadbetter
Point on 4-26, but none were positively identified during the surveys.
The week of April 25- 27 marked the high point for total birds observed in the
grass. Peak numbers of all but one species followed the pattern that Widrig determined for
spring migration (Widrig 1979). Dunlin were present in large numbers in all of the first
three weeks of the study, but peaked in the third week of the study, a full month past the
peak migration for that species as determined by Widrig but near the time Buchanan (1997)
found large concentrations in 1994. Total numbers of birds observed in Spartina grew
steadily and peaked in the third week (5534 total birds) then dropped-off in the forth week
(Tables 2 & 3). Large numbers of Western Sandpipers, normally expected near the last of
April and the first of May, were never observed during the course of the study though there
was a small increase observed over the time of the study. This could account for the drop
in total numbers observed during the fourth week of the study.
21
Table 2
Total birds in Spartina by week and species
DUNL
DOWI spp WESA
GRYL
BBPL
SEPL
UID
TOTAL
Week 1
4/11-4/13
910
85
33
25
13
0
0
1066
Week 2
4/18-4/20
1117
725
75
105
37
0
300
2359
Week 3
4/25-4/27
2105
1185
100
45
1
8
2090
5534
Week 4
5/2-5/4
257
500
117
75
0
17
900
1866
TOTAL
4389
2495
325
250
51
25
3290
10,825
North Site, Stony Point, and North Nemah were respectively the top three sites in
total numbers of birds seen in the grass (Tables 3 & 4). These were also the sites where the
largest numbers of birds were observed in general during the survey period.
Table 3
Total birds in Spartina by site and week
f= falling tide
r= rising tide
NS
NN
SC
113
SP
TOTAL
Week 1
4/11-4/13
f
765
r
0
r
75
f
97
f
129
1066
Week 2
4/18-4/20
r
614
f
180
f
45
r
0
f
1520
2359
Week 3
4/25-4/27
f
5148
r
0
r
85
f
116
r
185
5534
Week 4
5/2-5/4
r
448
f
1225
f
174
r
19
f
0
1866
TOTAL
6975
1405
379
232
1834
10,825
22
Table 4
Total birds in Spartina by site and species
NS
NN
SC
113
SP
TOTAL
DUNL
2812
215
167
45
1150
4389
DOWI spp.
1243
460
202
35
555
2495
WESA
100
105
5
40
75
325
GRYL
145
75
5
4
21
250
BBPL
7
0
0
11
33
51
SEPL
18
0
0
7
0
25
UID
2650
550
0
90
0
3290
TOTAL
6975
1405
379
232
1834
10,825
The S-curve site also had large concentrations of birds present, but the paucity of solid
Spartina accounts for the diminished use of the grass by the birds. However, birds were
observed feeding in the narrow band of arrowgrass at the upper marsh boundary on the last
two weeks (with about 4000 present on April 26). The 113th site had the least number of
birds both in the grass and at the site in general. Most of the birds observed from this site
stayed well off to the east near Porter Point, the Bear River, and the southern end of Long
Island, traditionally some of the best areas for shorebirds in the southern reaches of the bay.
Arguably the most interesting data collected during this study was related to bird
use of Spartina during the tidal cycle. Across all sites birds overwhelmingly used the grass
areas more on the falling tide than on the rising tide, with birds observed about 6.5 times
more often on falling tides (Table 5). At the North Nemah site no birds were observed in
Spartina on rising tides at all, and at both the 113th and Stony Point sites birds were not
observed in the grass on one rising tide each. Observations for the individual species
23
Table 5
Total birds in Spartina by site and tide
NS
NN
SC
113
SP
TOTAL
Rising
1062
0
160
19
185
1426
Falling
5913
1405
219
213
1649
9399
also followed the same pattern, except in the case of Semipalmated Plovers. Both total
numbers and number of instances for every species (except Semipalmated Plover) found in
Spartina were substantially higher on the falling tide (Table 6).
Table 6
Total birds in Spartina by species and tide
DUNL
DOWI spp WESA
GRYL
BBPL
SEPL
UID
TOTAL
Rising
182
458
17
95
7
17
650
1426
Falling
4207
2037
308
155
44
8
2640
9399
Generally, the birds began to arrive at the sites as the tide was passing through the
band of Spartina on the falling tide. These birds were usually very active as they followed
the ebbing tide through the grass; getting up, flying back and forth, and landing again at or
near the tide line. Most of the birds arriving to feed at our sites did so before the tide
reached the outer edge of the vegetated band. Often, birds were observed moving
significant distances laterally along the shoreline, actively feeding in and at the edge of the
Spartina band, even when open mud became available in front of the grass along the way.
As the birds moved through the grass, they spread out more and more from their original,
often tightly packed groups until they were finally beyond the grass.
24
The birds behaved very differently on the rising tide. If birds were observed using
the grass on incoming tides, they normally did so in smaller numbers than the groups that
were originally observed beyond the vegetation upon arrival. Generally, many more birds
flew from the site when the majority of open mud at the edge of the Spartina had been
covered than continued to move through the grass following the flooding tide line. Only
twice, both at North Site, did we observe larger numbers of birds (>300) move through the
entire band of vegetation with the rising tide (Figure 7). We never were able to document
where the birds flew when they left early, but we surmise that they were moving to roost
sites early, not to other places in the bay to continue foraging.
Figure 7. Shorebirds at North Site. This picture shows part of a mixed group of shorebirds (about 300 in all)
that was witnessed following the tide line in with the rising tide at North Site. This group was observed
foraging about equally in the openings between the grass and in the Spartina itself as it moved towards us.
In three instances we found large, mixed groups of shorebirds feeding and roosting
in pastures near our sites when open mud was still available at other, nearby areas in the
bay. A similar pattern was noted by Herman and Bulger (1981) during the spring in Grays
25
Harbor, Washington and by Brennan et al. (1985) with wintering Dunlin at various sites in
Western Washington. Only a few birds were recorded moving into the upper marsh at our
sites to roost on the rising tide, though birds were observed roosting at the sites at the
beginning of falling tide. The difference could be due to a tidal magnitude effect or to bird
movement during the high tide period.
We also observed one instance involving Peregrine Falcons that was of particular
interest. On April 18th while at North Site, we witnessed two peregrines fly over a large
group of shorebirds near the outer edge of the Spartina meadow. One bird flew inland
towards us with an unidentified shorebird in its talons. The other bird circled higher before
flying over us accompanied by two Bald Eagles. When the falcons were first spotted, we
noticed that many groups of shorebirds flew up from in front of the meadow and landed in
the grass, presumably seeking cover. This is of obvious interest because of the possibility
that shorebirds can use Spartina as cover from predators.
26
DISCUSSION
Shorebirds may be adversely affected by invasive vegetation for three primary
reasons. First, the birds could be excluded from the grass by physical characteristics, such
as high stem density. Second, they could be psychologically predisposed to feeding only in
open areas with undisturbed sight lines. Finally, they may be unable to fulfill their daily
nutritional requirements due to a reduction of invertebrate prey bases within the grass.
The first and second possibilities can be examined through direct observation of bird
movements and use of colonized estuaries and through long-term population studies of
birds in estuaries with differing amounts of exotic vegetation. The third requires both the
examination of differences in available prey between vegetated and unvegetated areas as
well as thorough investigations into the feeding behavior of the birds at each location in
question.
Spartina and Shorebirds
It was expected that some use of areas covered by Spartina would be documented
during the survey period. However, the variety and number of birds observed utilizing the
grass was unexpected. Previous studies investigating the possible effects of the grass on
shorebirds have implied a negative impact on overall numbers within and use of estuaries
where Spartina is spreading.
Goss-Custard and Moser (1988) studied wintering Dunlin populations in different
estuaries across Britain and found that numbers of shorebirds decreased significantly over
the period between 1973-74 and 1985-86. The largest decreases were seen in estuaries
where Spartina anglica had spread most significantly, though initial decreases in numbers
27
(1973-74 to 1977-78) were found to be independent of Spartina colonization while
continued decreases (1977-78 to 1985-86) were correlated with its spread. The numbers of
birds utilizing other estuaries where the grass was static or decreasing in density were not
found to be increasing over the same period, thus giving the impression that the birds were
not moving to alternate estuaries to winter. The belief is that both feeding area and time in
which to feed during the winter months are being lost as the grass colonizes the higher
intertidal areas, thus contributing to the observed decrease in wintering, British Dunlin. It
was also noted that the grass had spread most in traditional high density feeding areas.
Davis and Moss (1982) studied the populations of four shorebird species as well as
the spread of Spartina anglica in the Dyfi estuary in central Wales from 1969-1981.
Though only a 10% increase in Spartina was noted, a significant decrease in three of the
wader species was observed. As with the findings of Goss-Custard and Moser (1988), the
grass spread most in traditional high-density shorebird areas. Numbers of Dunlin, Ringed
Plover (Charadrius hiaticula), and Oystercatcher (Haematopus ostralegus) steadily
decreased in all seasons after a peak in the early 1970’s. Though a temporal correlation
between an increase in Spartina and a decrease in waders was observed, no mechanical
explanations were offered to explain the correlation.
Differences in shorebird feeding ecology and invertebrate communities were
studied by Millard and Evans (1982) within Spartina anglica and on adjacent mudflats at
Lindisfarne, England in the winter of 1973-74. Redshanks, Tringa totanus, were
commonly observed feeding in Spartina, with up to 15% of the estuary’s population
feeding in the grass at all tidal levels, though it was found that they were mainly utilizing
small areas of open mud within the meadows. Dunlins were observed avoiding the grass
28
altogether. The researchers concluded that the difference in feeding ecology was probably
due to flocking behavior as well as availability of desired prey.
Many species of shorebirds were found to be increasing in numbers in Langstone
Harbour, England as the Spartina anglica meadows began to die back naturally during the
1970’s (Haynes 1982). The increase in population was attributed to the increase in mudflat
foraging areas and a beneficial change to the invertebrate community following the erosion
of the Spartina meadows.
Evans (1986) observed similar results in Redshanks and other birds in the first two
years after areas were treated with the herbicide ‘Dalapon’ in Lindisfarne, England. Again,
it was concluded that changes in the invertebrate community following Spartina removal
led to increased wader utilization. It was also suggested that desirable invertebrate species
rebounded quickly in treated areas and that residual physical characteristics of the
meadows led to greater prey availability and accessibility and, hence, greater bird use up to
two years post treatment. However, continued growth of grass beyond two years resulted
in decreasing use due to the supposed effects height (visibility) and shoot density (ability to
land in and move through Spartina) have on waders. Contrary to these findings, the results
of Goss-Custard and Moser’s study (1988) suggest that die back of cordgrass in an estuary
had no discernable effect on wader numbers even if the estuary had traditionally held larger
numbers of birds before colonization by the grass. They hypothesized that the invertebrate
community suffered from residual effects of the original impacts experienced during
Spartina colonization, thus preventing birds from using these areas.
Nairn (1986) concluded that Spartina was a possible threat to Irish shorebirds based
on a review of the English studies. No studies had been conducted in Ireland, so no
29
firsthand evidence was used to reach this conclusion. The English studies are still the most
detailed, in-depth research projects attempting to define the possible effects of Spartina on
shorebird populations.
The most complete study conducted in the United States to determine the effects of
invasive Spartina on shorebirds was conducted at the Willapa National Wildlife Refuge
from 2000-2001 (Jaques 2002). The study is not as robust as those conducted in England,
though results indicate that shorebird use of sparse to moderate density Spartina in Willapa
Bay, Washington is less than use of open mud plots on flooding tides. The results,
however, are limited; only five surveys were conducted from December 12, 2000 to March
23, 2001, none of which was during the period of greatest shorebird use of the bay -from
mid-April to mid-May. Further, it was assumed that the birds did not use more dense
stands of cordgrass, so such areas were not surveyed, and no consideration was made for
possible tidal effects.
The field research contained in this paper is the only other study, found by the
author, which attempts to examine the possible effects of Spartina alterniflora colonization
on shorebird feeding behavior in Washington State.
Spartina and Invertebrate Communities
The effects that Spartina invasion has on native invertebrate communities have
been studied in greater depth than the effects invasion has on shorebirds. Studies have
mainly focused on the invasive S. alterniflora (Lana & Guiss 1991, Atkinson 1992,
Zipperer 1996, Luiting et al. 1997) and S. anglica (Millard & Evans 1982, Jackson et al.
1985, Hedge and Kriwoken 2000). It is thought that changes in the physical nature of the
30
habitat from unvegetated to vegetated may disrupt historic, invertebrate communities.
These changes could, in turn, lead to disruptions in predator-prey relationships, energy
flow, or other physical properties of the system. Examination of differences in invertebrate
communities arising from colonization may help shed light on possible effects to birds,
fish, crabs, and shellfish.
A preliminary study to quantify differences in invertebrate communities between
plots colonized with Spartina alterniflora and adjacent mudflats was conducted in Willapa
Bay, Washington in 1987 (Atkinson 1992). The study was limited in scope, though it did
point to the conclusion that colonization had a negative effect on invertebrate life. Two
subsequent studies, also in Willapa Bay, were conducted by students at the University of
Washington (Zipperer 1996, Luiting et al. 1997). Both studies reached similar conclusions
that differed from those of Atkinson. Both researchers found that the two communities
were very similar on a coarse taxonomic scale. Density of organisms and species richness
between vegetated and unvegetated plots was dependent on seasonal variations. The
invertebrate community associated with Spartina was dominated by buried deposit feeders,
suspension feeders, and predators where as the unvegetated community was dominated by
surface feeders. Corophium spp., crustaceans, and mollusks were found in greater densities
on the open mudflats, but the polychaete Capitella capitata and dipteran larvae were more
common in the vegetated plots. The main factors affecting the communities were sediment
grain size, shoot density, and below ground biomass.
A similar study was conducted in Paranagua Bay, Brazil, investigating invertebrate
communities within native stands of Spartina alterniflora and adjacent mudflats (Lana &
Guiss 1991). The researchers found both diversity and abundance of invertebrates to be
31
significantly higher in the Spartina alterniflora plots than on the adjacent mudflats, though
seasonal differences were documented. Corophium spp. were found more frequently on
the mudflats while polychaetes were more common in the vegetation. Unlike the Zipperer
and Luiting studies, however, Lana & Guiss found that suspension feeders were more
common in the unvegetated plots. At all times the greatest invertebrate densities were
correlated with the greatest detritus availability.
Studies to determine the differences in invertebrate communities inhabiting areas
colonized by the invasive Spartina anglica and native mudflats in England and Tasmania
have reached similar conclusions. Hedge and Kriwoken (2000) found significantly higher
diversity and abundance of almost all invertebrate life in a Tasmanian Spartina marsh
during the winter. However, seasonal variation in invertebrate communities has been
documented in the USA (Zipperer 1996, Luiting et al. 1997), England (Jackson et al. 1985),
and Brazil (Lana & Guiss 1991), so this study should be viewed in the narrow context of
one season. In the UK Jackson et al. (1985) documented high densities of invertebrates,
most notably the polychaete Neries diversicolor, within a Spartina anglica salt marsh,
though Millard & Evans (1984) recorded mixed results elsewhere in the UK. The latter
found that some Spartina plots had greater diversity and/or abundance than adjacent
mudflats but some did not. Corophium spp., however, were always less abundant in the
marsh.
Shorebird Feeding Ecology
Shorebirds in the Western Hemisphere rely on a diversity of prey in their diet
(Skagen & Oman 1996). Many species of shorebirds utilize dual foraging strategies (visual
32
and tactile) to increase the prey available to them (Hayman et al. 1986, Sutherland et al.
2000). They also show great variation in feeding patterns based on variations in tides,
seasons, and moon stages (Dodd & Colwell 1998). Due to their migratory nature, they are
required to utilize different prey sources as they move south and then back north during the
course of a year. Skagen and Oman (1996) found that the most commonly cited prey for
the ten most studied shorebird species were tellinid and venerid clams, gammarid and
corophid amphipods, and nereid polychaete worms. Different species of these common
prey resources can be found at most intertidal areas that waders utilize during their
movements.
Many researchers have found that waders are opportunistic feeders that take
advantage of the most abundant or most available prey resource at a given site (GossCustard et al. 1977(1), Goss-Custard 1977, Evans 1979, Brennan et al. 1990, Botton et al.
1994, Skagen & Knopf 1994, Smith & Nol 2000, Davis & Smith 2001). Thus, the highest
density of foraging birds is expected to occur at sites with the highest densities of prey. It
has also been noted that the birds may employ selective foraging techniques for more
desired prey in addition to opportunistic foraging when multiple prey sources are abundant
(Buchanan et al. 1985, Skagen & Oman 1996). Birds will utilize different prey at different
sites within the same general region (Buchanan et al. 1985, Brennan et al. 1990) and
different prey at different habitats within the same area (Smith & Nol 2000). Overall,
however, high regional similarities in diet occur within the same species and between
coexisting shorebirds of different species (Skagen and Oman 1996). Shorebirds have been
documented moving to different sites when foraging conditions become unfavorable at
preferred sites due to changes in habitat availability (Skagen & Knopf 1994, Warnock &
33
Takekawa 1995) or to changes in weather (Warnock et al 1995) though site fidelity has
been reported as generally high both during migration (Herman & Bulger 1981) and on
wintering grounds (Brennan et al. 1985, Warnock & Takekawa 1996). Shorebirds have
also been recorded moving and/or changing prey bases when prey availability changes
(Evans 1979), and moving from high-density bird and prey sites to lesser density sites to
avoid competition (Botton et al. 1994).
There is little doubt that shorebirds are highly adaptable in their feeding ecology
both during migration and on their wintering grounds. They are able to utilize the most
abundant and/or available prey sources at any given location during most of the year.
However, in certain situations shorebirds are forced to rely on a much narrower prey base.
This often happens during the winter months when colder temperatures cause prey
availability to be low (Evans 1979) and at migration staging areas that the birds are drawn
to because of the abundance of one prey source, such as the Delaware Bay in the spring.
Goss-Custard (1977) and Goss-Custard et al. (1977(2)) noted that shorebirds could be
adversely affected by a loss of habitat on their English wintering grounds when prey
availability is low, and Tsipoura and Burger (1999) hypothesized that a decline in
horseshoe crab eggs, the main prey of spring, migrant shorebirds in the Delaware Bay,
could have similar affects on shorebirds there. Thus, if prey resources are naturally limited,
shorebirds may not be able to compensate for certain changes to their preferred foraging
areas.
Studies documenting shorebird feeding ecology in Western Washington are limited.
No studies were found from Willapa Bay. Brennan et al. (1990) described the diet of
Dunlins during the winter of 1980-81 (December- March) in Puget Sound and Grays
34
Harbor. Birds were found utilizing different prey resources at each of the four study sites.
Polychaete and annelid worms were most common at one site, cumaceans at another,
Corophium spp. at the third, and dipteran larvae at the forth. Buchanan et al. (1985)
examined weight change in Dunlins as it relates to food habits and prey availability at the
same four sites in Puget Sound and Grays Harbor and during the same winter as the
Brennan study, and found that polychaete worms, the most abundant prey resource at all
sites, were only consumed in proportion to its availability at one site. The study suggests
that prey choice may involve a quality factor, and weight retention during the winter may
be correlated with distance to available roosting sites. Both of these studies document that,
though shorebirds in western Washington may not always choose prey based on
availability, they employ a diversity of items in their diet throughout the region.
Wilson (1994) examined the impact that shorebirds have on the invertebrate
community at Grays Harbor during spring migration and found polychaete worms and
Corophium spp. to be the most commonly taken prey at his study site, implying that
shorebirds feed opportunistically on multiple prey species during spring stopovers. He
concluded that invertebrate populations were not significantly depleted by the very large
number of birds present from late-April to mid-May. Other researchers have found that
shorebirds significantly deplete prey at some sites over the course of a season (GossCustard 1977) or for a short period during and just after migration periods (Mercier &
McNeil 1994). Wilson explains the differences in results as a function of length of stay
rather than density of birds. Because stopover times of spring migratory shorebirds along
the West Coast are typically less than 4 days (Iverson et al. 1996, Warnock and Bishop
1998), the birds are not able to significantly reduce their invertebrate prey.
35
CONCLUSION
Based on the research and literature review presented in this paper, it can not be
considered a forgone conclusion that Spartina alterniflora colonization in Willapa Bay is a
serious threat to the migrant shorebirds that utilize the bay’s intertidal mudflats. It is also
not a forgone conclusion that the colonies are benign in that regard. Some studies
attempting to detect the possible effects invasive Spartina has had on shorebirds have
concluded that the grass has a negative impact on the birds. However, these studies are
generally limited in scope, and do not offer definitive proof that documented declines in
shorebirds are caused by the invading grasses. Likewise, there is little information from
Willapa Bay that would lead to this conclusion.
Documentation of shorebird use of native Spartina marshes is limited. Large
numbers of shorebirds were reported moving between beach and salt-marsh habitats in
Delaware Bay by Botton et al. (1994), though specific use of the marshes was not
determined, and Burger et al. (1997) documented extensive use of Delaware Bay saltmarshes as foraging sites for shorebirds (7 species were found to be abundant, including:
Dunlin, dowitcher, and Semipalmated Sandpiper). Tsipoura and Burger (1999) also found
that marsh habitats were used frequently by foraging shorebirds in Delaware Bay, most
notably by Semipalmated Sandpipers (Calidris pusilla). Thus, the current literature,
though limited, suggests that shorebirds may rely on salt marshes as critical feeding areas
within Spartina alterniflora’s native range. Why then would it be assumed that similar
and, often, the same species would not utilize the grass where it is an exotic?
Many shorebird species have experienced population declines in North America
and worldwide in the recent past (Howe et al. 1989, and Buchanan 2002). The causes of
36
these declines are not fully understood, but many are the result of habitat loss. However,
since populations are declining worldwide and Spartina invasion is not a worldwide
phenomenon, there can be little certainty that the grass is the primary cause of decline in
colonized estuaries. There are also problems associated with actually documenting
shorebird use of the meadows due to the fact that the grass itself tends to obscure birds
from sight. Survey methods not designed to account for visibility, tide stages and direction,
time of year, as well as other variables may fail to gather reliable information on utilization
of Spartina marshes by shorebirds. Thus, to assume that Spartina is the cause of shorebird
population declines in colonized estuaries from the few studies conducted to date would be
premature.
Shorebirds will make use of an area for foraging if an abundant food source is
available. The Spartina colonized mudflats in Willapa Bay have been demonstrated to
contain rich invertebrate communities comprised of many of the prey items that shorebirds
commonly utilize such as polychaete worms and dipteran larva (Zipperer 1996, Luiting et
al. 1997). Often these prey are more common within the Spartina than on the open
mudflats. Hence, if the birds can use the area covered by grass, they will have access to an
abundant food source.
The results of the fieldwork presented in this paper suggest that spring, migrant
shorebirds in Willapa Bay may utilize Spartina covered areas to a significant extent,
especially on the falling tide. More research is needed to refine and extend these results,
and significant effort should be directed towards discovering the nature of the use. Care
should be taken to account for variables such as season (with emphasis on spring and
winter use by shorebirds), time of day, and tide stage, magnitude, and movement.
37
One might conclude based on the data presented in this paper that spring migrant
shorebirds in Willapa Bay are able to meet their nutritional requirements. Why else would
large numbers of birds forage in the meadows on the ebbing tide but not on the flooding
tide even when open mud is available elsewhere in the bay? If the birds are not fully
satiated by the time the flooding tide has reached areas covered by Spartina, one would
assume that they would forage in the meadow as they do on the ebbing tide or move to
areas free of vegetation to feed until all possible areas are covered by water. Individual
birds will ‘fill up’ at different rates during the peak feeding time, and many observers have
noted birds moving to roosting sites before all suitable foraging areas have been made
inaccessible by the tide in areas free of vegetation (personal observations and personal
communications). In these cases one would be forced to assume that the birds that leave
are full and those that stay are not. This phenomenon may explain why the meadows are
used less frequently on the flooding tide.
The use of pesticides in attempts to control or eradicate invasive species is a risky
undertaking. Unintended impacts to non-target species are difficult to eliminate, especially
in aquatic and coastal environments where wind and water can cause greater problems with
chemical drift. In any situation where chemical application is considered as a management
tool, there should be overwhelming evidence that irreparable harm will be caused by the
unwanted organism before control is attempted. Though the Spartina colonization of
Willapa Bay is a subject of concern, there is limited research available that demonstrates
that the presence of the grass will cause a significant, negative impact to the native flora
and fauna of the estuary. There is even less evidence to suggest that shorebirds using the
bay as a migratory stopover or wintering ground have been or will be significantly
38
impacted. Without further research documenting such impacts, current management
practices with the aim of eradication may require reconsideration. Ultimately, Spartina
occupation of Willapa Bay will impact shorebirds adversely only if their food supply is
limited by the plant; the fact that shorebirds utilize patches of Spartina argues against that
as an inevitable result of the colonization.
39
Works Cited
Aberle, Barbara L. 1993. The biology and control of introduced Spartina (cordgrass)
worldwide and recommendations for its control in Washington. Master’s Thesis, Evergreen
State College. 176p.
Andrews, R.S. 1965. Modern sediments of Willapa Bay, Washington; a coastal plain survey.
University of Washington, Department of Oceanography. Seattle, Washington. Technical
Report #118.
Atkinson, James B. 1992. A preliminary investigation of benthic invertebrates associated with
intertidal mudflats and intertidal mudflats infested with Spartina at one location in Willapa
Bay, Washington. . Courtesy of Willapa NWR. Ilwaco, Washington.
Bertness, M.D. & A.M. Ellison. 1987. Determinants of pattern in a New England salt marsh
plant community. Ecological Monographs 57: 129-147.
-------. 1988. Peat accumulation and the success of marsh plants. Ecology 69: 703-713.
-------. 1991. Zonation of Spartina patens and Spartina alterniflora in a New England salt
marsh. Ecology 72: 138-148.
Botton, Mark L., Loveland, Robert E., & Jacobsen, Timothy R. 1994. Site selection by
migrating shorebirds in Delaware Bay, and its relationship to beach characteristics and
abundance of horseshoe crab (Limulus polyphemus) eggs. The Auk 111(3): 605-616.
Brennan, Leonard A., J.B. Buchanan, S.G. Herman, & T.M Johnson. 1985. Interhabitat
movements of wintering dunlins in Western Washington. The Murrelet 66: 11-16.
-------, Finger, Michael A., Buchanan, Joseph B., Schick, Charles T., & Herman, Steven G.
1990. Stomach contents of dunlins collected in Western Washington. Northwest Naturalist
71: 99-102.
Buchanan, Joseph B., L.A. Brennan, C.T. Schick, M.A. Finger, T.M. Johnson, & S.G. Herman.
1985. Dunlin weight changes in relation to food habits and available prey. Journal of Field
Ornithology 56(3): 265-272.
------- & Joseph R. Evenson. 1997. Abundance of shorebirds at Willapa Bay, Washington.
Western Birds 28: 158-168.
-------. 2002. Shorebirds: plovers, oystercatchers, avocets and stilts, sandpipers, snipes, and
phalaropes. In Volume IV: Birds. Washington Department of Fish and Wildlife. Found at
http://www.wa.gov/wdfw/hab/phs/vol4/birdrecs.htm.
40
Burger, Joanna, Niles, Larry & Clark, Kathleen. 1997. Importance of beach, mudflat and marsh
habitats to migrant shorebirds on Delaware Bay. Biological Conservation 79: 283-292.
Chapman, V.J. 1960. Salt marshes and salt deserts of the world. Leonard Hill Ltd, London.
Chung, H.C. 1989. Ecological engineering of coastlines with salt marsh plantations. In
Ecological Engineering: An Introduction to Ecotechnology, eds. W.J. Mitsch and S.E.
Jorgensen (John Wiley and Sons, Inc., New York), pp. 255-289.
Colwell, Mark A. & Dodd, Sarah L. 1995. Waterbird communities and habitat relationships in
coastal pastures of northern California. Conservation Biology 9(4): 827-834.
------- & -------. 1997. Environmental and habitat correlates of pasture use by nonbreeding
shorebirds. The Condor 99: 337-344.
Daehler, Curtis C. & Strong, Donald R. 1996. Status, prediction and prevention of introduced
cordgrass Spartina spp. invasions in Pacific Estuaries, USA. Biological Conservation 78:
51-58.
Davis, Craig A. & Smith, Loren M. 2001. Foraging strategies and niche dynamics of
coexisting shorebirds at stopover sites in the southern Great Plains. The Auk 118(2): 484495.
Davis, Peter & Moss, D. 1984. Spartina and waders—the Dyfi estuary. In: Spartina anglica in
Great Britain. ed. Pat Doody. Nature Conservancy Council, Attingham Park, pp. 37-40.
Dodd, Sarah L. & Colwell, Mark A. 1998. Environmental correlates of diurnal and nocturnal
foraging patterns of nonbreeding shorebirds. The Wilson Bulletin 110(2): 182-189.
Evans, P.R. 1979. Adaptations shown by foraging shorebirds to cyclic variations in the activity
and availability of their intertidal invertebrate prey. In Cyclic phenomena in marine plants
and animals: proceedings of the 13th European marine biology symposium, eds. E. Naylor
& R.G. Hartnoll (Pergamon Press Ltd., Oxford, England), pp. 357-366.
-------. 1986. Use of the herbicide ‘Dalapon’ for control of Spartina encroaching on intertidal
mudflats: Beneficial effects on shorebirds. Colonial Waterbirds 9(2): 171-175.
Frenkel, R.E. 1987. Introduction and spread of cordgrass (Spartina) into the Pacific Northwest.
Northwest Environmental Journal 3: 152-154.
Goss-Custard, J.D., R.E. Jones & P.E. Newbery. 1977(1). The ecology of the Wash I:
distribution and diet of wading birds (Charadrii). Journal of Applied Ecology 14: 681-700.
-------, R.A. Jenyon, R.E. Jones, P.E. Newbery & R. LE B. Williams. 1977(2). Ecology of the
Wash II: seasonal variation in the feeding condition of wading birds (Charadrii). Journal of
Applied Ecology 14: 701-719.
41
-------. 1977. Ecology of the Wash III: density-related behavior and the possible effects of a
loss of feeding grounds on wading birds (Charadrii). Journal of Applied Ecology 14: 721739.
------- & Moser, M. E. 1988. Rates of change in the numbers of Dunlin, Calidris alpina,
wintering in British estuaries in relation to the spread of Spartina anglica. Journal of
Applied Ecology 25: 95-109.
Harrington, B. & E. Perry. 1995. Important shorebird staging sites meeting western hemisphere
shorebird reserve network criteria in the United States. Unpublished Report produced for
the USFWS.
Hayman, Peter, Marchant, J. & Prater, T. 1986. Shorebirds: An identification guide. Houghton
Mifflin Company, Boston.
Haynes, F.N. 1982. Spartina in Langstone Harbour, Hampshire. In: Spartina anglica in Great
Britain. ed. Pat Doody. Nature conservancy Council, Attingham Park. pp. 5-10.
Hedge, Paul & Kriwoken, Lorne K. 2000. Evidence for effects of Spartina anglica invasion on
benthic macrofauna in Little Swanport estuary, Tasmania. Austral Ecology 25: 150-159.
Herman, Steven G. 1980. The Naturalist’s Field Journal. A manual of instruction based on a
system established by Joseph Grinnell. Buteo Books, Vermillion SD. ISBN 0-931130-13-1
200pp.
Herman, Steven G. & Bulger, John B. 1981. The distribution and abundance of shorebirds
during the 1981 spring migration at Grays Harbor, Washington. Contract # DACW67-81M-0936. U.S. Army Corps of Engineers.
Howe, Marshall A., Giessler, Paul H., & Harrington, Brian A. 1989. Population trends of North
American shorebirds based on the International Shorebird Survey. Biological Conservation
49: 185-199.
Iverson, George C., Warnock, Sarah E., Butler, Robert W., Bishop, Marry Anne & Warnock,
Nils. 1996. Spring migration of Western Sandpipers along the Pacific coast of North
America: a telemetry study. The Condor 98:10-21.
Jackson, D., Mason, C.F., & Long, S.P. 1985. Macro-invertebrate populations and production
on a salt-marsh in east England dominated by Spartina anglica. Oecologia 65: 406-411.
Jaques, Deborah. 2002. Shorebird status and effects of Spartina alterniflora at Willapa NWR.
Courtesy of Willapa NWR. Ilwaco, Washington.
42
Lana, Paulo da Cunha & Guiss, Claudia. 1991. Influence of Spartina alterniflora on structure
and temporal variability of macrobenthic associations in a tidal flat of Paranagua Bay
(southeastern Brazil). Marine Ecology Progress Series 73: 231-244.
Luiting, Victoria T., Cordell, Jeffrey R., Olson, Annette M., and Simenstad, Charles A. 1997.
Does exotic Spartina alterniflora change benthic invertebrate assemblages. Proceedings of
the Second International Spartina Conference. Olympia, Washington.
Mercier, Francine & McNeil, Raymond. 1994. Seasonal variations in intertidal density of
invertebrate prey in a tropical lagoon and effects of shorebird predation. Canadian Journal
of Zoology 72: 1755-1763.
Millard, A. V. & Evans P.R. 1984. Colonization of mudflats by Spartina anglica; some effects
on invertebrate and shore bird populations at Lindisfarne. In: Spartina anglica in Great
Britain. ed. Pat Doody. Nature conservancy Council, Attingham Park. pp. 41-48.
Mobberly, D.G. 1956. Taxonomy and distribution of the genus Spartina. Iowa State
College Journal of Science 30: 47-574.
Nairn, R.G.W. 1986. Spartina anglica in Ireland and its potential impact on wildfowl and
waders—a review. Irish Birds 3: 215-228.
Partridge, T.R. 1987. Spartina in New Zealand. New Zealand Journal of Botany 25: 567-575.
Ranwell, D.S. 1967. World resources of Spartina townsendii (sensulato) and economic use of
Spartina marshlands. Journal of Applied Ecology 4: 239-256.
Sayce, Kathleen 1988. Introduced cordgrass, Spartina alterniflora Loisel., in
salt marshes and tidelands of Willapa Bay, Washington. U.S. Fish and Wildlife Service,
Willapa Bay National Wildlife Refuge, Ilwaco, Washington. 70 p. FWSI-87058(TS).
Skagen, Susan K. & Knopf, Fritz L. 1994. Migrating shorebirds and habitat dynamics at a
prairie wetland complex. Wilson Bulletin 106(1): 91-105.
------- & Oman, Heather D. 1996. Dietary flexibility of shorebirds in the western hemisphere.
Canadian Field-Naturalist 110(3): 419-444.
Spicher, D. & M. Josselyn. 1985. Spartina (Gramineae) in Northern California: Distribution
and taxonomic notes. Madrono 332: 158-209.
Smith, Adam S. & Nol, Erica. 2000. Winter foraging behavior and prey selection of the
semipalmated plover in coastal Venezuela. The Wilson Bulletin 112(4): 467-472.
Sutherland, T. F., Shepard, P. C. F. & Elner, R. W. 2000. Predation on meiofaunal and
macrofaunal invertebrates by Western Sandpipers (Calidris mauri): evidence for dual
foraging modes. Marine Biology 137: 983-993.
43
Teal J.M. & J.W. Kanwisher. 1966. Gas transport in the marsh grass, Spartina alterniflora.
Journal of Experimental Botany 17: 355-361.
Tsipoura, Nellie & Burger, Joanna. 1999. Shorebird diet during spring migration stopover on
Delaware Bay. The Condor 101: 635-644.
Valiela, I., J.M. Teal, & W.G. Deuser. 1978. The nature and growth forms in the salt marsh
grass, Spartina alterniflora. American Naturalist 112: 461-470.
Warnock, Nils, Page, Gary W. & Stenzel, Lynne E. 1995. Non-migratory movements of
dunlins on their California wintering grounds. The Wilson Bulletin 107(1): 131-139.
------- & Bishop, Mary Anne. 1998. Spring stopover ecology of migrant Western Sandpipers.
The Condor 100(3): 456-467.
Warnock, Sarah & Takekawa, John Y. 1995. Habitat preferences of wintering shorebirds in a
temporally changing environment: western sandpipers in the San Francisco Bay estuary.
The Auk 112(4): 920-930.
------- & -------. 1996. Wintering site fidelity and movement patterns of western sandpiper
Calidris mauri in the San Francisco Bay estuary. Ibis 138: 160-167.
Washington Department of Fish and Wildlife. 2002. Shorebirds: Plovers, oystercatchers,
avocets and stilts, sandpipers, snipes, and phalaropes. Volume IV: Birds. Prepared by
Joseph B. Buchanan. Found: www.wa.gov/wdfw/hab/phs/vol4/birdrecs.htm
Washington State 54th Legislature. 1995. Revised Code of Washington, Chapter 17.26.
Washington State Department of Agriculture. 1999. Report to the legislature: Progress of the
Spartina and Purple Loosestrife eradication and control programs. Prepared by B. Reeves,
Statewide Spartina Control Coordinator, Washington Department of Agriculture.
Washington State Department of Agriculture. 2001. Report to the legislature: Progress of the
Spartina eradication and control program. Prepared by K.C. Murphy, Statewide Spartina
Control Coordinator, Washington Department of Agriculture.
Washington State Department of Agriculture. 2002. Report to the legislature: Progress of the
2002 Spartina eradication program. Prepared by K.C. Murphy, Statewide Spartina Control
Coordinator, Washington Department of Agriculture.
Washington State Department of Fish and Wildlife (Fish Program). 2001. Aquatic Nuisance
Species Management Plan. Coordinated by Pamala Meacham of the Washington State
Department of Fish and Wildlife for the Washington Aquatic Nuisance Species Committee.
44
Widrig, Ralph S. 1979. The shorebirds of Leadbetter Point: A twelve month census with notes
on other records from Willapa Bay, Washington. Independent Publication.
Wilson, Herbert W. Jr. 1994. The effects of episodic predation by migratory shorebirds in
Grays Harbor, Washington. The Journal of Experimental Marine Biology and Ecology 177:
15-25.
Zipperer, Victoria T. 1996. Ecological effects of the introduced cordgrass, Spartina
alterniflora, on the benthic community structure of Willapa Bay, Washington. Master’s
Thesis. University of Washington.
45
Appendix A
Tides, magnitudes, and survey times for each site by date.
F or R indicate whether surveys were done on the falling or rising tide.
Tide times and magnitudes are approximations from nearby tide tables.
High Tide
Magnitude
NORTH SITE (NS)
F
R
4/11/2003
4/18/2003
9:00
15:50
8.0
9.2
F
4/23/200
9:30
7.5
R
5/2/2003
15:20
7.9
Low Tide
Magnitude
16:00
0.9
9:20
-2.0
16:00
0.9
8:50
-0.7
Start
11:30
11:30
12:30
11:30
End
14:30
14:00
14:00
13:40
High Tide
Magnitude
S-CURVES (SC)
R
F
4/13/2003
4/19/2003
11:50
3:50
8.6
10.5
R
4/26/200
11:10
7.4
F
5/3/2003
3:10
9.0
Low Tide
Magnitude
6:20
2.7
11:10
-1.9
5:40
2.7
10:30
-0.8
Start
7:00
6:30
6:30
6:00
End
8:20
8:00
8:00
7:00
High Tide
Magnitude
STONY POINT (SP)
F
F
4/13/2003
4/20/2003
11:20
4:10
8.7
10.4
R
4/27/2003
11:40
7.7
R
5/4/2003
16:40
7.2
Low Tide
Magnitude
17:50
0.2
11:00
-1.7
5:30
2.0
10:20
-0.7
Start
12:00
6:45
6:40
12:30
End
15:30
8:30
8:50
14:00
46
Appendix A continued
High Tide
Magnitude
NORTH NEMAH (NN)
R
F
4/11/2003
4/18/2003
10:50
15:50
7.3
9.0
R
4/25/2003
22:50
7.7
F
5/2/2003
15:20
7.7
Low Tide
Magnitude
16:10
0.9
21:50
1.3
16:20
0.9
21:00
2.4
Start
16:00
17:20
17:00
17:00
End
19:30
20:00
19:15
19:40
High Tide
Magnitude
113TH (113)
F
4/12/2003
10:40
8.1
R
4/19/2003
17:10
8.6
F
4/26/2003
11:10
7.4
R
5/3/2003
16:30
7.6
Low Tide
Magnitude
18:00
0.5
11:10
-1.9
18:00
1.0
10:30
-0.8
Start
12:15
13:30
12:30
12:30
End
14:50
15:15
14:45
14:30
47
