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Macalester College
DigitalCommons@Macalester College
Honors Projects Biology Department
5-1-2007
Impact of agriculture and urban development onthe community structure of wetland birds in EastCentral MinnesotaChrista R. von BehrenMacalester College, [email protected]
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Recommended Citationvon Behren, Christa R., "Impact of agriculture and urban development on the community structure of wetland birds in East CentralMinnesota" (2007). Honors Projects. Paper 5.http://digitalcommons.macalester.edu/biology_honors/5
Impact of agriculture and urban development on the community
structure of wetland birds in East Central Minnesota
Christa von Behren
Advisor: Mark Davis
Macalester College
Spring 2007
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Abstract
Wetlands are one of the fastest disappearing habitats in America. Many wetlands are also being altered due to the effects of various types of land use. Because wetlands provide important habitat for many types of birds, these species can be affected by changes in wetlands due to land use. The impacts of several wetland features, including wetland size, proximity to other wetlands, and vegetation, on bird communities have been debated in the literature. While some studies have found landscape-level features, such as connectivity to other sites to be the most important factors for explaining bird diversity, others have found within-patch characteristics to be more important. It is also unclear how these variables affect rates of nest predation in wetlands. The purpose of this study was to analyze the effects of several wetland features on wetland bird assemblages and nest predation rates at several spatial scales. Bird surveys, vegetation surveys, and measurements of nest predation were conducted at the Cedar Creek Natural History Area in East Bethel, Minnesota. Landscape analyses were conducted at four different spatial scales. Results showed that wetlands are used extensively, not only by species that breed in wetlands, but by species that breed in other environments as well, particularly by woodland birds. Results also indicated that diversity in vegetation structure is associated with an increase in the number of species using wetlands. Low bird species richness in wetlands was associated with increased amounts of agriculture and urban development, which was due to the reduction in trees in agricultural and developed areas. Unlike studies of upland species, birds responded the same way to urban development as to agriculture in the landscape. Features at both the habitat level and at broader landscape scales were found to be significantly correlated with features of the bird communities, indicating the importance of implementing conservation plans at multiple spatial scales. Results suggest that for restoration and construction of wetlands, increasing the variation in both vertical and horizontal structure within the wetland and in the surrounding landscape will increase the bird diversity within the wetland. The results of this study suggest that further encroachment of development and agriculture on wetlands in East Central Minnesota will lead to a decline in wetland bird diversity, particularly with respect to woodland birds that use the wetlands for foraging purposes. The data suggest that woodland obligates will disappear first from the area, followed by sensitive wetland obligates.
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Table of Contents Page
Foreword…………………………………………………………………………………..5
Part I: A landscape approach to wetlands and wetland birds: a literature review…………………………………………………….…....6
Introduction……………………………………………………………..………....6 Wetland types and functions in the landscape…………….…….………………...8 Wetland types and functions……...…………………...………………......8 Geology and hydrologic cycles…………………………………....…..…..9 Disturbances in wetlands……………………...…………………...……..10 Wetlands in the landscape…………………………………………...…...12 Wetlands at the landscape level: Metapopulations and habitat connectivity………………………...……..14 Wetlands as bird habitat………………………………………………….………17 Habitat selection by birds……………………………………….………..19 Wetland use and selection by birds: Importance of size……………………………………….……………….21 Wetland use and selection by birds: Importance of local and landscape features……………………………...24 Nest predation in wetlands…………………………………………………….…28 Urban development and habitat fragmentation……..……....................................29 Wetland conservation ……………………………………….…………………...33 Summary…………………………………………………………………………34 Part II: Association of local and landscape features with the community structure of wetland birds in East Central Minnesota………………………………………………………………….……………..36
Introduction………………………………………………………………………36 Methods………………………………………………………………..…………40 Study sites………………………………………………………....……..40 Bird surveys……………………………………….……………………..40 Measuring predation…………………………………….……………….41 Habitat analysis…………………..………………....................................42 Landscape analysis...………………………...………………………...…42 Data analysis…………………………….……………………………….43 Results ………………………………………………………………….………..44 Foraging guilds…………………………………………………………..46 Habitat guilds…………………………………….....................................47 Nest predation………………………………………………………..…..48 Combined 2001 and 2006 results………………………………………...49 Water Chemistry…………………………………………………………49
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Discussion…………………………………………………………………….….50 Effects of urban development and agriculture…………………………...50 Habitat guilds………………………………………………….....51 Effects of paved road…………………………………………….54 Foraging guilds…………………………………………………..55 Wetland connectivity………………………………………………..…...56 Effect of wetland size and shape……………..…………...……………...58 Effect of vertical and horizontal diversity...……………………………..58 Effect of wetland cover diversity…...……………………………………59 Nest predation……………………………………………………...…….60 Water chemistry and hydrologic cycles………………………...………..61 Conservation implications………………………………...……………..62 Literature Cited…………………………………………………………………..65 Tables ………….………………………………………………………………...71 Figures……………………………………………………………………………78
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Foreword
For most of my life, I never really thought much about birds. I knew that my
grandmother loved them and they woke me up early every time I went camping with my
family. I didn’t start to appreciate birds until I got to college. All it took was one
ecology class and I was hooked. After taking Ornithology I was ready to put my bird
knowledge to work. I got the opportunity to use my new bird identification skills in the
summer of 2006 as part of an ecology internship with Professor Mark Davis at the Cedar
Creek Natural History Area. Margaret Pettygrove and I decided to conduct a research
project on wetland birds in the area. We had a great time getting wet and muddy, and
even once losing the truck to the mud in an attempt to observe wetland birds. I decided
to continue work on the project during my senior year at Macalester and incorporate data
collected in previous years at CCNHA. This thesis is the product of the work conducted
over the summer and during the school year.
I would like to thank Professor Mark Davis for the opportunity to work at Cedar
Creek this summer and conduct this research. I would also like to thank Mark for all of
the help and advice on this project during the school year. I am grateful to Margaret
Pettygrove for getting up very early with me to survey birds all summer. This project
would not have been possible without her help. I would also like to thank Jerald Dosch
for initially getting me interested in birds and for help during the writing process. I
would also like to thank everyone at Cedar Creek, especially Martha Phillips for
providing water chemistry data.
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Part I: A landscape approach to wetlands and wetland birds
Introduction
Birds have long been used as indicators of environmental change due to their
sensitivity to environmental variables. For hundreds of years canaries have been carried
into coal mines and used to indicate the presence of deadly gases. More recently,
ecologists have monitored the populations of many species of birds to indicate changes in
their habitats. Bird populations have even been monitored to indicate subtle changes in
protected and monitored landscapes (Fitzpatrick, 2004).
Bryce et al. (2002) studied birds in the Willamette Valley in western Oregon to
develop criteria for detecting environmental change and determining specific
environmental conditions from bird presence. Another goal of the study was to evaluate
different bird species for their value as indicators. The greatest species diversity was
found near streams that had experienced the least human impact. Thirty two of the sixty
two bird species studied clearly responded to the disturbance gradient in the plots where
surveys were conducted. In general, a trend of decreased species diversity with increased
disturbance was found. The authors concluded that several species are sufficiently
responsive to change in habitat conditions to be used in the creation of an index for
monitoring environmental change in the Willamette Valley (Bryce et al., 2002).
In a study of the impacts of human disturbance on bird communities in western
Sikkim, India, Chettri et al. (2005) found that there was a significant correlation between
bird species richness and tree diversity in the birds’ habitats. Different bird feeding
guilds also showed different preferences for habitat. A feeding guild is a group of bird
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species that all feed in the same or similar ways. Results suggest that birds in this area
are dependent on the compositional complexity of trees, shrubs, and herbs. Human
disturbances did not have negative effects for all birds. The authors found that the
opening of some areas created a mid-successional stage which was beneficial for some
common species. They conclude that complexity in forest structure in this region is
necessary to maintain suitable habitat conditions for birds in different feeding guilds.
They recommend the incorporation of guild monitoring as an indication of environmental
conditions (Chettri et al., 2005).
In a study conducted in Pennsylvania, O’Connell et al. (2000) also studied the use
of bird guilds in environmental monitoring. Like Bryce et al. (2002), they used bird
guilds to create an index to be used for monitoring environmental change. Birds were
assigned to response guilds. Each response guild was a group of species of birds that
responded to the change in availability of a particular resource. They found that the
response guilds were useful in classifying the amount of disturbance in a habitat.
Response guilds also reflected some physical and chemical conditions present in the
studied habitats. The authors suggested that the developed index based on response
guilds could be used in place monitoring strategies that involve more intense surveying of
bird populations. The index could be used for evaluating environmental conditions at
landscape scales. For more accurate analyses of environmental conditions in landscapes,
they recommended the use of the index in combination with other types of indicators
(O’Connell et al., 2000). Because birds are commonly used to indicate environmental
change, it is important that their habitat requirements and community dynamics are
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understood. Because many birds use wetlands for breeding and foraging, they have the
potential to be useful indicators of changes in wetland environments.
Wetland types and functions in the landscape
Wetlands are broadly defined as any habitats that are wet for some period of time,
and are often viewed as the transition zones between aquatic and terrestrial environments
(Tiner, 1999). Although scientists often disagree on precisely which features define a
wetland (Hammer, 1997), they are found mainly in areas where surface water collects or
where groundwater discharges (Tiner, 1999). Wetlands vary enormously in their
hydrology, ranging from mainly terrestrial in nature to aquatic (Weller, 1999). As a
result, the determination of a wetland boundary is somewhat arbitrary due to this
enormous variation. As a consequence of hydrological variation, wetlands vary in the
communities of plants and animals that they support (Tiner, 1999).
Wetland types and functions
Hydrologic regime and vegetation characteristics are both used to classify
wetlands. Swamps are dominated by hardwood vegetation, which can be in the form of
shrubs, hardwood trees, or coniferous trees. In contrast, marshes are dominated by
herbaceous vegetation, with little woody vegetation. Swamps and fens typically develop
in shallow basins, while marshes usually have deeper water (Wovcha et al., 1995).
Hydrophytes, plants that live in anaerobic conditions due to standing water or excess soil
water, are often used to determine the presence of a wetland in an area. However,
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because many of these plants can grow in terrestrial habitats as well, their use as wetland
indicators can be problematic (Tiner, 1991).
In the prairie pothole region of the upper Midwest, many wetlands are
depressional ponds that occur as a result of former glaciations in the region. As the
glaciers receded, blocks of ice were often buried by till. When this ice melted, the
overlying sediment collapsed, leaving holes in the landscape that became wetlands when
filled with water (Wovcha et al., 1995). These wetlands can serve many hydrological
functions, including storage of water received from surface and groundwater, as well as
atmospheric and groundwater recharge. One study (Winter and Woo, 1990) found that
prairie pothole wetlands are not part of the natural system of surface water drainage, and
there is usually no through flow of surface water through stream channels that connects
wetlands. Winter et al. (1984) explained that this lack of surface through flow is why
prairie pothole wetlands can serve water storage functions. The water regime of a
wetland, including the length of time that it holds water, determines the composition of
the plant community in the wetland (Kantrud et al., 1989; Stewart and Kantrud, 1972).
Geology and hydrologic cycles
Because open water in a wetland and vegetation affect the wetland bird
community (Weller, 1999), the geology and subsequent hydrologic conditions that exert
control over the wetlands’ ecology (Hammer, 1997) could affect bird communities. The
underlying geology of a wetland plays a crucial role in the control hydrological function.
The topographic location of the wetland in the landscape affects its interaction with the
local groundwater system. Salinity, which is an important determinant of the wetland
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vegetation composition, is controlled by the geological structure and resulting
hydrological functions. Salinity can also fluctuate throughout the year in a wetland. The
salt concentrations are much higher when the wetland is at its driest in the summer, and
decrease when it receives large amounts of water (LaBaugh et al., 1996). Since salinity
affects the composition of the wetland vegetation, it may indirectly affect the wetland
bird community as well.
Hydrologic regime is a crucial element of a wetland that can exert control over the
system’s ecology. Ultimately, the hydrology of a wetland determines how the system
will function and what organisms can live there (Hammer, 1997). The vegetation present
in a wetland is determined by the flooding regime, drainage characteristics, soil
saturation, soil type, and water table location below the surface. Wetlands are constantly
exchanging water with the atmosphere, surface water, and ground water. The extent to
which each of these water sources interacts with a wetland varies considerably among
locations and to a large extent determines the type of wetland present (Carter, 1999).
Understanding the interactions between the geology, hydrological dynamics, and plant
ecology in a wetland could help to explain the distribution of wetland birds.
Disturbances in wetlands
Disturbances play an important role in many ecosystems because they create
patterns in the vegetation and are natural sources of habitat heterogeneity (Turner et al.,
2001). Disturbance regimes can also affect the suitability of wetland habitats for birds.
Steen et al. (2006) suggest that hydrologic change in wetlands in the Great Lakes region
could have large effects on bird communities. They argue that processes that
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homogenize the environment, such as water regulation, will reduce the bird population by
reducing heterogeneity, as this feature creates habitat for many species of birds.
Reducing water fluctuations would also likely cause some microhabitats to disappear
entirely. Microhabitats, such as those created by submergent vegetation, that are
specifically associated with wetlands may disappear, and the birds that depend on them
would be left without suitable habitat at that site. Using information from a literature
review, the authors placed wetland bird species into different risk categories according to
their expected responses to water stabilization. Surveys were also conducted, and the
resulting data were used to construct models to determine the accuracy of the assigned
risk categories. The survey data suggest that the proposed risk categories accurately
reflect the response of birds to hydrologic change. Therefore, their results suggest that
hydrological change in the region could be a driver of population trends of these bird
species. The authors hypothesize that anthropogenic regulation of water levels in
wetlands could affect bird populations more than natural mortality factors do because the
birds have not adapted to anthropogenic factors (Steen et al., 2006).
The drawdown of water in managed wetlands was also found to impact diversity
of water birds by Taft et al. (2002). They found that wetlands with periodic water
drawdown supported more bird species than did flooded wetlands. Dabbling and diving
ducks stopped using wetlands at the end of the dewatering cycle. The authors measured
the water depth at which species richness was greatest. In winter, the water bird diversity
was greatest at 10 to 20 cm in depth. At this depth, habitat diversity was also greatest.
These results suggest that the richest bird assemblage coincides with the greatest habitat
diversity. They authors also found that lowering water levels 10 cm below the traditional
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level significantly increased the number of species and individuals that used a wetland.
They emphasized that the topography underlying the wetland is important because it
influences water depth. Results suggest that the availability of shallow water may be the
limiting factor for several species in winter, and the authors recommend that grassland
managers increase the amount of shallow water available (Taft et al., 2002).
Murkin et al. (1997) also discussed the importance of regular wet-dry cycles in
wetlands for maintaining bird habitat. They monitored several species of birds during
water drawdown and reflooding events. While dry cycles and subsequent reflooding may
temporarily reduce the habitat suitability for some birds the authors emphasize the
importance of these events in maintaining long-term habitat suitability. The presence of
emergent vegetation is dependent on periodic draw downs of water. When water is
permanent and the wetland starts to evolve into more of a lake than a marsh, emergent
vegetation is lost. Because emergent vegetation is crucial for some birds, their habitat is
also lost with the establishment of permanent water. Regular drawdown events will
maintain vegetation diversity and allow for high use of the wetlands by birds. The
authors also emphasize the importance of preserving a variety of wetlands within a
matrix to provide different habitat types for birds (Murkin et al., 1997).
Wetlands in the landscape
Wetlands are naturally patchy, fragmented habitats in the landscape (Hanski,
2005). A landscape is defined by Turner et al. (2005) as an area that is spatially
heterogeneous in at least one factor of interest. A landscape can span meters or
kilometers, and can include both aquatic and terrestrial systems. Landscape ecology
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emphasizes the interactions between spatial pattern and ecological processes. Recently
landscape ecology has emerged as an important field of study that has become integrated
with land management (Turner et al., 2001).
The suitability of a landscape for providing habitat for organisms is species-
dependent. Many different factors determine suitability, and a landscape cannot be
simply classified as suitable or unsuitable. There is a range of habitat suitability, and
different patches within a landscape can vary considerably in their suitability for any
species. Individuals can be found in habitat patches that are less suitable than others and
survive and reproduce in these locations (Turner et al., 2001).
Scale is an important concept in landscape ecology. The term scale refers to the
dimension of an object or process in space or time. There is no single scale that is
appropriate for the study of all ecological problems and processes. Within a landscape,
processes that occur at fine scales can be considered the mechanisms of landscape
dynamics. The broad-scale patterns in the landscape are the constraints which limit the
processes that can occur within that landscape (Turner et al., 2001). Spatial scale is
important to consider in studies of wetland birds because different species have home
ranges at different landscape scales. While some species may only use a portion of a
wetland, others can have home ranges spanning several wetlands in a landscape (Weller,
1999). For wetland communities to be fully understood, they will need to be studied at
multiple spatial scales.
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Wetlands at the landscape level: Metapopulations and habitat connectivity
In fragmented landscapes, ecological studies are often focused on metapopulation
dynamics. As wetlands are naturally fragmented and patchy systems (Hanski, 2005), a
metapopulation approach is often used when studying wetlands. A metapopulation is an
assemblage of many populations of a single species that exist at several sites (Hanski,
2005). If a population is fragmented, containing a network of subpopulations, and each
subpopulation has a small probability of extinction, local extinctions may be balanced by
recolonization from neighboring subpopulations. Thus, local extinctions and
recolonization at the subpopulation level may be dynamic, but regionally stable at the
metapopulation level (Turner et al. 2001). Metapopulations may have important
functions in human-dominated landscapes. Patches in these landscapes are usually not
large enough to maintain isolated populations of species. However, networks of patches
may be able to preserve metapopulations. Verboom et al. (2001) emphasize the need for
indices that can assess whether spatial conditions can allow metapopulations to exist
(Verboom et al., 2001). Unlike relatively stable, large populations, the persistence of a
metapopulation is due to asynchrony in the dynamics of the subpopulations. Species
persist as a result of a balance between extinction and colonization in the
metapopulations. However, some density dependence exists at the subpopulation level,
and is an important factor for determining the long-term persistence of the
metapopulation (Hanksi, 1998).
While metapopulations allow for population persistence in fragmented
landscapes, total habitat area is still important for the success of the population. The
extinction risk of local subpopulations decreases as patch area increases, since larger
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patches allow for larger subpopulations to persist. For this reason, the author
recommends that conservation efforts focus on preserving the largest amount of habitat
possible (Hanski, 1998).
The connectivity of wetlands in the landscape is an important feature for the
survival of metapopulations of wetland birds. Hanski (2005) defines habitat connectivity
in terms of migration of individuals to different patches. According to Taylor et al.
(1993) and With et al. (1997), landscape connectivity is defined by the functional
relationships among habitat patches due to their spatial distribution and the movement of
species in response to landscape structure (Taylor et al., 1993; With et al., 1997).
Migration among patches is critical for the survival of metapopulations. Different habitat
patches vary in their spatial connectivity. The more connected and easy to reach the
patches within a wetland complex are, the easier it is for birds to find food and other
resources within the complex (Haig et al., 1998).
Connectivity can be a difficult variable to measure because it is the result of the
movement of organisms, not a measurement fixed in space. While the connectivity of
two patches is clearly related to the distance between them, Hanski emphasizes that
distance to a neighboring patch is not a good measurement of connectivity. Near-by
patches will not necessarily have populations that can supply individuals to other patches,
perhaps because the patch is unsuitable for the subpopulation. The patch could also be
suitable, but happens to not be occupied at the time. The near-by patch would not be
supplying individuals to other patches and therefore are not connected through migration
(Hanski, 2005). Turner et al. (2001) suggest that the average distance to other patches
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can be used as a surrogate measure for connectivity, and is more accurate than simply
measuring the distance to the nearest patch.
Wetlands are naturally fragmented habitats compared to other habitat types. A
single wetland is generally small in size, at least compared to a typical upland habitat,
and separated from other wetlands (Hanski, 2005). However, wetland complexes,
landscapes containing many connected wetland patches, are thought to be important for
many bird populations (Haig et al., 1998). While many bird species migrate large
distances annually, some water birds also move among several wetland patches within a
single breeding season. These movements may be important for offspring survival. It
has been suggested that environmental variability among wetland patches within a
complex is important for these birds. Changes in hydrological regime can create
problems for breeding birds, but variability among patches increases the likelihood that
they will be able to find a patch with favorable conditions. Haig et al. (1998) recommend
a consideration of wetlands not as isolated patches, but rather as connected mosaics for a
better understanding of how they are used by wetland birds (Haig et al., 1998).
Some wetlands can also become seasonally connected hydrologically. Leibowitz
and Vining (2003) studied the temporal connectivity of wetlands through surface water in
the prairie pothole region. They found that several wetlands in their study area became
seasonally connected due to precipitation and local relief characteristics. The authors
suggest that this intermittent connectivity could affects metapopulations in the area.
While most metapopulation studies have focused on movement of individuals over land
or through flight, it is thought that surface water connectivity could facilitate the
movement of other species. This type of movement may be especially important for local
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seed banks. It is not known how such movement and connectivity affects bird
populations. However, temporal connectivity of surface water could alter habitat
suitability by changing the hydrochemistry of a wetland and the plants found there,
affecting a food source for many bird species (Leibowitz and Vining, 2003).
Wetlands as bird habitat
Wetlands provide habitat for many different kinds of birds and other animals. A
habitat can be defined as simply the home for populations of living organisms (Hanski,
2005). A habitat contains the conditions needed for individuals to survive and reproduce.
In conservation biology, habitat is the most important concept because it is the most basic
requirement for the survival of populations and species (Hanski, 2005).
A bird’s breeding habitat must provide all of the resources that the bird needs to
live and reproduce. These resources include food, water, nest sites, roosting places, and
cover. In every landscape birds have a variety of habitat sites to choose from. Birds are
clearly good at selecting appropriate habitat, as they are rarely found outside of suitable
areas. For some species, the habitat requirements are very specific and only one type of
habitat can be used. Other birds are generalists and can use a variety of habitat types. In
all habitats potentially suitable for birds, the presence or absence of a bird depends on
some limiting factor. There will be some requirement that is rare compared to others,
such as holes in trees for roosting, available prey, or absence of predators. These rare
requirements must be available, and a bird will not choose an otherwise suitable habitat if
it lacks this limiting factor that is needed for survival (Temple, 2004).
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Wetlands provide many important resources for birds, and many birds use them as
breeding habitat. They provide water, which is a necessity for birds for drinking and
bathing. Open water also provides feeding opportunities for some birds, in particular for
diving ducks. Deep water can provide an escape route from predators. Wetlands can
provide resting places and ideal habitat for breeding and molting. In addition, wetland
vegetation can provide locations for escape from predators (Weller, 1999).
Wetlands provide important food resources for many species of birds, including
species that don’t breed in wetlands. A wide variety of food sources, both plant and
animal can be found in wetlands. Much of this diversity in food is a product of the
different conditions produced by hydrological cycles. Both seeds and tubers are
commonly found bird foods in wetlands. Wetland vertebrates, such as fish, frogs, and
small mammals, are also food sources for some wetland birds. The huge number and
diversity in invertebrates is food for many types of birds, especially during the breeding
season (Weller, 1999).
For many birds, wetlands meet the specific requirements for breeding habitat.
Because of the heterogeneity in habitat type in many wetlands, they provide breeding
territory and nesting sites for many species of birds with very different breeding and
nesting behaviors. Ducks use open water for their courtship displays while Red-winged
Blackbird males sing from their territories in the vegetation to attract females. Nests are
placed in all parts of the wetland, from open water to the dry edges of the wetland
(Weller, 1999).
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Habitat selection by birds
Because wetlands are such variable and patchy systems, birds may have several
types to choose from in a landscape. It is not known exactly how birds select their
habitats, but there are clearly both inherited and learned components. It is thought that as
a result of natural selection, birds inherit knowledge of the general range of their habitat
requirements (Temple, 2004). By the very precise selection of habitat, however, it is
clear that some aspects of habitat selection are learned in the early stages of life. A bird
first learns of appropriate habitat characteristics when living in its parents’ territory.
Once it has selected a territory of its own, it learns the precise location of that habitat and
may return to that location year after year (Temple, 2004). While many of the
requirements for suitable habitats are understood, the process of habitat selection by birds
remains one of the central questions in avian ecology (Battin and Lawler, 2006).
Many ecologists have studied patterns of habitat selection by birds. Ambuel and
Temple (1983) studied bird habitat selection in woodlands in Wisconsin. They found that
bird community composition was correlated with patch area (Ambuel and Temple, 1983).
In contrast, Sodhi et al. (1999) found that habitat selection by American Redstarts
(Setophaga ruticilla) in aspen-dominated forest fragments in Alberta was based on
vegetative characteristics. Willow abundance was particularly important in explaining
habitat selection patterns. The birds preferentially selected sites with high willow
abundance and avoided areas dominated by the common and dominant tree and shrub
species. However, shrub cover was found to be an important characteristic of selected
territories, along with the presence of willows. The authors hypothesize that shrub cover
20
is important for reducing predation and parasitism on American Redstart nests (Sodhi et
al., 1999).
Habitat selection patterns have also been studied in wetlands. Weller (1999)
suggests that for wetland birds, the amount of open water in a wetland or wetland
complex is important for habitat selection. Murkin et al. (1997) studied habitat selection
by birds in prairie wetlands over a period of ten years. They looked for cues used by
blackbirds, waterfowl, and American Coots (Fulica americana) when selecting habitats.
The study was conducted in Manitoba in a series of experimental marshes where the
water level was controlled. They found that use of marshes by birds varied according to
hydrological cycles. The seasonal wet-dry cycles affected the availability of cover and
food, among other important resources. There are likely intercorrelations among the
factors driving the selection of particular habitats.
The authors found that the two species of blackbirds, the Yellow-headed
Blackbirds (Xanthocephalus xanthocephalus) and Red-winged Blackbirds responded
differently to changes in marshes, and suggest that this difference is due to competition
between the species. As Picman et al. (1993) also found, the Yellow-headed Blackbirds
preferred the center of the marsh in this study, likely because of reduced nest predation
rates in this area. American Coots preferred habitats with large areas of open water and a
lot of emergent vegetation. In the summer, dabbling ducks also selected habitat with an
abundance of emergent vegetation. Later in the year these ducks tended to forage in
agricultural fields. They continued to use areas with open water and vegetation, likely
because of its value of protection from predators and bad weather. In contrast to the
dabblers, diving ducks used habitats with deep water and little emergent vegetation in the
21
summer. In the fall however, diving ducks became more associated with emergent
vegetation, likely for weather protection.
The wet-dry cycles in the marshes had a large impact on bird use. Use of the
marshes was generally lower during the dry cycles than during other parts of the year.
When the wetland basin is dry, birds nesting there must have access to another water
source. The response by birds to the return of water to the basin is not the same with
each reflooding event. The characteristics of the wetland will differ based on the amount
of water that comes back into the basin. Quick flooding may inhibit the growth of
emergent plants and not attract the birds that rely on this type of vegetation. However
diving ducks that require large amounts of open water will be more attracted to the area
(Murkin et al., 1997). It is clear that the scale of a habitat patch varies for different
species of birds, and that different birds respond to different cues when selecting habitat.
Wetland use and selection by birds: Importance of size
The importance of the size of a wetland for determining the suitability habitat for
birds has been debated by scientists. Naugle et al. (1999) found that some species are
area-dependent. The authors studied the influence of scale in the use prairie wetland
habitats by birds. They hypothesized that different species of birds would show different
perceptions of landscape structure and respond to features at different landscape scales.
They found that for several bird species there was an interaction between wetland area
and internal habitat characteristics which determined suitability of habitat for that species.
The space requirement for a species could not be determined without consideration of the
habitat features within that space. For example, Black Terns (Chlidonias niger) were
22
found to require large areas consisting of several wetlands. However, area requirements
fluctuated in response to the structure of individual wetlands. The distribution of Yellow-
headed Blackbirds and Pied-billed Grebes was (Podilymbus podiceps) explained solely
by patch area and within-patch characteristics, with no influence of surrounding
landscape features. Large wetland area was important for grebes, while the blackbirds
were correlated with the amount of vegetation cover. In contrast, the surrounding upland
characteristics were important in determining Black Tern distribution. The researchers
used the amount of anthropogenic grassland in the upland matrix to estimate pesticide
and fertilizer runoff into the wetlands. Habitat suitability models for the three studied
species showed diverse perceptions of landscape structure, as the authors had predicted.
Wetland size was found to be an important feature that determines habitat suitability for
some species, but other features are important as well (Naugle et al., 1999).
Findlay and Houlahan (1997) also found that the size of a wetland is an important
feature determining habitat value for many species. In their study of southeastern Ontario
wetlands, they found that species richness of birds, mammals, reptiles, amphibians, and
plants increased with wetland area. They conclude that bigger is better from a
biodiversity perspective (Findlay and Houlahan, 1997).
In contrast, Semlitch and Brodie (1998) found that the number of wetlands in a
landscape is more important for birds than the size of an individual wetland. They argue
that small wetlands play large roles in metapopulation dynamics, and that loss of small
wetlands will reduce the biodiversity of both flora and fauna in a landscape. The loss of
any wetland will reduce the number of species in the area. The ability of species to travel
between large wetlands will be hindered by the loss of small wetlands by increasing the
23
distance between wetland sites. Stopover sites where birds can go en route to a larger
wetland will be lost. With reduced migration among wetlands, a diminishing population
in one wetland site will be less likely to be rescued by immigration of individuals from
neighboring populations. The authors recommend that regulations consider not only
wetland size but wetland density in a landscape when determining sites for conservation
(Semlitch and Brodie, 1998).
Paracuellos and Tellería (2004) found that the structural features of a wetland are
important in addition to size. They found that avian richness increased with wetland size
and suggest that this correlation is due to the tendency of habitat heterogeneity to increase
with wetland area (Paracuellos and Tellería, 2004). Riffel et al. (2006) also found
heterogeneity to be important. Forested depressional wetlands were favored by many
birds, both those species characteristic of forests and those characteristic of wetlands.
Golet et al. (2001) found habitat heterogeneity to be an important habitat feature
for birds in maple swamps in Rhode Island. They found that the maple swamps were
dominated by bird species that typically hold territories in the forest interior but also
tolerate edge habitats. Swamp size and edge development were strongly correlated with
bird species richness. Like Paracuellos and Tellería (2006), the authors suggest that
species richness increased with swamp area because habitat heterogeneity increased with
swamp area. The abundance of forest interior specialists was correlated with the amount
of upland forest. These species were negatively correlated with the presence of wetland
within the forest; however edge species were positively correlated with this feature.
There was likely an overall positive effect of the availability of wetland on the total
abundance of birds in the landscape. Swamps likely provide resources for the interior-
24
edge species that upland forests do not. There was also greater bird abundance in areas
with thicker peat deposits. This trend may be due to the correlation of other habitat
features important to birds that are related to peat depth. The authors conclude that there
may be no inherent value to large swamp size. While more birds were found in larger
swamps, no species were found to use exclusively the larger swamps but were present in
small swamps as well (Golet et al., 2001).
Wetland use and selection by birds: Importance of local and landscape features
In a study of Great Lakes coastal regions, specific habitat components were found
to important in explaining the distribution of both wetland and upland species (Riffel et
al., 2006). Upland birds were more likely to use wetlands with large amounts of
graminoid vegetation. Wetland birds were associated with greater water depths and
diverse forest canopies. There was a relationship found between Song Sparrows
(Melospiza melodia) and Downy Woodpeckers (Picoides pubescens) and wetland
features, suggesting that these species benefit from forested wetlands, even though they
are not typically considered to be wetland species. It is clear that the birds benefited from
features not found in their typical habitats. The authors suggest that wetland birds search
for good habitat sites hierarchically, by first selecting large forest tracts, and then
searching for wetlands within them. They recommend the conservation of large forest
tracts with many wetland patches (Riffel et al., 2006).
It is not clear whether landscape or local structural features are more important in
predicting habitat suitability for birds. Naugle et al. (2001) studied the roles of local and
landscape features in predicting habitat suitability for 20 wetland bird species. They
25
conducted bird surveys, vegetation analyses, and landscape analyses. Then, they
simulated the loss of small wetlands in the landscape and studied the resulting effects on
the habitat suitability of the larger wetlands. They found that most of the permanent
wetlands were larger, while most temporal or seasonal wetlands were small. The scale of
importance was found to be species-dependent. For some species, like the Virginia Rail
(Rallus limicola) and Marsh Wren (Cistothorus palustris), habitat suitability depended on
local vegetative characteristics within wetlands.
For other species, such as the Northern Pintail (Anas acuta) and Black Tern,
landscape-level features at broader spatial scales determined the suitability of the habitat.
For these species, it is important to identify the landscape characteristics that determine
habitat suitability, because a wetland with suitable vegetative and hydrological
characteristics will still not be a good habitat for these species if it is surrounded by
unfavorable landscape characteristics. For example, results showed that the Northern
Pintail and Black Tern were more likely to use a wetland surrounded by grassland than
one surrounded by tilled matrix. Preserving a large number of wetlands in a landscape is
important because of the unpredictable nature of wetland hydrology. If one wetland in
the landscape currently has unfavorable hydrologic conditions, birds are still likely to
find suitable habitat in the landscape if there are many wetlands.
Results of the modeling experiments demonstrated the importance of small
wetlands for the suitability of larger wetlands (Naugle et al., 2001). The authors found
that the species most vulnerable to the loss of small wetlands are those that exploit
resources over a broad spatial scale. For Black Terns, feeding occurs over a large area.
When wetlands are small, they become increasingly dependent on wetlands in the
26
surrounding matrix. For this species, preservation of a high density of wetlands in a
landscape is important. For the Northern Pintail, breeding occurs in small wetlands
adjacent to a larger wetland where the birds feed. A large wetland with good food
resources will not be suitable without a nearby smaller wetland. Protected wetlands are
usually large, permanent wetlands, creating a challenge for the preservation of smaller
wetlands. The authors emphasize the importance of preserving landscapes with high
densities of well connected wetlands (Naugle et al., 2001).
Whited et al. (2000) found that wetland connectedness was the most significant
predictor of species richness of birds in depressional wetlands in three ecoregions of
Minnesota. The authors defined connectedness as a contiguous polygon of relatively
unimpacted land cover surrounding the wetland site. This feature is a measure of the
remnant patch size that surrounds each wetland site. Open water was an important aspect
of connectedness in some wetland complexes, but was less important in others. In one of
the ecoregions, high levels of connectedness corresponded with complexes that had
enough landscape diversity to provide requirements for waterfowl during different life
stages. Isolated sites were usually dominated by generalist bird species, while complexes
of well connected sites supported many wetland specialists. Connectedness appeared to
be a good surrogate measure of other wetland variables. Connectedness was also found
to be a better predictor of bird assemblages in agricultural regions that wetland area was
(Whited et al., 2000).
Twedt et al. (2002) found that structural features were important for the
colonization of bottomland hardwood forests by birds. They studied several sites in
Mississippi and Louisiana between two and ten years after replanting. Birds did not
27
colonize stands before the tenth year after trees had been planted. Maximum tree height
and tree cover, which were measurements of vertical structure, were closely linked to the
colonization of the sites by birds. The results indicate that reforestation with fast-
growing, early successional tree species is most beneficial for birds. The authors
recommend replanting in managed areas with a mixture of predominantly fast-growing
species (Twedt et al., 2002).
The relevant scale and features of the landscape, vegetation, and hydrology that
are important for birds depend on the life history traits of each species. Craig and Beal
(1992) studied marsh bird communities in Connecticut. They found that the feature of
importance for a bird species depended on what that species was using the wetland for.
Birds breeding in a wetland corresponded with different characteristics than birds that
foraged in the wetland but did not breed there did. Area was the most important factor
predicting the species richness of wetland breeders. For nonbreeders, the habitat mosaic
was more important than wetland area. There was an association between vegetation and
habitat heterogeneity and nonbreeder species richness. Most of these species used
mudflats for foraging and the marsh surface for resting and feeding. Wetland
connectivity was also important for these species. There was a strong correlation
between nonbreeder species richness and proximity to a neighboring marsh. For birds
searching for ephemeral prey sources, traveling between marshes is likely a more
effective foraging strategy than remaining at one wetland site is (Craig and Beal, 1992).
28
Nest predation in wetlands
Nest predation creates challenges for all breeding birds. One of the benefits of
nesting in marshes may be reduced nest predation rates. Picman et al. (1993) studied
predation on passerine nests in marshes. They compared nest predation rates in marshes
to rates in upland habitats. Overall, predation rates were lower in marshes than in
uplands. However, placement of nests within the marsh had a large effect on predation
rates. Rates were high in the shallow areas of the marsh, lower in areas of medium water
depth, and higher again in deep areas. Marsh Wrens were found to be responsible for
much of the nest predation in deep water areas where they nest. The predation rates on
experimental nests in deep areas were related to the density of Marsh Wrens. Predation
in deep areas was still lower than in shallow areas, however, due to the exclusion of many
upland predators, such as the raccoon (Stewart, 1999). There is a very diverse predator
community in upland areas, while in deep water areas Marsh Wrens are the only nest
predators. The authors also suggest that the lower predation rates in deeper water could
be due to efficient nest defense by Red-winged Blackbirds which nest in this area. While
water depth was important for nest predation rates, distance from the marsh edge was not
found to be important. They conclude that different nest predation patterns in marshes
and in uplands have led to the evolution of different reproductive strategies in these areas.
The high rate of polygyny among marsh birds may be due to safer nesting conditions,
giving males the opportunity to monopolize. They also hypothesize that predation by
Marsh Wrens has led to colonial nesting behavior by blackbirds in deep water areas
(Picman et al., 1993).
29
Nest predation can have a large effect on population dynamics. In their study of
birds in bottomland hardwood forests after reforestation, Twedt et al. (2002) found high
rates of nest predation in some areas. Predation was the primary cause of low nesting
success in these areas, and it was proposed that these areas are population sinks.
Reproductive success does not compensate for mortality in these areas, but there are high
rates of colonization by individuals from other patches (Twedt et al., 2002). Rodewald
and Yahner (2001) also found nest predation to have major effects on avian community
structure. They found greater rates of nest predation near agricultural disturbances
(Rodewald and Yahner, 2001).
Urban development and habitat fragmentation
Understanding the ecological effects of land-use changes remains an important
challenge to ecologists (Turner et al., 2001). Fragmentation of habitat is known to be a
major threat to bird populations in all habitats. Habitat fragmentation involves both loss
of habitat area and loss of connectivity. Habitats become increasingly fragmented as
patch size decreases and as isolation of the habitat patches increases. With increasing
fragmentation, connectivity among patches decreases. With this loss of connectivity,
birds can lose access to habitats. Migration among patches is greatly reduced, which can
eliminate the rescue effect and have consequences for populations (Hanski, 2005).
Sallabanks et al. (2000) studied the effects of fragmentation on bird populations in
bottomland forests in North Carolina. They found that proximity to edges usually had
positive effects on the abundance of species. Certain species appeared to prefer edges.
Surprisingly, results also showed that patch size was not proportional to bird density and
30
species richness. Species characteristic of one habitat were found in another habitat at
the edges, likely because at the edge there is greater proximity to the preferred habitat.
They hypothesize that the effects of fragmentation found in their study were dependent
on the nature of the edges of the fragments. For some species, the effects of patch size
and edge may be uniform across all landscapes, while for others the effects may be
landscape-specific (Sallabanks et al., 2000).
The effects of edges created by fragmentation were also studied by Dowd (1992)
in an assessment of the effects of development on bird species composition in forested
wetlands in Staten Island, New York. The immediate effect of development at the study
site was the fragmentation of a stream corridor by paved roads. Forest habitats are also
removed, creating edges. While the creation of edges results in poor breeding habitat for
many species, it is attractive to species that are more tolerant of disturbances. Two plots
surveyed in this study had the same area but different shapes. The plot that was long and
narrow, with more edge and a greater ratio of edge species to forest interior species. In
addition, more than half of the species in this plot were species attracted by urban
landscapes. In contrast, at the plot with a smaller amount of edge, less than one third of
the bird community was made up of species attracted to urban landscapes. Results of this
study indicated that large habitat patches in urban landscapes can provide nesting areas
for some species, but encroachment of development leads to dominance in the bird
community by species attracted to urban development (Dowd, 1992). DeLuca et al.
(2004) suggested that there is a disturbance threshold for birds. Beyond this amount of
disturbance bird communities will be negatively affected (DeLuca et al., 2004).
31
Road construction can be a cause of habitat fragmentation in many different
landscapes, including in wetland complexes. Findlay and Bourdages (2000) studied the
effects of road construction on biodiversity in wetlands in southeastern Ontario. They
found that for birds, reptiles, amphibians, and plants there were significant negative
effects on biodiversity that appeared only several years after the roads had been
constructed. For birds, they estimated that species loss can be detected within eight years
of construction. The authors suggested that this time lag after occurs because the
population decline in response to road construction occurs slowly. Researchers are not
able to notice a significant change in populations for several years. Because of the time
lag between road construction and the appearance of effects on populations it is important
to determine an appropriate temporal and spatial scale for study when conducting impact
assessments. The authors emphasized the importance of selecting appropriate indicators
for a given type of disturbance. The results of this study suggest that overall species
richness is not a very sensitive indicator of the effects of road construction. The use of
this indicator will likely underestimate the effects of road construction, at least in the
short term. The authors recommend the use of buffer zones between constructed roads
and wetland patches to mitigate the effects of the road (Findlay and Bourdages, 2000).
DeLuca et al. (2004) suggest that roads can create habitat for generalist species, which
can then outcompete wetland specialists.
Development in the surrounding landscape may also affect bird populations in
wetlands. Findlay and Houlahan (1997) found that forest density and paved road density
within two kilometers both affect the species richness of four taxa in wetlands. Both of
these factors had the largest effects 1000 to 2000 meters from the edge of a wetland.
32
With increased road density, bird, plant, reptile, and amphibian species richness
decreased. With increased forest cover, the species richness of reptiles, amphibians and
mammals increased, but bird species richness was not significantly affected (Findlay and
Houlahan, 1997).
Agriculture can also have large effects on wetlands and on wetland birds. The
runoff of chemicals and sediments into wetlands from agricultural fields is one of the
largest causes of wetland degradation (Stewart, 1999). Gleason et al. (2003) found that
sediment loading in wetlands can affect populations of plants and insects in the wetland.
Seeds and plants were buried by sediment, and the emergence of plant seedlings and
invertebrates was halted. Invertebrates were found to be more sensitive to burial by
sediment than were seeds. The sediment may disrupt cues for the invertebrates to emerge
from diapause, in addition to creating a physical barrier to emergence. A large sediment
load in a flooded wetland may also inhibit productivity in the wetland by reducing the
amount of light that penetrates to the aquatic plants for photosynthesis. Results of this
study show that agricultural activities close to wetlands could limit the reestablishment of
plants and invertebrates. More research is needed to determine an acceptable amount of
sediment loading for the emergence of these organisms, and to determine the relationship
between specific agricultural practices and sedimentation (Gleason et al., 2003). It is
unclear how sedimentation from agriculture affects wetland bird communities, but a
significant reduction in plant and invertebrate production in a wetland could affect the
food available for birds.
In wetland complexes, wetlands that are not adjacent to farmland can still be
affected by agricultural inputs. Whigham and Jordan (2003) emphasize that wetlands in
33
the prairie pothole region are rarely hydrologically isolated. They are connected through
groundwater or intermittent surface water, and each wetland therefore has the potential to
affect other systems (Whigham and Jordan, 2003). Nutrient deposition due to
agricultural practices can create special problems in this linked system. Wetlands
enriched with nitrogen and phosphorus have a high potential to discharge the nutrients to
downstream systems (Winter et al., 2001). It is not clear how the addition of nutrients
into wetland systems affect bird communities.
There are very complex relationships that exist between terrestrial and aquatic
systems. Studies of toposequences have shown that ecosystem adjacencies play
important roles in the transfer of nutrients (Turner et al., 2001). Adjacent cities and
agricultural areas can contribute to nonpoint source pollution that enters a wetland or
wetland complex. Nitrogen and phosphorus are two of the most important pollutants that
move from land to watersheds (Turner et al., 2001).
Wetland Conservation
In the past 200 years, half of the wetlands in the United States have disappeared
(Dahl, 1990). The Clean Water Act of 1972 made a major attempt to combat the rapid
loss of wetlands. The benefits of wetlands and the harmful effects of draining them have
only recently been realized. Restoration and creation of wetlands are now being used to
restore these benefits. The goals of wetland restoration and creation are to maintain and
restore certain ecological functions while accommodating human development. Usually
wetlands are created for the purpose of storing and filtering water, especially in and near
urban areas. While wetland restoration and creation can be very beneficial for many
34
different populations, there are many challenges that arise. Restoring one function, such
as water storage, could eliminate another ecological function of the wetland. Restoration
could also destroy habitat for some species, even though species diversity may be
enhanced and more habitat may be created. Site selection for wetland creation can also
be a challenge. The geological setting will determine what kind of wetland can be
created in an area and will determine the hydrological function of the wetland. It is also
important to establish plant communities that will enhance hydrological functions of the
wetland, stabilize the soil, and provide resources for animal communities (Kentula,
1999).
Recently, the Army Corps of Engineers has been charged with measuring the
value of wetlands that are not immediately adjacent to navigable rivers. The corps has
been asked to generate criteria by which to determine whether wetlands provide
significant enough benefits to water quality to prevent their development. Due to the
incredible diversity, the development of one set of criteria seems an unrealistic task, and
many fear that this will lead to a reduction in federal protection of wetlands. Scientists
say that birds and other indicators can help determine whether a wetland is positively
affecting water quality, but now they are being asked to determine whether these benefits
are important enough to merit protection (Stockstad, 2006).
Summary
The dynamics of wetlands and wetland bird communities are complex and
variable. Geological and hydrological characteristics create enormous differences in
wetland vegetation and water features. Different bird species respond to different
35
environmental variables at different spatial scales. It is still unclear how land-use
changes interact with habitat-level variables to affect wetland bird communities. The
dependence of different species on features at different spatial scales creates special
challenges for conservation. More research is needed to determine ways to restore and
construct wetlands to meet the needs of many different types of birds while also serving
hydrological functions. If birds are to be used as indicators of environmental change and
to evaluate the progress of conservation projects, their habitat requirements and responses
to land use patterns need to be better understood. To help provide this information, I
conducted a study near the Minneapolis-Saint Paul Twin Cities metropolitan area in East
Bethel, Minnesota to evaluate the association of local and landscape-level features of
wetlands with bird distribution in wetlands surrounded by a range of landscape types,
including other wetlands, wooded areas, agricultural fields, and urban development.
36
Part II: Impact of agriculture and urban development on the
community structure of wetland birds in East Central
Minnesota.
Introduction
Wetlands of the prairie pothole region provide important breeding and foraging
habitat for many species of birds. However, wetlands are also quickly disappearing
ecosystems in North America. They are second only to grasslands in percentage of total
area lost since European settlement (Weller, 1994). In the past several decades, there
have been efforts to restore altered wetlands and to construct them in locations where
they previously did not exist (Kentula, 1999). Wetlands are also often constructed for
hydrological purposes in, but usually fail to support a diverse bird community (Athanas,
1988). More research on the important wetland habitat characteristics for birds is needed
for wetland restoration and construction efforts to preserve habitat for diverse bird
communities as well as for hydrological function.
Several wetland characteristics have been found to affect the composition of
wetland bird communities. Many studies have shown that landscape characteristics
surrounding the wetlands, in addition to within-site characteristics are important
determinants of the community composition of many wetland birds. Some researchers
have found wetland complexes to provide habitat for many more bird species than
isolated wetlands do (Fairbairn 2001, Haig, et al., 1998, Naugle et al., 2000). However,
the important features and scales of analysis wetland complexes for birds are not
37
understood. Haig et al. (1998) described landscape connectivity as a functional
relationship among habitat patches caused by their spatial distribution and the movement
of organisms in response to landscape structure (Haig et al., 1998). Understanding the
characteristics of functional complexes of wetlands used by birds is important for the
conservation of wetland habitat. It is also necessary to understand the features at the
habitat level that are related to bird community composition.
The relative importance of wetland vegetation, size, and shape for bird
populations is not clear. Riffel et al. (2001) found most studied bird species to be
significantly correlated with increasing wetland area, and recommends emphasis on large
wetlands for bird conservation programs. In contrast, Sallabanks et al. (2000) found
wetland size to be less important in bird abundance than other characteristics.
Paracuellos and Tellería (2004) found wetland size to be an important determinant
of the number of bird species found in a wetland, and the degrees of wetland patchiness
and isolation were important as well. The authors suggest that bird species richness is
greater in large wetlands because habitat heterogeneity usually increases with wetland
area. They recommend focusing conservation efforts on maintaining a large number of
wetlands, paying special attention to large and clumped ponds (Paracuellos and Tellería,
2004). Haig et al. (1998) also found environmental variation to be important for bird
diversity, but in contrast to Paracuellos and Tellería, the authors focused on variation
within a complex of wetlands. Variability within a complex allows birds to easily find
new habitat when the conditions in their current patches change (Haig et al., 1998).
Naugle et al., (2001) found small wetlands within a complex to be crucial components of
bird habitat, greatly influencing the habitat suitability of nearby large wetlands. The
38
study also found that some species, such as the Virginia Rail, are hardly affected by
landscape characteristics at all, but instead habitat suitability is determined by vegetation
characteristics within a wetland.
Urbanization is responsible for fragmentation of many wetland complexes. An
investigation of landscape variables around wetlands found urban development to affect
the wetland bird community (DeLuca, et al., 2004). They found disturbance thresholds at
which the birds responded to. No change in the bird community was seen until this
threshold was reached. Beyond the threshold there were drastic changes in the bird
community. These effects of development were scale-dependent; the threshold response
was only observed when disturbance occurred at the habitat scale. At the watershed scale
the threshold response decayed. Roads were also found to have an effect on the bird
community. The researchers suggest that wetland assessments should include an analysis
of landscape-level characteristics, in addition to the standard within-site characteristics
(DeLuca et al., 2004).
Nest predation creates challenges for all breeding birds, including those in
wetlands. Comparatively lower rates of nest predation in wetlands compared to uplands
may be one of the benefits of nesting there (Picman et al., 1993). A previous study found
water depth to be an important determinant of nest predation rates in wetlands (Stewart,
1999). It is unclear how nest predation rates in wetlands are affected by environmental
features at the habitat and landscape level.
In East Central Minnesota, wetlands are the remnants of the Pleistocene
glaciations. There are many different kinds of wetlands that are classified by their
hydrology, underlying geology, and vegetation characteristics. Swamps, which are
39
dominated by woody vegetation and organic, poorly drained soils, are common in this
area. Marshes are also common. They consist of emergent, non-woody vegetation and
usually occur on sandy soils. There are also bogs with floating mats composed of
partially decomposed organic matter called peat. Some wetlands contain open water all
year, while others are only seasonally flooded (Wovcha et al., 1995).
The purpose of this study was to document the association of different habitat and
landscape features with bird assemblages and nest predation rates in wetlands in and
around the Cedar Creek Natural History Area in East Bethel, Minnesota. The main goals
of the study were to determine the effects of the scale of analysis on the correlations
between bird communities and environmental features, and to determine the effects of
urbanization and agriculture on wetland bird communities. It was hypothesized that the
lowest species richness would be seen in wetlands surrounded by the most urbanization
and agriculture, due to the greater habitat fragmentation and lower habitat connectivity
likely to occur in these areas. Wetlands in more remote areas were expected to show
higher richness and diversity due to greater habitat connectivity. It was also expected
that wetland size and shape, as well as proximity to other wetlands would influence the
composition of wetland bird communities. Nest predation rates were expected to be
higher in sights with little diversity in the vertical structure of the vegetation, due to the
lack of well-concealed locations for nests.
40
Methods
Study Sites
This study was carried out during the summer of 2006 at the Cedar Creek Natural
History Area (CCNHA), a National Science Foundation Long Term Ecological Research
(LTER) site in East Bethel, Minnesota (Figure 1). Twelve wetland sites were selected;
eight on Cedar Creek property, and four in the immediate surrounding area of East
Bethel. An attempt was made to select for heterogeneity among sites in terms of
vegetation, hydrology, and level of development. At each site, a 150 by 75m census plot
was established, with one of the plot edges adjacent to the road or path at the wetland
edge. This plot served as the boundary for the vegetation analysis and the space within
which birds were recorded during surveys.
Bird Surveys
A point-count method was used to survey birds. Surveys were conducted at each
site once a week, between seven and nine a.m. for four weeks between June 29 and July
25, for a total of four surveys at each site. At each site surveys were conducted from two
fixed points along one edge of the 150 by 75m box. Observers stood at each point for ten
minutes and recorded all the birds seen or heard within the site boundaries during that
time. Birds flying over or through the site, clearly without using the space, were not
recorded.
41
Measuring nest predation
Artificial nests made of tennis ball halves with Spanish moss glued to the outside
were used to measure nest predation rates in the eight wetland sites on Cedar Creek
property. Five nests were placed in each site, 1 to 2 meters above the ground in trees or
shrubs. The nests were attached to the vegetation with wire. A quail egg and a clay egg
were placed in each nest (Figure 2). The clay eggs were made of gray clay with black
spots to mimic the quail eggs. The clay and quail eggs were both approximately the size
of warbler eggs. The clay eggs were used in addition to the quail eggs so that different
types of predation marks could be seen. Double incisor marks in clay eggs were evidence
of predation by a mammal, while pierced holes in quail or clay eggs were evidence of
predation by a bird. These marks on the clay egg allowed evidence of predation to be
seen and recorded even if both eggs were still in the nest.
Eggs were placed in the nests and checked every day for three days. They were
checked again after one week, and any remaining eggs were removed. For each nest
checked, the presence or absence of each egg was recorded. For eggs remaining in the
nests, any signs of predation, such as piercing, claw marks, or teeth marks were also
recorded. Predation was measured twice in each site with two sets of eggs.
Predation rates were calculated using the method described by Mayfield (1975).
The probability of egg mortality per day is equal to the number of eggs lost during the
observation period divided by the sum of the number of days that each egg was exposed,
or, egg days.
42
Habitat Analysis
The vegetation within each site was characterized along two 50 meter transects in
each plot. The two transects originated at the two corners of the 150 by 75 meter plot
that were closest to the road or path. Each transect was angled toward the center of the
site at 45 degrees. The length of the transects covered by vegetation of four different
height categories (<1m, 1-2m, 2-5, and >5m) was recorded. The type of vegetation was
also classified as cattail, other graminoid, other herbaceous, moss, trees, or shrubs. Bare
ground, open water, and floating vegetation were also recorded. Each of these substrate
types provides foraging habitat for different types of birds. The percentage of the
transect covered by each vegetation type was calculated, and this number was used as an
estimate of the amount of the wetland site covered by that vegetation type. Because trees
were scattered on the edges of the wetlands, they were not well represented in the
transects. To account for this, the area covered by trees was visually estimated for each
wetland and the percent coverage was calculated from this estimate.
Landscape Analysis
Landscape analyses were conducted at four different scales using Cedar Creek
GIS, Google Earth, and Adobe Illustrator. The total area surveyed in the landscape
analysis was approximately 42 square kilometers. The landscape surrounding each site
was analyzed within 100, 250, 500, and 1000 meters of each site. For the eight sites on
Cedar Creek property, the analysis was conducted in Cedar Creek GIS. For each scale,
the analysis was completed within a circle with that radius around the site. The lengths
of paved and unpaved road within the circle were recorded. The area in square meters
43
and the percentage of the circle covered by wetland, permanent lake, grassland,
woodland, farmland, and development were also recorded. The number of wetland
patches within the circle was counted and the average distance between the study site and
all other wetland patches in the circle was calculated.
Water chemistry data collected from wetlands at CCNHA were also mapped and
compared to patterns of bird distribution. Data on water chemistry were provided by
Martha Phillips, an investigator at CCNHA.
Data Analysis
The birds observed were analyzed by species, foraging guilds, and habitat guilds.
Birds were divided into wetland obligates which nest in wetlands, woodland birds, and
other non-wetland obligate and non-woodland birds. Birds were also divided into seven
foraging guilds; ground and litter foragers, ground probers, foliage gleaners, dabblers and
grazers, excavators, aerial insectivores, and fish eaters, according to the guild
descriptions given by Alcock (2004). Life history information on the Cornell Lab of
Ornithology website’s bird guide (2003) was used to separate the species into guilds.
Species that were only observed once were excluded from the analysis. The purpose of
using guilds was to increase the applicability of the results to other studies. Studies
conducted in other locations will have different species of birds, but will likely have
species that fall into the same guilds as in this study.
Data were analyzed using reciprocal averaging, an indirect gradient analysis
method. Reciprocal averaging is a graphical ordination technique that displays rows and
columns of data as points in a vector space. The distance between the points indicates the
44
strength of the association between them (Davis et al., 2000). Reciprocal averaging
analyses were conducted in PC-ORD. Site coordinates produced from the gradient
analysis were correlated with the habitat and landscape measurements for each site in
Microsoft Excel to determine which environmental variables were associated with the
axes of the indirect gradient analysis at the significance level p < 0.1. The variables
found to be significantly correlated with the axes were used in a canonical
correspondence analysis (CCA) with the significantly correlated bird species.
Data were also analyzed using indirect gradient analysis and canonical
correspondence analysis with wetland bird data collected in a previous study at CCNHA
in 2001 by Aletia Van Brocklin and Louise Bier (Van Brocklin, 2002). Three of the
environmental variables, percent herbaceous vegetation, percent woody vegetation, and
percent open water, were measured in both 2001 and in 2006. In addition, diversity of
these three cover types was calculated in 2001 using the Shannon diversity index. Cover
type diversity was also measured for the 2006 data. These analyses were done to
determine if wetland birds responded to wetland cover diversity in the two years. Water
chemistry data were compared to the ordination plots of the indirect gradient analysis of
species.
Results
Twenty-one of the sixty-six environmental variables measured were found to be
significantly correlated with the axes in the indirect gradient analysis of species (six at the
0.1 level, eight at the 0.05 level, and seven at the 0.01 level). Significant (p < 0.1) results
of the correlations between environmental variables and the X and Y values assigned to
45
the sites by the indirect gradient analysis of birds are shown in Tables 1 and 2. Table 1
shows the environmental variables and bird variable at the habitat level that were
significantly correlated with the axes. The significance levels of the correlations are also
shown.
A total of forty-four species of birds were observed in the twelve sites. Forty of
these species were observed more than one time throughout the survey period. The total
number of birds observed and identified in each site ranged from 38 to 151. The number
of bird species observed in each site ranged from twelve to twenty-five. The four species
with a total of only one individual observed in all surveys were not included in the
analyses. Of the forty species analyzed with indirect gradient analysis, 14 were found to
be significantly correlated with the X or Y axes. Thirty-six species were not significantly
correlated with the axes. The fourteen significantly correlated species and their spatial
relationships to each other and to the axes are shown in Figures 4 and 5. Figure 4 shows
the distribution of species with the X-axis, and Figure 5 shows the distribution of the
species with the Y-axis. The closer two points are to each other, the more similar those
two species are in terms of the sites where they were observed. Similarly, the closer
together the sites are on the plot, the more similar they were in terms of bird composition.
All of the species classified as woodland obligates (green) were located in the top left
quadrant. The two wetland obligates (purple) are in the top two quadrants.
Figure 6 shows the X-axis of the indirect gradient of species with an arrow
indicating the direction of urban development and agriculture. The figure shows that the
Killdeer (Charadrius vociferus), Mallard (Anas platyrhynchos), and Tree Swallow
(Tachycineta bicolor) were positively associated with development and agriculture, while
46
the majority of the species shown are negatively correlated with development and
agriculture. High levels of development and agriculture in the landscape are associated
with a high number of bird individuals at a wetland site, while low levels are associated
with high bird species richness.
Figure 7 shows the ordination plot of the indirect gradient analysis of species.
The figure shows the locations of the study sites in the vector space, in addition to the
bird species. The clustering of species indicates their occurrence in similar habitat
conditions during the surveys. Similarly, the closer together the sites are, the more
similar they are in terms of the bird compositions observed in them.
Foraging Guilds
Table 3 shows the species that comprised the seven foraging guilds. The ground
and litter forager and foliage gleaner guilds were the most represented foraging guilds,
with 10 species from each guild observed. The species in each guild are shown in Table
3. Table 4 shows the correlations of the environmental variables with the X and Y axes in
the indirect gradient analysis of foraging guilds. The table shows that 15 of the
environmental variables were significant at the p < 0.05 or p < 0.01 levels.
Figure 8 shows the first axis of the indirect gradient analysis of foraging guilds.
Three of the guilds, foliage gleaners, ground and litter foragers, and dabblers were found
to be significantly correlated with the axes in this indirect gradient analysis. The dabblers
were positively associated with the axis, while foliage gleaners were negatively
associated. Environmental variables associated with short vegetation and agriculture and
47
urban development in the landscape were positively associated with the X-axis. Ground
and litter foragers were negatively correlated with the Y-axis.
Figure 8 also shows that, like in the ordination of species, the variables associated
with wetland connectivity within 250 meters were positively associated with the first axis
in the ordination of foraging guilds. The amount of surrounding farmland and paved road
were also positively correlated with the axis, both within 100 meters and within 250
meters.
Habitat Guilds
Of the 40 species analyzed, nine were classified as woodland obligates, and seven
as wetland obligates. One species, the Veery (Catharsus fuscescens), was classified as
both a woodland and wetland obligate based on information provided by Cornell Lab of
Ornithology’s online bird guide (Bird Guide, 2003). Table 5 shows the species classified
as wetland and woodland obligates. Table 6 shows the environmental variables that were
significantly correlated with the axes in the indirect gradient analysis of wetland and
woodland obligates. Figures 9 and 10 show the biplots of the canonical correspondence
analyses of the woodland and wetland obligate guilds. The circles on the graphs indicate
the general range of each guild in the vector space. In this ordination, wetland obligates
were negatively correlated with the X and Y axes at the p < 0.05 level. Woodland
obligates were positively correlated with both axes, at a significance level of p < 0.01.
Figure 9 shows that at the habitat level, the two guilds were associated with opposite
vegetation features. Wetland obligates were associated with grasses and vegetation less
48
than one meter in height, while woodland obligates were associated with taller
vegetation.
Figure 10 shows that woodland obligates shown in green were negatively
associated with the X-axis, and most were clustered in the top left quadrant. The
majority of the wetland obligates were also negatively correlated with the X-axis as well,
although some were positively correlated. These species were not grouped as tightly
together as the woodland obligates were. Development, farmland, and number of
wetlands were positively associated with the X-axis, as were wetland obligates.
Nest Predation
Rate of nest predation was not found to be significantly correlated with either of
the axes (p > 0.1), in the indirect gradient analysis. There was also no statistically
significant relationship between nest predation rates and environmental variables (p >
0.1). There was little difference in predation rate between the clay eggs and quail eggs.
However, nest predation rate varied significantly among sites. Table 7 shows the
probability of predation per day on each nest at each site. The results from Trial 1 and
Trial 2 are shown.
Results from the ANOVA test showed that there were variations in the average
rates of nest predation among the sites, at the p < 0.001 level. The predation rates of sites
3, 7, and 8 were significantly higher than the average rates of the other sites. Sites 3 and
8 had higher average rates than site 7 did. Sites 10 and 12 had significantly lower
predation rates than the other sites.
49
Combined 2001 and 2006 results
The indirect gradient analysis of the combined data set showed that 11 birds were
significantly correlated with the axes. These birds and their distributions are shown in
Figure 11. Figure 11 shows the biplot first axis of the ordination plot of the indirect
gradient analysis of species observed in 2001 and in 2006 with the cover diversity
gradient. The graph shows that higher cover diversity was negatively associated with the
X-axis. Of the species significantly associated with the axis, the majority were positively
associated with high cover diversity. Only the Red-winged Blackbird and Black Tern
were associated with low cover diversity.
Water Chemistry
Figure 12 shows a map of CCNHA with the water collection sites and measured
pH and the bird survey sites marked. Only three of the sampled for water chemistry data
were also wetland bird sites. South of Fawn Lake Drive, there was a trend found in the
water pH of the sites measured. Near site 1, pH was 7.085, close to neutral, and had
declined to 4.895 near site 12. In this region of the study area, water becomes more
acidic from north to south. There was no similar trend observed north of Fawn Lake
Drive. For the three bird observation sites where water samples were collected from,
there was a trend between the distribution of sites based on bird species data and water
pH. This trend is shown in Figure 13. The axis shown is the X-axis of the indirect
gradient analysis of species. There is a trend toward higher pH with higher levels of
urban development and agriculture in the landscape.
50
Discussion
Results show that considerable avian use of wetlands is by birds that breed in
other habitats. As Table 4 shows, ten woodland obligates were observed in the wetland
sites. It is not surprising that the presence of these species was associated with the
vertical structural diversity within a site and with the presence of trees in the surrounding
landscape. Twedt et al. (2002) also found that birds that breed in woodlands used
wetlands within a woodland matrix. The presence of woodland birds clearly contributes
to the high species richness in forested wetlands.
Effects of agriculture and urban development
Figures 7 and 8 show that of the fourteen birds significantly correlated with the
axes in the indirect gradient analysis, ten were negatively associated with farmland and
development.
Also in these figures, the association of the Tree Swallow, Mallard, and Killdeer
with farmland, development, and paved road suggests that these species are not
negatively affected by these types of land use in the proximity of their habitats, and
perhaps are even positively affected by them. Mallards were strongly associated with the
X-axis, as were these environmental variables, suggesting that they are able to breed with
the amount of farmland and development currently in and around CCNHA. The thirty-
six species that were not associated with the axes in the indirect gradient analysis are also
not likely affected by the amount of farmland and urban development in the area. These
species were also not associated with within-habitat characteristics, suggesting that they
51
are more generalist species than the fourteen that were significantly correlated with the
axes.
The number of wetlands within 250 meters and the total wetland area within 1000
meters were also positively correlated with this axis. Although it is counterintuitive that
there was more wetland area in developed areas than in remote places, it is possible that
the amount of urban development near CCNHA may currently be low enough that
wetland area has not yet been affected. There could also be constructed ponds and
drainage ditches filled with water in these areas. It is also possible that the low wetland
area and density recorded in undeveloped areas is due to the resolution of the photos used
in the analysis. Patches were typically recorded as wetlands if there was some open
water present. Forested wetlands or seasonally dry wetlands could have been present in
undeveloped areas but were not counted due to their low visibility in the aerial photos.
It is likely that the Mallards are responding to the higher amount of wetland
habitat in developed areas and not specifically to development. However, Picman et al.
(1993) found that dabbling ducks fed on emergent wetland vegetation in the summer and
foraged in agricultural fields later in the year, while remaining near areas with open water
and vegetation. Perhaps for these birds, wetlands in farmland matrices offer ideal habitat
by providing an abundant food source and protection. Perhaps the strong association of
Mallards with farmland, in addition to wetland number and area, indicates that Mallards
are specifically selecting wetlands surrounded by farmland.
Habitat Guilds. Figure 13 shows that as a group, woodland obligates were
negatively associated with farmland and development. As a group, the wetland obligates
52
were not as constrained by environmental variables as woodland obligates were. The
woodland obligates were more negatively associated with urban development, paved
road, and agriculture than wetland obligates, suggesting that they are more sensitive to
increases in farmland and development. However, only the Mallard was positively
associated with farmland and development. The negative association of woodland birds
with urban development, paved road, and agriculture is likely due to the lower density of
trees in these areas. Table 3 shows that many of the woodland obligates are foliage
gleaners or excavators, so the absence of trees would make it more difficult for these
species to find food. Figure 11 shows that foliage gleaners were positively associated
with the area covered by trees within 500 meters and within 1000 meters. This suggests
that woodland birds, which include many foliage gleaners, seek out wooded areas at
broader landscape scales. Perkins et al. (2003) also found that the amount of woodland
present at broad spatial scales was important for some species of woodland birds in
landscapes dominated by agriculture. They found that the Eastern Wood-pewee and
Great Creasted Flycatcher were especially dependent on the amount of woodland within
500 meters of the study sites.
Four of the wetland obligates, the Veery, Swamp Sparrow, Marsh Wren, and
Virginia Rail, were also negatively associated with farmland and development as Figure
10 shows. These results suggest that the breeding habitats of some wetland obligates
could be diminished by the expansion of farmland and development in the area. Of these
species, the Marsh Wren was found to be associated with vegetation 1-2 meters tall and
2-5 meters tall. For this species, the negative association with farmland and development
could be a reflection of the lack of variability in the structure of the vegetation in these
53
areas. Marsh Wrens glean insects off of marsh vegetation (Bird Guide, 2003). Like the
woodland obligates, the Marsh Wren likely can only use wetlands with structural
vegetation that provides its primary food source, although the structure that it requires is
not woody vegetation, like the majority of the foliage gleaners. From these data, it could
be hypothesized that with the expansion of farmland and development, woodland
obligates will first disappear from wetlands, followed by sensitive wetland obligates.
Since there was no development recorded within 100 meters of the wetland sites,
it is unclear how development immediately adjacent to wetlands affects wetland bird
communities. As development likely causes a reduction of variety in the structure of
vegetation at a fine scale and the reduction of woodland density at broader scales, it can
be hypothesized that development adjacent to wetlands would lead to a decrease in the
species richness of birds in the wetland.
Chapman and Reich (2007) found that sensitive native bird species from different
habitat types, woodland, savanna, and grassland, decreased in abundance across a
gradient of urbanization in the Twin Cities area. Results of this study suggest that for
wetland birds as well, abundance could decrease with increasing urbanization. Chapman
and Reich (2007) suggest that there is a disappearance threshold associated with
development, especially for sensitive savanna and grassland birds. The birds would
become less abundant as development intensifies until a threshold is surpassed and
species are eliminated from the region (Chapman and Reich, 2007). There may be a
similar trend for some wetland species as well. DeLuca et al. (2004) found a
development threshold within 500 meters of wetlands for wetland birds in the
Chesapeake Bay area.
54
It is interesting and somewhat surprising that agriculture and development were
associated with the same axes and bird species in all ordination plots. This suggests that
in this area birds using wetlands are responding the same way to adjacent agriculture as
to development. Chapman and Reich (2007) found that bird diversity of upland species
was much higher in agricultural areas near reserves than in developed areas. Another
study at CCNHA also found that upland birds were strongly negatively affected by
surrounding urbanization and roads, but they used agricultural fields as habitat
(Goldsmith, 2007). In contrast, my results suggest that birds using wetlands are
negatively affected by both development and agriculture in the surrounding landscape. It
is possible that the lack of difference in the associations of development and agriculture
with the bird communities was due to the low amount of development in the area
compared to agriculture. Even the sites that were surrounded by the highest amount of
development were still surrounded by even larger areas of agriculture.
Effects of paved road. Paved road was significantly associated with the bird
distribution at multiple landscape scales. The majority of the species that were
significantly correlated with the axes were negatively associated with paved roads.
Findlay and Houlahan (1997) also found that wetland bird species richness decreased
with density of paved roads. In contrast, Findlay and Bourdages (2000) found that
species richness is not very sensitive to paved road density.
Results of this study suggest that paved road density at multiple landscape scales
affects species richness of birds in wetlands, however the mechanism is unclear. It could
be that that, since paved road density is associated with more developed areas, the lower
55
number of species is due to the reduction in woodland area, which was also found to be
associated with development. In this case, paved road and development function as one
environmental variable. However, since the graphs show that paved road and
development are associated differently with the wetland bird distribution, it is likely that
paved roads have effects on the bird community that are different from the effects caused
by development in general.
The mechanism by which the length of paved road affects bird distribution could
differ at the different landscape scales. Paved roads could fragment the landscape and
separate wetland patches, creating challenges for species such as the Virginia Rail, that
don’t move among patches during the breeding season (Bird Guide, 2003). Roads
dividing wetland patches into fragments could reduce the habitat size for these birds, and
possibly limit their access to resources. Paved road immediately adjacent to wetlands
could also cause chemicals and sediments to run into the site. It is unclear how this
would affect the bird distribution. At broader spatial scales, noise pollution caused by
paved road could disturb breeding birds. Findlay and Bourdages (2000) found that there
was a time lag between road construction and visible effects on bird communities. It is
therefore important that studies of the effects of paved roads on wetland communities
occur over a variety of both spatial and temporal scales.
Foraging guilds. Figures 9, 10, and 11 show that the four foraging guilds found to
be significantly correlated with the axes varied widely in their distribution in the vector
space. While foliage gleaners were strongly associated with trees at broad scales and
dabblers were correlated with farmland, development, and short vegetation, Figures 10
56
and 11 show that the ground and litter foragers and ground probers were not strongly
associated with any of the environmental gradients. Ground probers were slightly
positively associated with short vegetation, development, and farmland, while ground and
litter foragers were slightly negatively associated with these features. While it is unclear
from these results what environmental variables the distribution of these groups are
associated with, the figures suggest that they are not strongly affected by farmland and
development in the landscape, but rather, they are more generalists than birds in other
foraging guilds.
While Figures 6 and 7 show that few species were associated with farmland and
development, greater numbers of bird individuals were seen in these sites. It is possible
that this trend is due to sampling bias. Birds are easier to detect and count in open areas.
In sites with high structural diversity most birds were identified by their calls. It is much
more difficult to count birds by this method, and birds present but not calling were not
observed. At site 7 where the fewest birds were seen, almost all identification was by
call. Due to a thick layer of shrubs and trees adjacent to the road, very few birds in the
site could actually be seen.
Wetland connectivity
There was a large effect of scale in this study. Environmental variables were
found to be important at both the habitat and landscape scales. In this study, number of
wetlands within 250 meters, average distance to wetlands within 250 meters, and wetland
area within 1000 meters were found to be correlated with the axes. It is surprising that
number of wetlands and average distance to wetlands were both positively correlated
57
with the X-axis. A high number of wetlands within a landscape suggest high
connectivity of wetlands, while higher distance between wetlands suggests low
connectivity. However, number of wetlands was correlated the p < 0.01 level, while
average distance to wetlands was only correlated at the p < 0.1 level, suggesting that the
number of wetlands is a more important variable. Several other studies have found
wetland connectivity to be important in explaining bird distribution (Fairbairn, 2001;
Haig et al.; 1998, Naugle et al.; 2000). Semlitch and Brodie (1998) found that for species
with home ranges spanning a large area containing many wetlands, the loss of small
wetlands makes travel between larger patches more difficult. Naugle et al. (2001) also
concluded that species that use resources over large spatial scales will be the most
vulnerable to the loss of small wetlands.
Trends in wetland connectivity variables were not uniform across scales. Wetland
density was found to be important only within 250 meters, while wetland area was only
important within 1000 meters. The lack of a clear trend could be partially due to the lack
of independence at some of the sites at the broader scales. For these sites, the area
analyzed within 500 meters and within 1000 meters overlapped, making the analyses for
these sites very similar to each other. These results could also indicate that in this area
the level of connectivity is not the most important feature for determining habitat
suitability for wetland birds. Other habitat and landscape features could be more
important, and the birds may be responding primarily to these features. It is also possible
that connectivity among wetland patches in this area is high enough that the birds are not
significantly affected by varying connectivity among patches. Perhaps, like for
58
development, there is a threshold for loss of connectivity, beyond which birds are
significantly affected, but that threshold has not been surpassed in the CCNHA area.
Effect of wetland size and shape
It was expected that wetland size and shape would have an effect on bird
distribution; however no relationship was found between wetland size, amount of edge,
and the axes in the ordination plots. In contrast, Riffel et al. (2001) found large wetlands
to be important for several bird species. It is possible that area was not found to be
important in this study because the effect was masked by other measured environmental
variables. Perhaps features such as woodland in the proximity of the wetland or
vegetation features within the wetland are more crucial for birds than wetland size. This
would mean that even if a certain wetland size is preferable for a species, it would not be
able to use the site unless another environmental feature is present. These results
question the use of wetland area as the primary criterion for the selection of preservation
sites.
Effect of vertical and horizontal structural diversity
The variables found to be significant at the habitat level were mainly associated
with the vertical structure of the vegetation. Twedt et al. (2002) also found that in
bottomland swamps that had been reforested, colonization by woodland breeding birds
occurred much more in sites with fast-growing, tall vegetation than in sites with shorter
vegetation. This suggests that the presence of some birds is at least partially dependent
on the within-patch structure. While the woodland obligates were mostly found to be
59
positively associated with greater structure within the wetland, some other species were
found to be associated with short vegetation and graminoid vegetation. Several variables
were found to be significant at the landscape level as well. The figures show that most of
the bird species observed were negatively associated with development and agriculture.
Results suggest that to provide habitat for a diversity of bird species that use
wetlands, heterogeneity in the types of wetland patches within a landscape is important.
Some species, like the Mallard and Killdeer were found mostly in sites with open water
and little tall vegetation, while other species were found in sites with more diversity in
the structure of the vegetation. Killdeer are not typically associated with open water
(Bird Guide, 2003), so this relationship is surprising. It is likely that these birds are
responding to the lack of structural vegetation that was found to accompany open water.
Results also suggest that both local and landscape features are important to consider.
Woodland birds were mainly associated with trees at the landscape scales, while the
wrens were also strongly associated with vertical structure within the wetland patch.
DeLuca et al. (2001) also found that environmental variables were important at both local
and landscape levels.
Effect of wetland cover diversity
The 2001 study also found that structural variation within wetlands is important in
explaining bird distribution in wetlands. The analysis of the data compiled from both
years also illustrated the significance of cover diversity. The majority of species that
were significantly correlated with the axes were positively correlated with a high cover
diversity index number. Because the index was calculated using measures of structural
60
diversity, this suggests that more bird species from the two years of data are associated
with areas of high structural diversity. The three birds that were negatively associated
with the diversity index were the Red-winged Blackbird, Black Tern, and Veery. As
terns typically have habitat ranges over large spatial scales and require open water and
open space in the landscape (Naugle, et al., 1999), it is logical that their distribution is not
explained by high within-patch structural diversity. The Red-winged Blackbirds often
breed in areas of high cattail density (Weller, 1999), so a high diversity in the structure of
vegetation would likely not be necessary for their breeding success as long as their
preferred vegetation type is available. However, it is surprising that the Veery was not
found to be positively associated with high structural diversity in this analysis.
Nest predation
It was surprising that, while nest predation rates varied significantly among sites,
predation rates were not significantly correlated with any of the ordination axes or with
any measured environmental variables. Picman et al. (1993) suggest that the main
predators in marshes are Marsh Wrens, however we found no correlation between
predation rate and distribution of the Marsh Wren. The two sites with the highest nest
predation rates were both bogs with floating mats and less structural vegetation than most
of the other sites. It is possible that the high predation rates in these sites were due to the
lack of structure, however associations with these variables were not statistically
significant due to the small number of sites.
It is also possible that nest predation rates varied significantly in features at scales
smaller than those that were measured. A single wetland patch was the smallest scale
61
that variables were measured at. However, other studies have found nest predation rates
to vary among locations within a single wetland (Picman et al., 1993; Stewart, 1999). It
is possible that differences in predation rates could be explained by differences in
features in the immediate vicinity of the nests, which were not measured. For future
studies, I recommend measuring environmental conditions at much finer scales, as well
as at broad scales to determine the scale at which nest predators respond.
Water chemistry and hydrologic cycles
Figure 17 shows a clear trend in pH, from near neutral to more acidic going south
from Fawn Lake Drive. While pH was not measured at the sites where bird surveys were
conducted, this trend suggests that water at sites 1 and 11 are close to neutral, with water
at site 2 being more acidic and water at sites 3 and 12 still more acidic. Figure 4 shows
that there is a slight trend in the location of these sites on the graph of the indirect
gradient analysis of species.
There was a trend in acidity among the three sites where both water data and bird
data were collected. Figure 16 shows that acidity decreases along the first axis in the
ordination of species. Following this trend, Figure 4 suggests that woodland obligates are
associated with sites with acidic water, while wetland obligates are associated with water
with more neutral pH. To determine if there is a significant relationship between bird
distribution and water pH, in future studies pH should be measured at the sites where bird
data is collected. Sites 3 and 12 were also very similar in their vegetation compositions,
as were sites 1 and 11, which likely has a larger effect on the distribution of bird species.
However, the pH of the water and soil in the site could play a large role in determining
62
the vegetation composition. Other features of water chemistry, such as salinity should
also be measured. Salinity could affect vegetation growth and subsequently, bird use in
wetlands. The salinity of wetland water could also be affected by land use in the
surrounding landscape, especially by agricultural uses.
Although water level was not directly measured in this study, changes in the water
levels at some of the wetland sites were observed. Some sites that had standing water in
the initial surveys were completely dried out by the end of the study. Changes in the bird
communities at some of the sites over this period were noticed as well. The Virginia
Rails and Spotted Sandpipers were only seen in the last two weeks of the study. In
contrast, Veerys were observed in the first week of the study in the majority of the sites,
but was observed in none of the sites during the last two weeks. I recommend that in
future studies water level be monitored throughout the course of the observation period
so that the effects of hydrological fluctuations can be observed.
Conservation Implications
Results of this study have implications for wetland conservation, restoration, and
construction projects. Most importantly, results show that it is necessary to consider
environmental variables at multiple scales when planning wetlands projects. Conserving
and restoring appropriate habitat for wetland birds is especially important, as Valiela and
Martinetto (2007) found that wetland birds have decreased in abundance by 34 percent
over the past 50 years.
Results of the habitat-level analyses suggest that within a patch bird species
richness can be maximized by the inclusion of horizontal and vertical structural variation
63
in the vegetation. Vegetation two meters or taller may attract woodland bird species.
Vegetation taller than 1 meter can also provide habitat for wetland obligates such as the
Marsh Wren and Veery that use habitats with some vertical structure. However,
landscape features are important to consider for the distribution of birds within the
wetland patch as well. While diversity in the structure of vegetation may allow a variety
of birds to use the site, results suggest that woodland in the proximity of the wetland is
necessary woodland birds to use the wetland. However, a large density of woodland in
the surrounding area may limit the ability of some species, such as Mallards, to use the
site. These results suggest that sites for restoration should be carefully chosen based on
the characteristics of the surrounding landscape and the goals for restoration or
construction. To provide habitat for the greatest possible diversity of birds, sites in the
proximity of woodland should be selected. However, more open areas should be selected
to provide habitat for some wetland species.
Campbell and Ogden (1999) suggest that cattails, bulrushes, and common reeds
are the vegetation types typically used in constructed and restored wetlands because of
their vigorous growth and wide availability. While these species may me ideal for the
desired water filtering and storage functions (Campbell and Ogden, 1999), results of this
study suggest that wetlands dominated by these species may not provide ideal habitat for
a wide diversity of birds because they provide very little variety in both horizontal and
vertical structure. Engelhardt and Richie (2001) also hypothesized that with higher
vegetation species richness more wildlife could be supported in wetlands. They found
higher overall biomass in wetlands with a greater diversity of vascular plants (Engelhardt
and Ritchie, 2001). If one of the goals of wetland construction or restoration is high
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biodiversity of birds, then plans should involve planting species to provide vertical
structure in the vegetation.
My results suggest that, in contrast to what Findlay and Bourdages (2000) found,
birds can be indicators of environmental changes in wetlands due to increasing urban
development, paved road lengths, and agriculture in the landscape. While Findlay and
Bourdages (2000) concluded that birds are not very sensitive to construction of paved
roads, this study suggests that birds are responding to paved road in the proximity of the
wetlands, as paved roads were associated with low bird species richness.
From the results of this study, it can be concluded that in wetlands, environmental
variables at both habitat and landscape scales are important for explaining the distribution
of birds. It appears that high bird species richness is dependent on the structure of the
vegetation within the habitat and on woodland in the surrounding landscape. Birds using
wetlands near CCNHA seem to be responding the same way to agriculture as to
development. I hypothesize that if agriculture and development continue to expand in
this region, the bird species richness in many of the wetlands will decrease primarily due
to loss of wooded areas.
65
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Variable X Y Significance Graminoid vegetation + p < .1 Snags + p < .05 Vegetation < 1m tall + p < .05 Vegetation 1-2m tall - p < .05 Vegetation 2-5m tall - p < .1 Number of birds + p < .05
Table 1: Habitat variables and bird variables correlated with the X and Y axes. The table shows that 4 environmental variables and 1 bird variable were correlated with the X-axis in the indirect gradient analysis. Only 1 environmental variable was correlated with the Y-axis. Two of the variables were significantly correlated at the p < 0.1 level, and 4 were significant at the p < 0.05 level.
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Scale Variable X Y Significance100m
Farmland + p < .01 Paved road + p < .1 250m
Farmland + p < .01
Number of wetlands + p < .01
Distance to wetlands + p < .1
Trees - p < .1 500m
Farmland + p < .01 Development + p < .01 Paved road + p < .05 Trees - p < .05 1000m
Farmland + p < .05 Development + p < .01 Paved road + p < .01 Wetland area + p < .1
Trees - - p < .05, p < .1
Table 2: Landscape variables significantly correlated with the axes in the indirect gradient analysis of species. The table shows that a total of 15 variables at landscape scales were found to be significantly correlated with the axes. Two variables were significant within 100m, four withing 250m and within 500m, and five within 1000m.
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Ground/Litter Forager
Ground Prober
Foliage Gleaner Excavator
Aerial Insectivore Dabbler
Fish Eater
Veery Swamp Sparrow
Common Yellowthroat
Downy Woodpecker Tree Swallow Duck
Great Blue Heron
Red-winged Blackbird
American Robin House Wren
White-breasted Nuthatch Barn Swallow
Canada Goose
Green Heron
American Goldfinch
Spotted Sandpiper Marsh Wren
Eastern Wood Pewee
Ovenbird Killdeer Gray Catbird Eastern Kingbird
Lark Sparrow Northern Flicker Blue Jay
Eastern Phoebe
Song Sparrow Virginia Rail
Scarlet Tanager
Great Crested Flycatcher
Vesper Sparrow Cedar Waxwing
Alder Flycatcher
Brown-headed Cowbird
Black-capped Chickadee
Eastern Bluebird
Mourning Dove Rosebreasted Grosbeak
American Crow Northern Cardinal
Table 3: Seven observed foraging guilds. The ground/litter forager and foliage gleaner guilds were the most represented guilds with 10 species in each. The excavator, dabbler, and fish eater guilds each had two species, and therefore were the least represented guilds in terms of number of species.
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Scale Variable X Y Significance Habitat
height <1m + p < .05 height 1-2m - p < .1 100m
Farmland + p < .05 250m
Farmland + p < .05
Number of wetlands + p < .05
Distance to wetlands + p < .05
500m Farmland + p < .01
Development + p < .01 Paved road + p < .05 Trees - p < .05 1000m
Farmland + p < .1 Development + p < .01 Paved road + p < .05
Table 4: Environmental variables significantly correlated with the axes in the indirect gradient analysis of foraging guilds. While 15 variables were significantly correlated with the axes in the indirect gradient analysis of species, 13 were significant in the analysis of foraging guilds.
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Wetland Woodland
Marsh Wren Ovenbird Swamp Sparrow Scarlet Tanager
Mallard Black-capped Chickadee
Green Heron Rose-breasted Grosbeak
Virginia Rail Downy Woodpecker
Veery White-Breasted Nuthatch
Eastern Wood Pewee
Great Crested Flycatcher
Veery
Table 5: Wetland and woodland obligates observed. A total of six wetland obligates and nine woodland obligates were observed in the study.
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Scale Variable X Y Significance Habitat
Graminoid vegetation + p < .1
height <1m + p < .05 height 1-2m - p < .1 height 2-5 - p < .05 100m
Farmland + p < .05 250m
Farmland + p < .01
Number of wetlands + p < .01
Distance to wetlands + p < .1
500m Farmland + p < .01
Development + p < .05 1000m
Development + p < .05 Table 6: Environmental variables significantly correlated with the axes in the ordination of woodland and wetland obligates. A total of 11 variables were significantly associated with the axes in the ordination of woodland and wetland obligates.
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Trial 1 Site 1 Site 2 Site 3 Site 7 Site 8 Site 10 Site 11 Site 12
Nest 1 0 0 0.5 0.5 1 0.5 0.142857 0Nest 2 0 1 1 0 1 0.142857 0.142857 0.5Nest 3 0.142857 0 1 0.5 1 0 0 0Nest 4 0.142857 0.5 1 1 1 0 0.142857 0Nest 5 0 1 1 0 0 0
Trial 2 Site 1 Site 2 Site 3 Site 7 Site 8 Site 10 Site 11 Site 12
Nest 1 1 0 1 0.142857 0.142857 1 0 0Nest 2 0.333333 0.5 1 0.5 0.5 1 0.142857 0Nest 3 0.142857 0.333333 0.5 0.5 0.142857 0 0 0Nest 4 0 0.142857 0.5 1 0.142857 0 0 0.142857Nest 5 0 1 0.142857 0 0 0
Tables7a and 7b: Probabilities of nest failure per day in Trials 1 and 2. The tables show the probabilities of nest failure that were calculated for each nest in each site using the method described by Mayfield (1975). Nests were considered failures only when there was evidence of predation on both the quail and clay eggs.
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Figure 2: Quail and clay eggs in an artificial nest. The nests were placed in trees 1-2 meters above the ground at the edge of the wetland sites. A quail egg and a clay egg were placed in each nest. Photo by author.
Figure 3: Landscape analysis within 500m of Site 2 using Cedar Greek GIS. The green triangle shows the approximate location of the sample plot within the wetland. The yellow circle has a radius of 500 meters with the center the triangle. The red lines stretch from the triangle to other wetlands within the circle. The length of these lines were averaged to give an average distance from the study site to other wetlands as a measure of wetland connectivity.
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Graminoid vegetationVegetation < 1m tall Farmland (100, 250, 500, 1000m) Urban development (500,1000m) Paved road (100, 500, 1000m) Number of wetlands (250m) Distance to wetlands (250m) Wetland area (1000m)
Vegetation 1-2m tallVegetation 2-5m tall Trees (250, 500, 1000m)
KILL MALL TRES RWBL VESP HOWR
DOWO
WBNU
BCCH MAWR
SOSP
EAPH COYE
MODO
X-axis
Woodland Obligate Wetland Obligate
Other
Figure 4: X-Axis of the indirect gradient analysis of species with environmental variables significantly correlated with the axes. The axis represents a gradient in the environmental features. The variables listed on the right side of the axes are positively associated with that end of the axis and the species on that end For example, a high abundance of vegetation 1-2m tall was associated with the left end of the axis and with the presence of the House Wren, and a low abundance vegetation 1-2m tall with the right end of the axis and with the Killdeer. High graminoid abundance was associated with the right end of the axis, and low abundance with the left end of the axis. Species labeled in green are woodland obligates and those labelled in purple are wetland obligates. HW=House Wren, DW=Downy Woodpecker, WB=White-breasted Nuthatch, CH=Black-capped Chickadee, MW=Marsh Wren, EP=Eastern Phoebe, CY=Common Yellowthroat, MD=Mourning Dove, SS=Song Sparrow, VS=Vesper Sparrow, RW=Red-winged Blackbird, TS=Tree Swallow, ML=Mallard, KD=Killdeer.
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WBNU
MAWRHOWR DOWO
KILL
BCCH
MALL
COYE
TRES
EAPH
RWBC
VESP
SOSP
MODOY-axis
High area trees (1000m)High cover snags
Woodland Obligate Wetland Obligate
Other
Figure 5: Y-axis of the indirect gradient analysis of species with environmental variables significantly correlated. A high abundance of snags was associated with the left end of the axis and with the presence of the House Wren, and a low abundance of snags with the right end of the axis and with the Mourning Dove. High tree abundance within 1000m was associated with the right end of the axis, and low abundance with the left end of the axis.
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Urban development and agriculture
Woodland Obligate Wetland Obligate Other
KILL MALL TRES RWBL VESP HOWR
DOWO
WBN
BCCH MAWR
SOSP
EAPH COYE
MODO
X-axis
Figure 6: Direction of development in the ordination plot of species. The red arrow shows the increase in urban development and agriculture toward the right end of the axis. A high number of bird individuals is associated with high development and agriculture, while high species richness is associated with low development and agriculture.
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1
23 4 5
6
7
8
9
1011
12
Mallard
NuthatchDowny
Chickadee
Marsh Wren
Dove
Killdeer
Tree Swallow
Red-winged
House Wren
Yellowthroat
Phoebe
Song Sparrow Vesper Sparrow
X-axis
Y-axis
Figure 7: Ordination plot of bird species. The plot shows that 14 bird species that were significantly correlated with the X and Y axes. The species labeled in green are woodland obligates, the purple are wetland obligates, and the black are generalists. The triangles represent study sites. The spatial relationship on the graph indicates the similarity of the species in terms of the locations where they were observed.
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Foliage Gleaner DabblerX-axis
Vegetation < 1m tall
Farmland (100, 250, 500, 1000m)
Urban development (500, 1000m)
Paved road (500, 1000m)
Number of wetlands 250m
Average distance to wetlands 250m
Vegetation height 1-2m
Figure 8: X-axis of the indirect gradient analysis of foraging guilds with significantly correlated environmental variables. Foliage gleaners and dabblers were the two guilds significantly correlated with the X-axis.
85
1
23
45 67
8910
11
12Graminoidheight <1m
height 1-2m
height 2-5mDowny Nuthatch
Grosbeak
Ovenbird
Flycatcher
Pewee
Chickadee
Tanager
Marsh Wren
Swamp Sparrow Mallard
Virginia Rail
Green Heron
Veery
X-axis
Y-axis
Figure 9: Canonical correspondence analysis of species in woodland and wetland habitat guilds with habitat-level variables shown as vectors. The woodland obligates are labelled in green and the wetland obligates are purple. The Veery was classified as both a woodland and wetland obligate. The circle on the left side of the graph represents the range of the woodland obligates, and the circle on the right side represents the range of wetland obligates. The vectors show that vegetation heights 1-2m and 2-5m were negatively associated with the X-axis, while vegetation height < 1m and graminoid vegetation were positively associated with the axis.
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# wetlands 250mDevelopment 500mFarmland 250mFarmland 100m
Development Farmland 500m
Downy Chickadee
PeweeOvenbir
Nuthatch
Tanager
Grosbeak
Flycatcher
Marsh WrenSwamp Mallard
Green Heron
Virginia Rail
Veery
Y-axis
Figure 10: Canonical correspondence analysis of species in woodland and wetland habitat guilds with landscape-level variables shown as vectors. The small circle in the top left quadrant represents the range of the woodland obligates, and the large circle represents the range of the wetland obligates. All six significant landscape-level variables are positively correlated with the X-axis.
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TernRed-wingedVeeryBarn Swallow
Vesper
Goldfinch
Catbird
Yellowthroat
Mallard KilldeerTree Swallow
High wetland cover diversity
Low wetland cover diversity
Figure 11: First axis of the CCA of species observed in 2001 and 2006 with the Shannon diversity index variable. The Shannon diversity index was calculated using the cover types herbaceous vegetation, woody vegetation, and open water. H’ is negatively associated with the axis.
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W50
W48
W57 W56
F84
W24
W47
W28
W16
W25
W15
P67
W75
12
3
2
111
8
7
10
4.895
5.565
5.9745
6.1875
5.94
7.085 6.27
5.475
5.32
5.2025
6.305
5.715
5.5525
pHBird sitesWater samples
Figure 12: pH of wetland water sites in CCNHA. The green arrow shows a pH gradient south of Fawn Lake Drive. The water samples get more acidic going southwest from the road. There was no similar trend north of Fawn Lake Drive. Water samples were only collected from three of the sites where bird surveys were conducted.
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Site 7 Site 12 Site 1
5.2 5.6 6.3
Urban development and agriculture
X-axis
Figure 13: First axis of indirect gradient analysis of species with the three sites with pH measurements shown. The pH of the water at these sites is shown in red. Water acidity is negatively associated with the axis. The water approaches neutral pH in the positive direction.