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Passerine Relationships with Habitat Heterogeneity and Grazing at Multiple Scales
in Northern Mixed-Grass Prairie
By Barbara Bleho
A Thesis
Submitted to the Faculty of Graduate Studies
In Partial Fulfillment of the Requirements
For the Degree of
Master of Natural Resources Management
Natural Resources Institute
University of Manitoba
Winnipeg, Manitoba
August 2009
Copyright © 2009 by Barbara Bleho
THE UNIVERSITY OF MANITOBA
FACULTY OF GRADUATE STUDIES
*****
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Passerine Relationships with Habitat Heterogeneity and Grazing at Multiple Scales
in Northern Mixed-Grass Prairie
By Barbara Bleho
A Thesis/Practicum submitted to the Faculty of Graduate Studies of The University of
Manitoba in partial fulfillment of the requirement of the degree of
Master of Natural Resources Management
© 2009
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ABSTRACT
Limited information exists on the relationships among grazing, scale, and patterns of
heterogeneity in grassland communities and few studies have explored how grazing and
habitat heterogeneity together influence grassland bird communities. I used mixed
models to analyze the influence of cattle grazing, habitat structure, and habitat
heterogeneity on avian richness, diversity, and abundance within two spatial scales (plot,
pasture) at Grasslands National Park, Saskatchewan, Canada. Grazing exclusion in the
park has resulted in an increasingly homogenized landscape, which may result in
exclusion of some grassland bird species due to the loss of appropriate habitat. In 2008,
I conducted avian and habitat surveys within nine ungrazed and four conventionally
grazed 300-ha pastures, each containing 10 plots of 100-m radius (3.2 ha). Grazing did
not have an effect on species richness or diversity, but both positively and negatively
influenced individual species. Grazing likely influenced the avian community indirectly
through changes in habitat; both habitat structure and heterogeneity were strongly
influenced by grazing, though relationships with grazing varied among habitat variables.
Avian species richness and diversity were greater at higher levels of habitat
heterogeneity, but species relationships with habitat heterogeneity varied. Species
richness and diversity increased with spatial scale, whereas habitat heterogeneity
decreased with spatial scale, suggesting that heterogeneity was relatively high at the
smaller plot scale and the larger pasture scale did not capture further variability in habitat
structure. Relationships among grazing, habitat, and birds existed primarily at the plot
scale, suggesting that other factors, such as landscape variables, were driving avian
community dynamics at the larger pasture scale. Overall, light-moderate grazing did not
have a negative effect on avian diversity. An appropriate grazing regime may benefit
some species and increase community diversity within the park.
i
ACKNOWLEDGEMENTS
I would like to thank Parks Canada for funding my research, as well as the Faculty of
Graduate Studies at the University of Manitoba for its generous support of my work
through its award of the Manitoba Graduate Scholarship.
I also wish to thank the researchers and staff at Grasslands National Park for their
guidance and support of this project. Special thanks go to Rob Sissons, Conservation
Biologist, Pat Fargey, Species at Risk Biologist, and Michael Fitzsimmons, Ecosystem
Scientist. Thank you also to Darcy Henderson, Grassland Ecologist with the Canadian
Wildlife Service, for statistical help, and to John Wilmshurst, Ecologist with Parks
Canada, for early feedback on my thesis proposal.
I thank my committee members, Spencer Sealy, Professor in Biological Sciences at the
University of Manitoba, and John Markham, Assistant Professor in Biological Sciences at
the University of Manitoba, for taking the time and having the patience to read through
the stages of my thesis and offer their invaluable feedback. I especially thank my
advisor, Nicola Koper, for her support and guidance through these last two years.
Finally, I would like to thank Tim Teetaert, Allison Selinger, Krystle White, and Laura
Murray for two wonderful summers in the field. This project could not have been
completed without their help.
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TABLE OF CONTENTS
ABSTRACT .......................................................................................................................... i
ACKNOWLEDGEMENTS ................................................................................................. ii
LIST OF TABLES ............................................................................................................... v
LIST OF FIGURES ............................................................................................................ vi
1.0 INTRODUCTION ......................................................................................................... 1
1.1 Background ............................................................................................................. 1
1.2 Problem statement ................................................................................................... 2
1.3 Research Objectives ................................................................................................ 2
1.4 Hypotheses .............................................................................................................. 3
1.5 Limitations .............................................................................................................. 3
1.6 Contributions ........................................................................................................... 5
1.7 Definitions of Terms ............................................................................................... 5
2.0 LITERATURE REVIEW .............................................................................................. 8
2.1 Status of North American grassland birds............................................................... 8
2.2 Natural history of grassland passerines ................................................................... 9
2.2.1 Horned Lark (Eremophila alpestris) ............................................................. 9
2.2.2 Sprague’s Pipit (Anthus spragueii) ............................................................. 10
2.2.3 Clay-colored Sparrow (Spizella pallida) ..................................................... 11
2.2.4 Brewer’s Sparrow (Spizella brewerii) ......................................................... 12
2.2.5 Vesper Sparrow (Pooecetes gramineus)...................................................... 13
2.2.6 Savannah Sparrow (Passerculus sandwichensis)........................................ 14
2.2.7 Baird’s Sparrow (Ammodramus bairdii) ..................................................... 15
2.2.8 McCown’s Longspur (Calcarius mccownii) ................................................ 17
2.2.9 Chestnut-collared Longspur (Calcarius ornatus) ....................................... 18
2.2.10 Red-winged Blackbird (Agelaius phoeniceus) ........................................... 19
2.2.11 Western Meadowlark (Sturnella neglecta) ................................................ 20
2.2.12 Brown-headed Cowbird (Molothrus ater) ................................................. 21
2.3 Habitat selection by grassland birds ...................................................................... 22
2.4 Habitat associations of grassland birds ................................................................. 26
2.5 Habitat heterogeneity and grassland bird diversity ............................................... 27
2.6 Measures of heterogeneity .................................................................................... 30
2.7 Effects of grazing on grassland birds .................................................................... 32
2.8 Spatial patterns in grazing ..................................................................................... 33
2.9 Grazing as a management tool .............................................................................. 36
2.10 Research and management needs for grassland bird conservation ..................... 38
2.10.1 Multiple-scale studies and ecosystem management .................................. 38
2.10.2 Habitat restoration and conservation........................................................ 39
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3.0 METHODS .................................................................................................................. 41
3.1 Study area .............................................................................................................. 41
3.2 Study design .......................................................................................................... 42
3.3 Avian surveys ........................................................................................................ 45
3.4 Habitat surveys ...................................................................................................... 45
3.5 Statistical analysis ................................................................................................. 46
4.0 RESULTS .................................................................................................................... 52
4.1 Avian community structure ................................................................................... 52
4.2 Influence of grazing on the avian community ....................................................... 52
4.3 Influence of grazing on habitat structure and heterogeneity ................................. 57
4.4 Relationships among birds, habitat, and grazing................................................... 62
4.4.1 Plot-scale relationships ............................................................................... 62
4.4.2 Pasture-scale relationships ......................................................................... 69
4.4.3 Grazing-habitat relationships...................................................................... 75
4.5 Scale effects........................................................................................................... 76
5.0 DISCUSSION .............................................................................................................. 80
5.1 Avian relationships with grazing, habitat structure, and habitat type ................... 80
5.2 Measures of heterogeneity .................................................................................... 87
5.3 Influence of grazing, habitat type, and scale on habitat structure and heterogeneity . 92
5.4 Effects of spatial scale on bird-habitat relationships ............................................. 96
5.5 Influence of habitat heterogeneity on grassland birds ........................................... 99
6.0 CONCLUDING REMARKS ..................................................................................... 105
7.0 MANAGEMENT IMPLICATIONS ......................................................................... 106
8.0 LITERATURE CITED .............................................................................................. 108
9.0 APPENDICES ........................................................................................................... 123
9.1 Appendix I ―Passerine species observed in the study area ............................... 123
9.2 Appendix II ―Example of a mixed-effects model ............................................. 124
iv
LIST OF TABLES
1. Influence of grazing treatment on avian community structure and species
abundance at plot and pasture scales...........................................................................53
2. Influence of habitat type on avian community structure and species abundance….…...55
3. Influence of grazing treatment on avian community structure and species
abundance in upland and lowland habitat………………………………………...…...56
4. Influence of grazing treatment on habitat structure and heterogeneity at plot and
pasture scales………...………………………………………………...……………….….….58
5. Influence of habitat type on habitat structure and heterogeneity……….…..…..59
6. Influence of grazing treatment on habitat structure and heterogeneity in upland
and lowland habitat…………………………………………………………….……..60
7. Influence of grazing treatment, habitat structure, and habitat heterogeneity on
avian community structure and species abundance at plot scale………………..….....63
8. Influence of grazing treatment, habitat structure, and habitat heterogeneity on
avian community structure and species abundance at pasture scale………...………...71
v
LIST OF FIGURES
1. Location of the study area…………………….……………………..……………...…….…..43
2. Layout of the study area……………………………………………..…………...…..……. ....44
3. Effects of spatial scale on avian community structure……….………………….………..77
4. Effects of spatial scale on habitat heterogeneity measured from standard deviation….78
5. Effects of spatial scale on habitat heterogeneity measured from coefficient of
variation….………………………………………………………………………………….... 79
vi
1.0 INTRODUCTION
1.1 Background
Grassland bird populations in North America have experienced severe declines in the past
50 years (Knopf 1996). Habitat loss appears to be the primary driver behind this decline
(Davis et al. 1999, Murphy 2003), followed by degradation of native grasslands (Askins
et al. 2007). Consequently, there has been much interest in the conservation and
restoration of grassland habitat to ensure the survival of these species (Johnson and
Schwartz 1993, McCoy et al. 2001, Brennan and Kuvlesky 2005).
Grazing has been proposed as a necessary component of sustainable grassland ecosystems
(Fuhlendorf and Engle 2001). North American grassland birds evolved under the periodic,
intensive grazing pressure associated with nomadic bison herds and are adapted to the mosaic
of habitats that selective grazing creates (Knopf 1996, Fuhlendorf and Engle 2001). Grazing
affects grassland birds indirectly through changes in habitat structure (Fondell and Ball
2003). Moderate grazing has been shown to increase habitat heterogeneity (Hartnett et al.
1997) and avian diversity (Kantrud 1981) in mixed-grass prairie; however, the mechanisms
behind these relationships are not well understood. Furthermore, both avian-habitat
associations and the effects of grazing on habitat heterogeneity can vary across spatial scales
(Fuhlendorf and Smeins 1999, Thogmartin and Knutson 2007), so effects of grazing on
grassland bird diversity may also vary across scales.
Cattle grazing at low-to-moderate intensities may be a useful management tool for
conserving birds in the mixed-grass prairie; however, the challenge remains to develop a
grazing regime that preserves or enhances the heterogeneity of the landscape needed to
support a variety of grassland bird species (Ricketts et al. 1999).
1
1.2 Problem statement
Limited information exists on the relationships among grazing, scale, and patterns of
heterogeneity in grassland communities (Fuhlendorf and Smeins 1999). Most studies to
date have addressed the topic of grazing in grassland ecosystems at single spatial scales
or have focused on the effects of grazing on either vegetation or bird diversity, but few
have considered how all of these variables interact. Moreover, most studies have been
conducted within intensely managed or small-scale (<100-ha) pastures, yet commercial
pastures in southern Alberta and Saskatchewan are on average over 400 hectares (Davis
et al. 1999, Koper et al. 2008). Cattle behaviour and grazing patterns differ among
pasture sizes (e.g., herd distribution, forage selection), so it is difficult to extrapolate
results from smaller-scale pastures to large pastures used in management (Hartnett et al.
1997, Winter et al. 2005, Briske et al. 2008).
1.3 Research Objectives
My main objective was to determine how habitat structure and heterogeneity affect
grassland bird diversity, and how these relationships are influenced by grazing.
My specific objectives were to identify:
1. What patterns of heterogeneity existed in habitat structure in the absence and
presence of grazing and at two spatial scales.
2. How grassland bird diversity and abundance were influenced by patterns of
heterogeneity in habitat structure at two spatial scales.
3. How relationships between habitat heterogeneity and grassland bird diversity were
influenced by grazing at two spatial scales.
2
1.4 Hypotheses
I predicted that grazed habitat would be more heterogeneous in structure than ungrazed
habitat due to the disturbance caused by grazing. I also predicted that heterogeneity
would increase with scale as more habitat types were captured in each experimental unit.
I predicted that avian richness and diversity would be greater in grazed than in ungrazed
habitat as a result of the heterogenizing effect of light-to-moderate grazing on habitat
structure, and that both would increase with scale as more microhabitats suitable for a
wider array of species became available. I also predicted that richness and diversity would
covary with habitat heterogeneity, both in the presence and absence of grazing, because
many grassland birds appear to select breeding habitat based on habitat structure (Chapman
et al. 2004, Fritcher et al. 2004, but see McCoy et al. 2001). However, I predicted that the
relationship between avian diversity and habitat heterogeneity would weaken as scale
increased and landscape variables became more influential in habitat selection.
1.5 Limitations
This study was conducted within the confines of a larger, adaptive management-based
study, which at times meant modifications of existing methods and the addition of new
methods where appropriate. Although three years of avian data were available, only one
year of habitat data was collected. Consequently, results were based on one field season.
Thus, I caution against the interpretation of results as cause-effect relationships between
grazing and avian community structure and between habitat heterogeneity and avian
community structure. Annual variations in external factors, such as climate or migratory
patterns of irruptive species, may confound relationships perceived to exist between
grazing and avian community structure or between habitat heterogeneity and avian
3
community structure because these factors also influence local population densities
(Cody 1981). Furthermore, factors such as food availability and predation risk can
influence avian habitat choice (Knapton 1994, Jones and Cornely 2002). Multiple years
of study are required to deduce cause-effect relationships; however, this study serves as a
starting point in detecting such relationships. Further, comparisons among the three years
of bird surveys show relatively consistent trends in the relationships between grazing and
avian community structure across years. This suggests that relationships in this study
were overriding potential annual variations in the system.
I did not consider avian nesting success and therefore draw no conclusions about the
quality of available habitat (i.e., source/sink dynamics). Detailed nesting studies can rule
out the possibility of birds inhabiting areas where reproductive success is low (Van
Horne 1983); however, when multiple species or large areas are of interest, as was the
case in this study, an approach based on species abundance data is often the only costand time-effective method of analysis (Madden et al. 2000). Moreover, it appears birds
are generally able to distinguish between source and sink habitats and select higher
quality habitats for nesting (Bock and Jones 2004); thus, abundance data may be a good
estimator of nesting success and habitat quality in many circumstances (e.g., Patterson
and Best 1996, Winter et al. 2005). Furthermore, in habitats where reproductive success
is low, a high abundance of individuals can offset nest failure, resulting in a greater total
of offspring produced than in habitats where few individuals nest, though reproductive
success is high (Best et al. 1997). Evidently, what constitutes high-quality habitat is not
always clear (McCoy et al. 1999); thus, abundance data should not be dismissed as a
potential indicator of population health.
4
1.6 Contributions
This study contributes to our understanding of the complex relationships among
grassland passerines, their habitats, and the effects of grazing on mixed-grass prairie
communities. Moreover, it contributes to our knowledge of how scale influences and
alters relationships among patterns (heterogeneity, diversity) and processes (grazing) in
mixed-grass prairie.
These results contribute to the development of a grazing plan for Grasslands National
Park of Canada that will create and maintain suitable habitat for grassland passerines,
including those at risk, within the park. Furthermore, insights gained here may serve as
useful suggestions for the improvement of range management practices. Fuhlendorf and
Engle (2001) proposed a paradigm that promotes heterogeneity on rangelands for both
biodiversity and sustainable livestock production. Results of this study support such a
paradigm.
1.7 Definitions of Terms*
Animal unit – 1 AU = 1000 lb; animal units are used instead of number of animals
because utilization rates vary among individual animals.
Conventional grazing – season-long (June-October) grazing at moderate grazing
intensity.
Grazing intensity – the cumulative effect grazing animals have on the land during a
particular time period (expressed as percent utilization in this study).
Light grazing – permits maximized growth of palatable species (increase in herbage
production); ~32% utilization and lower.
Moderate grazing – permits sustained growth of palatable species (no change in
5
herbage production); ~43% utilization.
Heavy grazing – does not permit sustained growth of palatable species (decrease in
herbage production, leading to eventual removal); ~57% utilization and higher.
The above percent utilization rates are averages based on 25 studies conducted
globally (Holechek et al. 2001). Regional correspondence between percent utilization
rates and grazing intensities depends on the ecosystem. In semi-arid grasslands,
moderate grazing intensity is approximately 35-45% utilization (Holechek et al.
1999).
Heterogeneity – variability or non-uniformity in a structure, system, or community. In
this study, heterogeneity represents the degree of uniformity in habitat structural
characteristics. Higher heterogeneity indicates lower uniformity or greater variability
(patchiness).
Percent utilization – the percentage of the current year‘s primary production consumed or
destroyed by livestock; one method of measuring grazing intensity (other methods
exist).
Selective foraging – herbivore consumption of plant species or growth stages
disproportionately to their availability.
Species (avian) richness – the number of species present within a study area or
community.
Species (avian) diversity – most commonly defined as the number of species and relative
abundance of each species within a study area or community (Wiens 1989b; definition
used in this study); occasionally interpreted as species richness, particularly where
relative abundance of individual species is low. May also be interpreted as species
6
evenness: simply the relative abundance of each species within a study area or
community.
Stocking rate – the amount of land allocated to each animal unit for the grazed period of
the year.
*Grazing definitions adapted from Holechek et al. (1999) and Holechek et al. (2001).
7
2.0 LITERATURE REVIEW
2.1 Status of North American grassland birds
Populations of North American grassland birds have severely declined in the past 50
years (Herkert 1995, Knopf 1996). Some species have declined by as much as 50% in 25
years (Johnson and Schwartz 1993). The mixed-grass prairie supports five of the six
passerine species endemic to North American grasslands (Knopf 1996). Of these,
Sprague‘s Pipit and McCown‘s Longspur are species at risk in Canada (COSEWIC
2008). All five endemic species are in decline, and, in Canada, this decline is significant
for Sprague‘s Pipit and McCown‘s Longspur at P<0.05, and for Chestnut-collared
Longspur at P<0.1 (Sauer et al. 2008; see Appendix I for scientific names). Secondary
endemic species (i.e., those species that have more flexible habitat requirements and
wider ranges, but still occur predominantly in open habitat) experiencing decline in
Canada are Horned Lark, Vesper Sparrow, Savannah Sparrow, Grasshopper Sparrow, and
Western Meadowlark, for which all except Vesper Sparrow this decline is significant
(P<0.05; Sauer et al. 2008).
Habitat loss appears to be the primary driver behind the decline in many grassland
bird populations (Davis et al. 1999, Murphy 2003). Total losses of native North
American grasslands range from 30-99% of historical distributions (Knopf and Samson
1997). Altogether, an estimated 30% of the mixed-grass prairie remains (Samson et al.
2004). Approximately 75% of native mixed-grass prairie in Canada and 80% in
Saskatchewan has been lost to human use (Holroyd 1995, Iwaasa and Schellenberg
2004). Urbanization, livestock ranching, conversion to crop fields, and invasion of
woody and exotic species have all been implicated in the loss, fragmentation, and
8
degradation of native grasslands (Herkert 1994, Warner 1994, Murphy 2003, Davis 2004,
Brennan and Kuvlesky 2005). The most severe declines in grassland bird populations are
occurring in areas of intensive agriculture (Igl and Johnson 1997). Local populations that
are relatively stable or increasing are often associated with contiguous tracts of native or
restored prairie (Johnson and Schwartz 1993, Herkert 1998, Davis 2004). For example,
Sprague‘s Pipit is most abundant in southeastern Alberta and southwestern
Saskatchewan, where large areas of native mixed-grass prairie remain (Davis et al. 1999),
and regional population trends for several species have improved where Conservation
Reserve Program lands have been established in the United States (Johnson and Schwartz
1993, Vickery and Herkert 2001).
2.2 Natural history of grassland passerines
2.2.1 Horned Lark (Eremophila alpestris)
Horned Lark is a common bird of open habitats (Kantrud 1981, Beason 1995). It is
widespread across the North American continent, with breeding populations in Alaska
and the Arctic Archipelago down to central Mexico (Beason 1995). This species is a
permanent resident in most of its range; however, most breeding populations in Canada
and Alaska migrate within North America. Horned Lark is one of the first birds to arrive
on its breeding grounds (Hales 1927). Migrating populations arrive in southern Canada
as early as February and in Manitoba in early April, with males arriving first and females
shortly thereafter (Beason 1995). Migration to wintering grounds occurs from late
October to late November (Dinkins et al. 2000).
Horned Lark is typically found in short, sparsely vegetated habitat and is
characteristic of short grasslands, agricultural lands, deserts, and tundra (Cody 1985b,
9
Beason 1995). This species responds positively to grazing, favouring moderately to
heavily grazed grasslands (Wiens 1973, Kantrud 1981). Breeding habitat is typically
barren, with extensive bare ground, vegetation no taller than a few centimetres, and little
to no woody vegetation (Wiens 1973, Beason 1995, Davis and Duncan 1999, Dinkins et
al. 2000).
Breeding territories are established soon after arrival on breeding grounds and are
uniformly dispersed. They are multi-purpose, used for both breeding and foraging
(Beason 1995). Territory size is influenced by habitat quality and population densities
and can range from 0.3-5.1 ha, but more typically from 0-3-1.7 ha in optimal habitat
(Beason 1995, Dinkins et al. 2000). Nest construction occurs from mid-March to late
June in Great Plains populations and the preferred nest site is on bare ground with no
vegetation covering the nest (With and Webb 1993, Beason 1995).
2.2.2 Sprague’s Pipit (Anthus spragueii)
Sprague‘s Pipit is endemic to North American grasslands (Mengel 1970). Formerly more
widespread, it is now absent in areas where it was once abundant, such as in parts of
Manitoba (Robbins and Dale 1999), though it remains common where grasslands still
exist (Davis et al. 1999). It is a short-to-medium-distance migrant, departing from its
wintering grounds in south-central United States and Mexico and arriving at its breeding
grounds in the northern Great Plains in late April to mid-May before moving south once
again in late September (Robbins and Dale 1999).
Sprague‘s Pipit prefers well-drained areas in open grassland with little to no shrubs
(Robbins and Dale 1999, Madden et al. 2000). It is most commonly associated with
medium-height grasses and moderate litter depth and prefers native over tame grasslands
10
(Davis and Duncan 1999, Madden et al. 2000). This species generally prefers light to
moderate grazing and is negatively influenced by heavy grazing (Davis et al. 1999, but
see Kantrud 1981).
Territories are multi-purpose, used for both breeding and foraging (Robbins and Dale
1999). Optimal habitat will typically be packed with territories, with reports of up to five
territories within a 5-ha site (Robbins and Dale 1999). The breeding season lasts
approximately three months in Saskatchewan. Nest construction begins in early to midMay (Robbins and Dale 1999). Nests are built in a depression on the ground, usually at
the base of a clump of grass, and covered with a dome of vegetation (Ehrlich et al. 1988,
Dechant et al. 1998d). Nest sites contain relatively tall, dense vegetation, low forb and
shrub cover, and little bare ground (reviewed in Dechant et al. 1998d).
2.2.3 Clay-colored Sparrow (Spizella pallida)
Clay-colored Sparrow is possibly the most common species in brush habitats of the
northern Great Plains, though its breeding range expands into southern Ontario to the east
and southern Northwest Territories to the north (Knapton 1994). Individuals arrive on
their breeding grounds in early May and depart for their wintering grounds in the
southern United States and Mexico in late August to late September (Knapton 1994).
More of a brush species than a true grassland species (Cody 1985b), Clay-colored
Sparrow has benefited from European settlement and subsequent fire suppression in the
prairies, which has permitted the spread of woody vegetation favoured by this species
(Knapton 1994). Ideal breeding habitats for Clay-colored Sparrow include open
shrubland, thickets along waterways, second-growth areas, and forest edges (Knapton
1994). It prefers native grassland over tame pastures or hayland (Dechant et al. 1998c).
11
This species tolerates light to moderate grazing (Kantrud 1981). Shrub cover is a strong
predictor of this species‘ presence in an area (Madden et al. 2000), likely more so than
grazing regime (Dechant et al. 1998c).
Territories are used for breeding only and are consequently small, ranging from 0.10.4 ha in size (Knapton 1994). Territories are typically situated adjacent to suitable
foraging areas, which consist of open habitat containing short, sparse vegetation (Dechant
et al. 1998c). Males defend their territories aggressively during establishment against
other conspecific males; otherwise, there is minimal interaction among individuals.
Some interspecific territoriality has been reported with Chipping Sparrows, Song
Sparrows, and Brewer‘s Sparrows (Knapson 1994). The breeding season runs from May
to early August. Nests are constructed primarily in low shrubs, especially western
snowberry (Symphoricarpos occidentalis; Knapson 1994, Davis and Duncan 1999),
though they may also be on the ground in residual vegetation (Dechant et al. 1998c) or at
the base of a shrub (Ehrlich et al. 1988).
2.2.4 Brewer’s Sparrow (Spizella brewerii)
Brewer‘s Sparrow is commonly found in the intermountain region of the American west,
though its breeding range extends east to the western limits of the Central Plains
(Rotenberry et al. 1999). Wintering grounds are located in the far southwestern United
States and north-central Mexico (Rotenberry et al. 1999). Spring migrants arrive in
southeastern Alberta in early May and depart from mid-August to September (Rotenberry
et al. 1999).
Brewer‘s Sparrow is a shrubland species, breeding strictly in shrub-steppe or
grasslands containing extensive shrub cover, primarily sagebrush (typically Artemisia
12
tridentata, but also Artemisia cana), and is generally considered a sagebrush-obligate
(Rotenberry et al. 1999, Walker 2004). This species tolerates light grazing (Wiens 1973).
Territory size varies greatly between years and both among and within sites, ranging
from 0.10-2.36 ha (Rotenberry et al. 1999). Territories are multi-purpose, used for both
breeding and foraging (Rotenberry et al. 1999). Foraging occurs primarily above-ground
in shrubs, but occasionally also on the ground (Walker 2004). Males defend their
territories aggressively against conspecifics and interspecific territoriality has been
recorded with Clay-colored Sparrow (Knapton 1994, Rotenberry et al. 1999). Pairs
typically form shortly after arriving on the breeding grounds and begin nest construction
soon afterwards. Nests are built above-ground in a relatively tall, dense shrub,
particularly sagebrush (Kantrud and Kologiski 1983, Rotenberry et al. 1999), surrounded
by other shrubs (Walker 2004).
2.2.5 Vesper Sparrow (Pooecetes gramineus)
Vesper Sparrow is a ground-dwelling species found in open habitats. Its breeding range
follows a horizontal band across southern Canada and north and central United States,
while its wintering grounds are situated in the southern United States and Mexico (Jones
and Cornely 2002). Spring migrants arrive on breeding grounds in Saskatchewan from
mid-April to early May and depart from September to early October (Dechant et al.
2000).
Vesper Sparrow is considered to be somewhat of a habitat generalist, occupying a
wide variety of grassland habitat types, including native prairie, shrub-steppe, meadows,
native and tame pastures, haylands (but see Kantrud 1981), croplands, and woodland
edges (Davis and Duncan 1999, Jones and Cornely 2002). Its preferred habitat consists
13
of dry, open areas of short, sparse, patchy vegetation (Dechant et al. 2000). In mixed
grasslands, this species is associates with low vegetation height (Davis and Duncan
1999). Though foremost a ground dweller, Vesper Sparrow will use shrubs and taller
vegetation as singing perches (Hales 1927).
Territory size ranges from 0.29-8.19 ha across this species‘ range, with larger
territories tending to have fewer food resources (Jones and Cornely 2002) or located in
agricultural areas (Dechant et al. 2000). Territory size in the central grasslands more
typically ranges from 0.29-3.2 ha (Dechant et al. 2000). Interspecific territoriality is rare,
though reports exist of occasional aggression against Savannah Sparrows (Jones and
Cornely 2002). Nest construction occurs in late May on northern breeding grounds.
Nests are built in shallow depressions on the ground and are usually concealed by
overhanging vegetation or a shrub (Dechant et al. 2000, Jones and Cornely 2002).
2.2.6 Savannah Sparrow (Passerculus sandwichensis)
Savannah Sparrow is widespread and abundant in open habitats throughout North
America (Wheelwright and Rising 1993). This species breeds throughout most of
Canada and the northern United States and winters in the southern United States down to
Central America (Wheelwright and Rising 1993). Spring migrants reach South Dakota
by early April and depart for wintering grounds beginning in early September
(Wheelwright and Rising 1993).
Savannah Sparrow breeds in a variety of open habitats, including native prairie,
weedy fields, swamps, tundra, and coastal marshes, and avoids areas with extensive tree
cover (Cody 1985b, Wheelwright and Rising 1993). However, some low woody
vegetation is desirable for perching by singing males (Swanson 1998). This species will
14
use croplands, but prefers native grasslands (Swanson 1998), though pastures seeded with
exotic species appear to be equally suitable (Davis and Duncan 1999). In the Central
Plains, Savannah Sparrow is a common summer resident of mixed- and tallgrass prairie
(Cody 1985b), where it is associated with intermediate vegetation height, density, and
litter accumulation (Swanson 1998). This species tolerates light grazing, but is greatly
reduced by heavy grazing (Kantrud 1981).
Territory sizes vary between habitats, regions, seasons, and years, and range from
0.05-1.25 ha across the species‘ continental range, though in Wisconsin, territories range
from 0.53-0.86 ha (Wheelwright and Rising 1993). Territories are multi-purpose, used
for both breeding and foraging, though foraging also occurs outside of the territory.
Interactions are generally restricted to the breeding pair until young are fledged, with
little interspecific interaction and no interspecific territoriality observed (Wheelwright
and Rising 1993), though spatial exclusion of this species may occur in the presence of
Vesper and Brewer‘s Sparrows (Cody 1985b). Nests are built in shallow depressions on
the ground in a clump of grass or at the base of a shrub and are well hidden by a canopy
of dried vegetation (Wheelwright and Rising 1993). Nest sites are characterized by dense
vegetation dominated by grass, with little forb cover (Wiens 1969).
2.2.7 Baird’s Sparrow (Ammodramus bairdii)
Baird‘s Sparrow is endemic to North American grasslands (Mengel 1970). Historically,
it was one of the most common grassland birds, but is now rare throughout its range, with
only pockets of locally abundant populations where optimal habitat exists (Green et al.
2002). Baird‘s Sparrow breeds in a restricted area confined to the northern mixed-grass
prairie of southern Canada and the Dakotas (Cody 1985b). Spring migrants arrive on
15
breeding grounds in early May (Saskatchewan) and depart for wintering grounds of the
southern United States and Mexico in mid-late September (Green et al. 2002).
Baird‘s Sparrow breeds primarily in mixed-grass and fescue prairie where shrub cover
is relatively low, vegetation is relatively tall and dense, and litter accumulation is
moderate (Kantrud and Kologiski 1983, Madden et al. 2000, Davis 2004), though Winter
(1999) reported that this species preferred low litter accumulation. Once considered a
native prairie specialist, Baird‘s Sparrow is now known to also use hayfields and seeded
pastures (Davis and Duncan 1999). This species is often associated with undisturbed
grassland (Green et al. 2002), but tolerates light to moderate grazing (Kantrud 1981).
Territory size ranges from 0.80-2.25 ha (Green et al. 2002). Males establish territories
shortly after arrival on the breeding grounds and aggressively defend their territories
against conspecifics (Green et al. 2002). Territories frequently exhibit a clustered
distribution (Ahlering et al. 2006). Interspecific territoriality with Grasshopper Sparrow
has been recorded (reviewed by Cody 1985b), though complete territorial overlap
between the two species has also been observed, as well as between Baird‘s Sparrow,
Savannah Sparrow and Chestnut-collared Longspur (Green et al. 2002). Nest
construction begins mid-late May (Green et al. 2002). Nests are built in a shallow
depression on the ground among grasses and may be hidden by overhanging vegetation
(Ehrlich et al. 1988), or may be left unconcealed (Davis and Sealy 1998). Preferred nest
sites generally have taller, denser vegetation, greater litter accumulation, and little bare or
exposed ground (Green et al. 2002).
16
2.2.8 McCown’s Longspur (Calcarius mccownii)
McCown‘s Longspur is endemic to North American grasslands (Mengel 1970), breeding
in a restricted area of the Central Plains confined to southeastern Alberta, southern
Saskatchewan, Montana, Wyoming, and North Dakota, and wintering in south-central
United States and north-central Mexico (With 1994a). This species‘ breeding range has
been drastically reduced and historically spanned further east into Manitoba and western
Minnesota, and south to Oklahoma; however, breeding populations are locally abundant
where suitable habitat exists (With 1994a). Spring migrants arrive on the breeding
grounds in April (Saskatchewan: mid-late April), with males preceding females by
approximately two weeks. Fall migrants leave Saskatchewan in mid-late September
(With 1994a).
McCown‘s longspur is a shortgrass species, restricted to open habitat with naturally
sparse vegetation and low litter accumulation or habitat rendered as such by grazing or
other disturbance (With 1994a, Dechant et al. 1999a). This species favours heavily
grazed pastures and is generally absent from idle mixed grassland left to grow tall and
dense (Kantrud 1981). Cultivated lands may also be utilized (Dechant et al. 1999a).
Territories range in size from 0.6-1.4 ha, with average size in Saskatchewan of 1.1 ha
(With 1994a). Males aggressively defend territories against conspecific males and
boundaries are firmly delineated with no overlap (With 1994a); however, territories have
a clustered distribution (Greer and Anderson 1989). Territories are multi-purpose, used
for both breeding and foraging, and are typically located on barren, south-facing slopes
(With 1994a). Mickey (1943) and Greer and Anderson (1989) did not find any
interspecific territoriality for this species; however, there have been subsequent reports of
17
aggression against intruding Lark Buntings, Horned Larks, and Chestnut-collared
Longspurs (reviewed in With 1994a). Nests are built in shallow depressions on the
ground out in the open or beside a clump of grass, shrub, cactus, or dung (Mickey 1943,
With and Webb 1993). Preferred nest sites are in dry upland with low vegetation cover
(Greer and Anderson 1989).
2.2.9 Chestnut-collared Longspur (Calcarius ornatus)
Chestnut-collared Longspur is a bird of the dry, treeless, open plains, and is most at home
on the ground (Hales 1927). It is endemic to North American grasslands (Mengel 1970),
breeding in a restricted area from southeastern Alberta to southwestern Manitoba in the
north and Montana and northern South Dakota to the south (Hill and Gould 1997).
Spring migrants arrive on breeding grounds in April and depart for wintering grounds in
south-central United States and north-central Mexico in mid-late September (Hill and
Gould 1997).
Typical breeding habitat consists of arid, short or mixed-grass prairie, preferably
recently grazed or mowed, with minimal litter accumulation and shrub cover (Kantrud
and Kologiski 1983, Davis and Duncan 1999, Davis et al. 1999). This species frequents
heavily grazed pastures (Kantrud 1981). Native grasslands are preferred, but haylands
and croplands are also used if vegetation is of suitable height and density (Dechant et al.
1998b, Davis et al. 1999). Chestnut-collared Longspur prefers denser, taller vegetation
than McCown‘s Longspur (Ehrlich et al. 1988).
Territory size ranges from 0.25-4 ha (Saskatchewan: 0.4-0.8 ha), with larger territories
situated in marginal habitats (Hill and Gould 1997). Territories are multi-purpose, used
for both breeding and foraging, though individuals may also leave to forage elsewhere
18
(Hill and Gould 1997). Territories do not overlap, but may exhibit a clustered
distribution, resulting in locally high population densities (Hill and Gould 1997).
Territorial boundaries are aggressively defended against conspecifics at least early in the
season (Hill and Gould 1997). Interspecific territoriality has been observed with Horned
Lark, Baird‘s Sparrow, McCown‘s Longspur, Western Meadowlark, Savannah Sparrow,
and Brown-headed Cowbird (Hill and Gould 1997). Nest construction occurs from late
April to early May. Nests are built in a shallow depression on the ground in sparse
vegetation, usually by a clump of grass or dung (Hill and Gould 1997). Nests may be
concealed by grasses or a small shrub or they may be left exposed (Dechant et al. 1998b).
2.2.10 Red-winged Blackbird (Agelaius phoeniceus)
Red-winged Blackbird is possibly the most abundant bird in North America and is
widespread across the continent (Yasukawa and Searcy 1995). It is a permanent resident
in most of the United States and Mexico, but most Canadian and northern United States
populations migrate to the southern United States in the fall (Yasukawa and Searcy
1995). Migrating populations arrive on breeding grounds by mid-May and depart
September-October (Yasukawa and Searcy 1995).
Red-winged Blackbird is primarily associated with freshwater marshes (Forcey et al.
2008), but will nest in a variety of wetland and upland habitats, including roadside
drainage ditches, saltwater marshes, tame pastures, agricultural fields, and suburban areas
(Yasukawa and Searcy 1995, Chapman et al. 2004). This species tolerates only light
grazing and is greatly reduced by heavy grazing (Kantrud 1981).
Breeding territories are clearly delineated and aggressively defended against both
conspecifics and other species (Yasukawa and Searcy 1995), though unsuccessfully
19
against Yellow-headed Blackbirds, which typically exclude Red-winged Blackbirds from
optimal habitat where the species coexist (Willson 1966). Territories exhibit a clumped
distribution for this colonial species (Ehrlich et al. 1988). Territory size varies greatly,
but is typically smaller in wetland habitat (mean = 0.16 ha) than in upland habitat (mean
= 0.29 ha). Territories are multi-purpose, used for both breeding and foraging, but
individuals will leave their territory to forage in communal areas and to seek out extrapair copulations (Yasukawa and Searcy 1995). Nest construction occurs from mid-May
to early June in Saskatchewan (Yasukawa and Searcy 1995). As with breeding habitats,
nest sites vary greatly. Nests are built on a variety of substrates, typically within a shrub
beside a creek or woven into emergent vegetation in wetlands (Hales 1927, Ehrlich et al.
1988). In upland habitat, nests are usually built on or near the ground (Yasukawa and
Searcy 1995).
2.2.11 Western Meadowlark (Sturnella neglecta)
Western Meadowlark is one of the most abundant and widespread birds of open habitats,
breeding from the western half of Canada down to the southwestern United States and
north-central Mexico and wintering in the western half of the United States down to
central Mexico (Lanyon 1994). This species is a permanent resident throughout most of
its range, but migrates from colder northern and central breeding grounds and from
higher elevations (Lanyon 1994). Populations breeding in the Canadian prairies arrive in
late March-early April and frequently remain until late October (Hales 1927).
Native grasslands and pastures are the preferred breeding habitats for Western
Meadowlark, but this species will breed in a variety of open habitats, including hay
fields, cropland borders, orchards, and seeded pastures (Lanyon 1994, Chapman et al.
20
2004). This species tolerates a wide range of grazing intensities and is found in an
equally wide range of vegetation heights and densities (Kantrud 1981, Dechant et al.
1999b).
Territories range in size from 1.2-13 ha across the species‘ range (Manitoba: mean = 7
ha) and are multi-purpose, used for both breeding and foraging (Lanyon 1994). Males
establish and defend territories up to four weeks prior to the arrival of females (Lanyon
1994). Interspecific territoriality is generally not observed, except with Eastern
Meadowlark (Sturnella magna) where these two species coexist (Wiens 1969, Ehrlich et
al. 1988). The breeding season runs from late March-August. Nests are concealed on the
ground, often in shallow depressions and usually in relatively dense vegetation
culminating in a domed canopy over the nest (Ehrlich et al. 1988, Lanyon 1994).
2.2.12 Brown-headed Cowbird (Molothrus ater)
Brown-headed Cowbird is believed to have originally been restricted to the Great Plains,
where this species followed herds of bison and fed on insects flushed by the ungulates‘
hooves (Goguen and Mathews 2001). Transformation of the land to agricultural and
suburban landscapes following European settlement has permitted the dispersal of this
species to previously uninhabited areas (Lowther 1993). Today it is widespread across
North America (Goguen and Mathews 2001), breeding throughout the United States to
central Canada in the north and south to central Mexico and wintering in the southern and
eastern United States to central Mexico (Lowther 1993).
Brown-headed Cowbird prefers open habitats interspersed with scattered trees or tall
woody vegetation (Lowther 1993) and is rare or absent where no trees or woody
vegetation exists (Mengel 1970); however, this species is found in a wide variety of
21
habitats where these conditions are met, including native prairies, native and seeded
pastures, croplands, roadsides, forest edges and clearings, wetlands, and suburban areas
(reviewed in Shaffer et al. 2003).
Brown-headed Cowbird is an obligate brood parasite, laying its eggs in other species‘
nests and leaving the hosts to care for the young (Ehrlich et al. 1988). Red-winged
Blackbird, Vesper Sparrow, and Clay-colored Sparrow are common host species (Ehrlich
et al. 1988). Territories are used for mating and egg laying, which occur in the mornings
(Lowther 1993). Afternoons are spent foraging both in and out of the territory, in short
vegetation, frequently alongside livestock (Morris and Thompson 1998, Goguen and
Mathews 2001). Brown-headed Cowbird is highly gregarious (Ehrlich et al. 1988).
Territories are defended against conspecifics, but there is some overlap (Lowther 1993).
There is no indication of interspecific territoriality for this species, though host species
often respond aggressively to Brown-headed Cowbirds near their nests (Lowther 1993;
Hill and Gould 1997).
2.3 Habitat selection by grassland birds
Grassland birds must select suitable breeding habitat to meet their nesting and foraging
needs. Many factors influence the decisions an individual bird makes when selecting
habitat, such as availability of nest sites, cover from predators, food availability, and
presence of foraging sites (Wiens 1989b). Optimal habitats provide the greatest food
availability and substrates for efficient foraging, lowest competition and predation, and
one or more (for polygynous species) preferred nest sites (Greer and Anderson 1989).
Birds use various cues to select suitable breeding habitat, including habitat structure
(Coppedge et al. 2008), plant composition (Rotenberry 1985), patch size and edge area
22
(Johnson and Igl 2001, Fletcher and Koford 2002, Davis 2004), microclimate (With and
Webb 1993), population density (Cody 1985b), and community assemblage (Wiens
1989b).
Breeding territories of most grassland passerines are multi-purpose in that they are
used for all breeding season activities (i.e., courtship, mating, nesting, foraging). Thus,
birds must seek out habitats that meet all these needs, which may require different
microhabitats (Derner et al. 2009). Great variability exists in territory size for most
species depending on year, location, population density, and resource availability;
however, there is a general tendency for territory size to increase as suitability of habitat
decreases (Schoener 1968, Cody 1985b). When population densities are high, optimal
habitat becomes saturated, forcing some individuals to occupy sub-optimal or marginal
habitat instead (Van Horne 1983). Marginal habitat is poorer in resources, so individuals
require larger territories to meet their needs (Cody 1985a). Thus, one would expect
fewer individuals to occupy a measured area of marginal habitat than an equal area of
optimal habitat (Wiens 1989b). This relationship between population density and habitat
quality is not always realized, however, particularly at high densities, when many
individuals are forced into marginal habitats so that densities there are also high (Van
Horne 1983). Van Horne (1983) proposed that such disassociation between population
density and habitat quality is more likely to occur in common species and in resourcelimited habitats, since rarer species are far less likely to saturate suitable habitat and
habitats with ample resources are less likely to become saturated. Studies of habitat
occupation in grassland birds indicate that habitats are frequently not saturated with
individuals despite their apparent suitability (Wiens and Rotenberry 1981, Greer and
23
Anderson 1989). Further, food resources do not appear to be a major limiting factor in
grassland communities (Wiens 1973, Wiens and Rotenberry 1979), as abundance of
invertebrates (the primary food source for young and a major portion of the adult summer
diet for most grassland passerines [Maher 1979, Nocera et al. 2007]) is high during the
breeding season (Wiens 1977). Also, many grassland passerines exhibit similar diets and
foraging ecologies (Maher 1979, Wiens and Rotenberry 1979), yet low interspecific
territoriality, which further suggests that food resources are not limited in these systems
(Wiens 1989b).
Bird-habitat relationships may also be distorted by site fidelity. Individuals may
return to a previous breeding area or territory even if more suitable sites are available
(Wiens 1989b). However, loyalty to a site appears to be decided based on past
reproductive success either for the individual bird (Haas 1998, Hoover 2003) or for
conspecifics breeding in the area (Bollinger and Gavin 1989, Nocera et al. 2006). Thus,
birds do not appear bound to particular sites, but instead choose to return to breeding
areas or territories that have resulted in successful breeding seasons in the past, whereas
sites that were not productive in the past may be abandoned (―win-stay, lose-shift‖
strategy [Schlossberg 2009]). Site fidelity is expected to be more common in species
breeding in relatively stable environments where past reproductive results should be good
predictors of future ones (Schlossberg 2009, but see Switzer 1993). In highly unstable
environments such as the central grasslands, site fidelity may be disadvantageous because
of the associated uncertainty of reproductive success between years in these ecosystems
(Cody 1985b). Instead, nomadism is expected to be more common in such environments,
with species settling in areas that provide currently suitable breeding habitat (Andersson
24
1980, Greenwood and Harvey 1982), often resulting in high annual variability in local
population densities (Jones et al. 2007).
Site fidelity has been documented for a number of grassland-breeding passerines,
including Horned Lark (Camfield et al. 2007), Clay-colored Sparrow (Schlossberg 2009),
Brewer‘s Sparrow (Rotenberry et al. 1999), Vesper Sparrow (Jones and Cornely 2002),
Savannah Sparrow (Wheelwright and Rising 1993), Chestnut-collared Longspur (Hill and
Gould 1997), Red-winged Blackbird (Beletsky and Orians 1987), Western Meadowlark
(Lanyon 1994), and Brown-headed Cowbird (Dufty 1982), though return rates vary
among years, breeding sites, sexes, and studies. Many of these species, however, are not
grassland endemics and return rates represent results from studies across their breeding
ranges, not specifically from populations breeding in the central grasslands, where annual
variations in environmental conditions and consequent unpredictability of reproductive
outcomes may be more extreme than in other breeding areas. Thus, whereas site fidelity
of Savannah Sparrow is high in some portions of its extensive breeding range (reviewed
in Wheelwright and Rising 1993), Jones et al. (2007) found this species to exhibit low
site fidelity in mixed-grass prairie. The authors also found site fidelity in mixed-grass
prairie to be low for Sprague‘s Pipit and Baird‘s Sparrow, both endemic to the Central
Plains (Mengel 1970), and for Grasshopper Sparrow, which exhibits site fidelity in other
portions of its breeding range (Dean et al. 1998). Vesper Sparrow, another wide-ranging
species, also appears to exhibit site fidelity in parts of its range, but not in the central
grasslands (Dechant et al. 2000). Winter (1999) concluded that Baird‘s Sparrow is a
highly nomadic species, with territory locations and population densities reflecting
annual habitat selection. Site fidelity also appears low for another grassland endemic,
25
McCown‘s Longspur (With 1994a). Thus, it appears that, whereas site fidelity is an
adaptive strategy for birds breeding in relatively stable environments (e.g., shrublands
and forests [Schlossberg 2009]), nomadism is more frequently observed in the central
grasslands, where individuals may benefit from opportunistically seeking out suitable
habitat in these highly unstable environments (Winter 1999, Jones et al. 2007). The
reoccupation of newly restored habitats (e.g., Conservation Reserve Program fields
[Johnson and Schwartz 1993]) and abandonment of formerly occupied habitats rendered
unsuitable (Vickery et al. 1992) by many grassland passerines further suggests that
fidelity to a site is overcome when new opportunities arise.
2.4 Habitat associations of grassland birds
Grassland birds appear to be associated more so with habitat structure than with plant
species composition (Sutter and Brigham 1998, Davis and Duncan 1999, Chapman et al.
2004, Coppedge et al. 2008, but see Rotenberry 1985). Further, plant species
composition and habitat structure tend to be correlated, making distinctions between the
effects of one or the other on avian communities difficult (Wiens 1989b). Thus, the
apparent associations between some bird species with specific plant species may be
attributable to the structural characteristics of the plant. Shrub-steppe birds (e.g.,
Brewer‘s Sparrow) in particular appear associated with specific shrub species (Wiens and
Rotenberry 1981); however, this may be due to the physiognomic differences among
shrub species. For example, Clay-colored Sparrow nests primarily in western snowberry,
which is denser than sagebrush and offers better concealment from predators (Knapton
1994). In the central plains, a preference for blue grama grass (Bouteloua gracilis) by
McCown‘s Longspur (reviewed in Dechant et al. 1999a) is likely due to this grass‘ short
26
height. Any associations with plains prickly pear cactus (Opuntia polyacantha) in the
northern plains may be attributable to this cactus species‘ predominance in the region.
Numerous studies have documented the relationship between grassland bird
abundance and habitat structure (e.g., Wiens 1973, Rotenberry and Wiens 1980, Kantrud
1981, Kantrud and Kologiski 1983, Herkert 1994, Patterson and Best 1996, Davis et al.
1999, Madden et al. 2000, Chapman et al. 2004, Nocera et al. 2007, Coppedge et al.
2008). In the mixed-grass prairie, avian species characteristic of both short- and tallgrass prairie are present, so habitat associations tend to follow a gradient from short to
tall vegetation (Knopf 1996). For example, Horned Lark, McCown‘s Longspur, and
Chestnut-collared Longspur are associated with short-to-medium-height vegetation
(Kantrud and Kologiski 1983, Davis 2004), whereas Savannah Sparrow and Baird‘s
Sparrow are associated with medium-height to tall vegetation (Wiens 1973, Davis et al.
1999). Clay-colored Sparrow and Brewer‘s Sparrow are associated with shrub cover
(Wiens 1973, Madden 2000), whereas Common Yellowthroat is primarily a riparian
species (Taylor 1986, Patterson and Best 1996). For all of these species to coexist,
variation in habitat structure must exist (Chapman et al. 2004, Fritcher et al. 2004, Derner
et al. 2009).
2.5 Habitat heterogeneity and grassland bird diversity
The habitat heterogeneity hypothesis predicts that species diversity is positively
correlated with habitat heterogeneity (MacArthur and MacArthur 1961). The theory
holds that as variability in a habitat increases, more microhabitats are created, which
permits more species to inhabit a given area, each species fitting into a specific niche
within the larger habitat. Literature regarding the effects of habitat heterogeneity on
27
grassland bird diversity is limited (Winter et al. 2005); however, the relationship between
habitat heterogeneity and species diversity has been extensively documented in other
taxa, including small mammals (Stinson 1978, Williams and Marsh 1998, Cramer and
Willig 2002), forest birds (MacArthur and MacArthur 1961, Hanowski et al. 1997,
Davidar 2001), waterfowl (Elmberg et al. 1994), reptiles and amphibians (Pianka 1967,
Ricklefs and Lovette 1999), and invertebrates (Siemann et al. 1998, Weibull et al. 2000).
Nevertheless, this relationship does not appear to be universal across taxa or locations.
Busack and Jaksic (1982) found no support for the habitat heterogeneity hypothesis in
their study of Iberian herpetofauna, nor did Ricklefs and Lovette (1999) in their study of
Lesser Antillean bats. Ricklefs and Lovette (1999) suggested that such disparities in the
relationship between habitat heterogeneity and species diversity among taxa may be due,
in part, to stronger habitat specializations in some taxa. Specialized taxa are predicted to
be more strongly influenced by habitat heterogeneity than taxa in which species tend to
be generalists. Many grassland bird species do exhibit strong habitat associations, so if
the above argument holds true, one would expect these species to be strongly influenced
by habitat heterogeneity as well.
Patterns of heterogeneity in grassland vegetation structure may influence patterns of
avian distribution and abundance because of the strong habitat associations of many
grassland birds (Fuhlendorf and Engle 2001, Fritcher et al. 2004, but see Wiens 1974);
however, these habitat associations are not always consistent (Maurer 1986). For
example, Sprague‘s Pipit is generally associated with taller vegetation (Kantrud and
Kologiski 1983, Knopf and Samson 1997), yet Kantrud (1981) found a higher abundance
of Sprague‘s Pipits in shorter grasses in mixed-grass prairie. Similarly, Baird‘s Sparrow
28
is generally described as a medium-tall grass specialist (Kantrud 1981, Davis et al. 1999),
yet Madden et al. (2000) found Baird‘s Sparrow associated with short, sparse vegetation.
Some species, such as Savannah Sparrow, are associated with a broad range of habitats
across their range (Vickery et al. 1999), but may exhibit specific habitat preferences
within a localized area. Consequently, it may be difficult to predict how habitat
heterogeneity affects an avian community without first knowing the avian-habitat
associations within the specific area of interest.
Spatial scale also influences the relationship between habitat heterogeneity and
grassland bird diversity. Each species operates within a boundary of scales in which it
perceives differences in its environment (i.e., heterogeneity); smaller scales are perceived
as homogenous and larger scales are beyond the lifetime range of the species (Kotliar and
Wiens 1990). Thus, habitat associations occur within this set boundary of spatial scales.
However, habitat associations at one spatial scale may disappear or reverse at other
spatial scales (Wiens 1989a). Wiens (1986) found that local-scale habitat associations in
a shrub-steppe bird community disappeared at larger scales. Thogmartin and Knutson
(2007), in their study of habitat associations of forest birds, and Böhning-Gaese (1997),
in her study of scale effects on European birds, came to similar conclusions. Specifically,
Böhning-Gaese (1997) found that within-habitat characteristics (e.g., vegetation height,
horizontal heterogeneity) were important at fine scales, whereas between-habitat or
landscape characteristics (e.g., habitat area, presence of water) became important at larger
spatial scales. This demonstrates the importance of understanding which spatial scales
are relevant in avian habitat selection and how they affect the community of interest,
since knowledge of relationships at one scale does not infer knowledge of relationships at
29
other scales (Winter et al. 2005). Which spatial scales are of ecological significance
depends on the species, patterns, and processes of interest. Relevant scales for grassland
birds include territory size (the area occupied and defended during the breeding season)
and home range (all areas occupied by a species throughout its life cycle), so habitat
characteristics at these scales may be important predictors of avian diversity (Koper and
Schmiegelow 2006).
2.6 Measures of heterogeneity
Various methods exist to measure heterogeneity in the landscape. One method is to
measure the extent of variability in the sampled data. Higher variability in the data
indicates a higher level of heterogeneity in the study system. Standard deviation (SD)
and coefficient of variation (CV) are two statistical measures frequently used to
determine the degree of variability in a data set. Standard deviation is a measure of
spread around a mean data point. A larger SD value indicates more variability in the data
because data points are spaced farther apart. In biological terms, this equates to more
heterogeneity in a given study variable. However, variables that have higher mean values
will generally also have higher variability around those mean values, so SD will be
higher. Hence, SD values tend to correlate with their respective means. Coefficient of
variation is a standardized value that controls for differences among mean values by
dividing SD values by their respective means. For this reason, CV is traditionally
preferred over SD as a measure of heterogeneity in ecological studies (e.g., Lueders et al.
2006). However, CV can be too extreme of a control for the correlation between mean
and SD, such as when two variables have the same SD, but different mean values. In this
scenario, the relative or proportional variation (as calculated by CV) will differ between
30
the variables even though absolute or measurable variation (as measured by SD) is the
same (Sørensen 2002). With respect to avian habitat selection, differences in the mean
values of habitat characteristics may have important biological consequences. Consider a
hypothetical example: at one site, the height of vegetation with a mean height of 2 cm
may vary between 1.5 and 2.5 cm, whereas at a second site, the height of vegetation with
a mean height of 20 cm may vary between 15 and 25 cm. If CV is used as the measure of
heterogeneity in this scenario, both sites will generate the same value (0.25) and will
appear equally heterogeneous, whereas if SD is used as the measure of heterogeneity, the
latter site will generate a higher value (5.0) and will appear more heterogeneous than the
former site (SD = 0.5). Arguably, from the perspective of a bird, the latter site may be
perceived as patchier than the former, which may simply be perceived as ―short‖ even if
proportionally it varies by the same amount. This effect is magnified when diverse or
multiple habitats are of interest. Within a single habitat type, variability in habitat
structure may be relatively low; however, if different habitat types are included in the
analysis, large differences in structure between the habitats (e.g., 2 cm versus 20 cm
mean vegetation height) could be misinterpreted to be similar when CV is used as the
measure of heterogeneity. Furthermore, when the two sites are grouped together into a
larger experimental unit, SD is averaged into an intermediate value (2.75), whereas CV
remains the same (0.25). Thus, there is the potential for any scaling effects that exist in
the system to go undetected. Three important criteria to consider when choosing a
heterogeneity measure are the identity of the study subjects, the location (ecosystem,
habitat) of the study, and the spatial scale(s) addressed by the study. Coefficient of
variation may be appropriate for single-scale studies within a single habitat type, whereas
31
SD may be more appropriate for multiple-scale studies and for studies of multiple habitat
types. Careful consideration of the biological meaning of the results is required when
deciding which measure is more biologically relevant in a particular study.
2.7 Effects of grazing on grassland birds
Grazing has been a major driver in the evolution of the prairies (Bock et al. 1993, Bragg
and Steuter 1996). Historically, large herds of bison (Bos bison) numbering in the tens of
millions roamed across the Great Plains (Knopf and Samson 1997). Many grassland
birds evolved under the periodic, intensive grazing pressure associated with these
nomadic herds and are adapted to the mosaic of habitats that selective grazing creates
(Knopf 1996, Knopf and Samson 1997, Fuhlendorf and Engle 2001); however, responses
to grazing vary among species (Kantrud and Kologiski 1983, Milchunas et al. 1998) and
locations (Kantrud 1981, Madden et al. 2000). This may be due to the variability of plant
responses to grazing. Studies suggest grazing affects grassland birds indirectly by
altering vegetation (Fondell and Ball 2003), since vegetation is a strong determinant of
avian abundance in the breeding season, when individuals seek out appropriate breeding
habitat (Wiens 1973, Fletcher and Koford 2002). Grazing has been shown to both
increase and decrease heterogeneity in vegetation, depending on the intensity of grazing
and level of plant productivity (Cid et al. 1991, Bakker et al. 2006). In mixed-grass
prairie, grazing at moderate intensities generally appears to increase plant species
diversity by reducing the competitive advantage of dominant species (Collins and Barber
1985, Hartnett et al. 1997, Harrison et al. 2003). Moderate grazing also creates structural
patches in the landscape because grazers are able to selectively forage on preferred
species (Hartnett et al. 1997). Through this behavioural mechanism, grazing promotes
32
both vertical and horizontal heterogeneity in vegetation structure by reducing vegetation
height, increasing basal cover of grass and cover of some forbs, and decreasing woody
species (e.g., shrubs) in grazed patches (Rotenberry and Wiens 1980, Klute et al. 1997,
Stohlgren et al. 1999). Additionally, grazing indirectly increases the amount of bare
ground because of the associated trampling of vegetation and deposition of animal waste
by grazers (Hartnett et al. 1997).
Because many grassland birds are associated with specific habitat characteristics, and
moderate grazing promotes habitat heterogeneity, it follows that a moderately grazed
landscape may also support a greater diversity of species, including those that prefer both
shorter or taller vegetation and various configurations of grasses, forbs, and shrubs.
Results from Kantrud (1981) and Kantrud and Kologiski (1983) support this hypothesis;
the authors found that, among sites being grazed at various intensities, avian diversity
was highest in moderately grazed mixed-grass prairie. Klute et al. (1997) found avian
diversity in tall-grass prairie to be higher in moderately grazed pastures than in ungrazed
Conservation Reserve Program fields. In contrast, intensive grazing generally reduces
avian diversity (Fondell and Ball 2003); heavily grazed habitat tends to be more
homogenous (Fuhlendorf and Smeins 1999), supporting only those species that prefer short
vegetation, though abundance of these species may be high (Klute et al. 1997).
2.8 Spatial patterns in grazing
Grazing herbivores make foraging decisions at six spatial scales: home range (landscape),
camp (e.g., pasture), feeding site (e.g., particular area within pasture foraged for a few
hours), patch, feeding station (group of plants within immediate reach), and bite (plants
ingested) (Vallentine 2001). These scales follow a hierarchical organization, in that
33
decisions made at larger scales limit the decisions that can be made at smaller scales
(Vallentine 2001). How these foraging decisions affect vegetation, and consequently
grassland bird diversity, depends on the pre-existing state of the vegetation.
Heterogeneity in vegetation structure exists in the absence of grazing due to various
abiotic (slope, soil, moisture content) and biotic (small mammal disturbances, insect
grazing) factors (Cid et al. 1991, Stohlgren et al. 1999, Vallentine 2001, Harrison et al.
2003, Joern 2005). Patterns of avian distribution and abundance are likely to be
influenced by inherent heterogeneity in vegetation because of the strong habitat
associations of many grassland bird species (Fuhlendorf and Engle 2001, Fritcher et al.
2004, but see Wiens 1974). Because of the influence that factors such as slope, quality or
desirability of forage, and distance to water have on grazing distribution (Adler et al.
2001, Vallentine 2001, Rook and Tallowin 2003, Fontaine et al. 2004), patterns of
heterogeneity in the presence of grazing may differ from those inherent in the vegetation.
Consequently, patterns of avian distribution and abundance may also differ in the
presence of grazing. However, if inherent heterogeneity is high, patterns of grazinginduced heterogeneity may be less pronounced and more difficult to predict (Vallentine
2001). Conversely, heavy grazing can override pre-existing heterogeneity in vegetation,
rendering it more homogenous (Fuhlendorf and Smeins 1999). Thus, knowing the preexisting level of heterogeneity is important when assessing the impacts of grazing on
grassland bird diversity.
Because grazing distribution results from foraging decisions, which operate at various
spatial scales (Vallentine 2001), it is not surprising that effects of grazing on vegetation
also depend on scale. In their study of Sonoran grasslands, Fuhlendorf and Smeins
34
(1999) found that grazing had positive, negative, and no influence on heterogeneity,
depending on the scale of observation; for example, among ungrazed, moderately grazed,
and heavily grazed sites, those that were moderately grazed had the lowest level of
heterogeneity at intermediate scales, but the highest level of heterogeneity at large scales.
Glenn et al. (1992) found that, in tallgrass prairie, heterogeneity was highest in ungrazed
treatments at local scales, but highest in grazed treatments at regional scales. Vegetation
structure can be homogenous within a feeding station, yet heterogeneous at patch scale,
because feeding stations are separated by undesirable vegetation. Conversely, vegetation
structure can be heterogeneous within a feeding station because only the most desirable
vegetation is selected per bite. At the patch scale, this can give the appearance of
homogeneity because fine-scale patterns of heterogeneity within feeding stations are
―lost‖ or averaged out across the larger scale (Wiens 1989a).
Furthermore, the process of grazing can itself vary across spatial scales. For example,
WallisDeVries et al. (1999) found that, although cattle selected among feeding stations in
an experimental mosaic of habitat patches, foraging within feeding stations was random
and homogenous. Similarly, Wallace et al. (1995) found that bison foraged randomly
within patches, but selected among feeding sites within the landscape. These results
suggest that heterogeneity at the smaller scales of feeding station and bite may be
―invisible‖ or irrelevant to the grazer because these scales fall below the range of scales
in which the grazer perceives differences in its environment (Kotliar and Wiens 1990,
Adler et al. 2001). Moreover, such observations are consistent with scale theory, namely
that not only do community characteristics vary with scale, but the processes that affect
community characteristics also vary with scale (Wiens 1981). Again, this demonstrates
35
the importance of understanding which scales are relevant to the species, processes, and
patterns of interest, while stressing the inability to extrapolate from one scale to another.
Although limited information exists on the influence of scale on interactions between
grazing and avian diversity, it is probable that the effects of grazing on grassland bird
diversity vary across spatial scales because: (1) effects of grazing on grassland vegetation
vary with scale (e.g., Glenn et al. 1992, Fuhlendorf and Smeins 1999, reviewed in Adler
et al. 2001); (2) many grassland birds exhibit strong habitat associations; and (3) habitat
associations of birds vary with scale in grasslands and other ecosystems (e.g., Wiens
1986, Böhning-Gaese 1997, Coreau and Martin 2007, Thogmartin and Knutson 2007).
2.9 Grazing as a management tool
Livestock production is currently one of the biggest land uses in the northern plains
(Lueders et al. 2006); most land unfit for agriculture due to poor soils is used as
rangeland (Iwaasa and Schellenberg 2004), particularly for domestic cattle (Bos taurus).
Cattle have largely replaced bison as the primary drivers of ecological disturbance in the
northern mixed grasslands (Knopf and Samson 1997, Willms et al. 2002). While the two
species have similar foraging ecologies (both are generalist grazers of predominantly
grasses [Hartnett et al. 1997]), resulting in similar grazing outcomes (reduction of
standing crop [Bragg and Steuter 1996]), there are some differences in their foraging
behaviours. Cattle and bison exhibit high dietary overlap, but cattle are more selective,
while bison have a narrower dietary breadth (Hartnett et al. 1997). Bison are highly
nomadic, whereas cattle are more sedentary (Hartnett et al. 1997). Bison use water more
efficiently, so will forage farther from water and for longer periods than cattle (Fleischner
1994). Historically large numbers of bison would have produced larger patches of grazed
36
habitat that were more intensively grazed than those produced by smaller herds of freeranging cattle; however, this intensive grazing would have been traded off with longer
rotation periods, giving the vegetation time to recover (Askins et al. 2008). Thus,
patterns of heterogeneity created by bison grazing would exist at a broader scale than
those created by cattle grazing (Milchunas et al. 1998). Despite these differences, it is
generally believed that surrogate grazing by cattle is more beneficial to grassland
communities than no grazing at all (Askins et al. 2008). Furthermore, the replacement of
bison with cattle has not had a major impact on native grasslands or their faunal
communities (Hartnett et al. 1997). Instead, it is the intensive management associated
with cattle production (e.g., addition of fences and roads, fragmentation of land, etc.) that
has had the most detrimental effects on these communities (Knopf and Samson 1997).
Some researchers have proposed that grazing is necessary for the sustainability of the
mixed-grass prairie (Bragg and Steuter 1996, Harrison et al. 2003); however, grazing
systems currently in use, such as rotational grazing, may not adequately create or
preserve suitable habitat for grassland birds because they promote uniform land use with
the goal to maximize forage intake by livestock (Derner et al. 2009), resulting in
homogenous habitat suitable for only a portion of the avian community (Holechek et al.
2001, Coppedge et al. 2008). Instead, systems that permit selective foraging would more
likely create the heterogeneous habitat required to support a diversity of species (Rook
and Tallowin 2003). Some researchers have argued that grazing is detrimental to
grassland communities and that areas of permanent grazing exclusion should be
established (e.g., Bock et al. 1993, Fleischner 1994); however, their arguments focus on
grazing practices that do not prioritize ecological conservation, but instead focus solely
37
on livestock productivity. The uniformity of grazing in some systems is probably more
detrimental to grassland birds than the actual presence of livestock, since grassland birds
appear indirectly responsive to grazing through changes in habitat (Knopf and Samson
1997, Fondell and Ball 2003) and not directly to grazing livestock per se (Bock and
Webb 1984). This argument is strengthened by the fact that declines in many grassland
bird populations began (and continue) after rangeland conditions were ‗improved‘ to
maximize sustainable livestock production (Fuhlendorf and Engle 2001). In the mixedgrass prairie, cattle grazing may be a useful management tool for grassland bird
conservation when implemented at low to moderate intensities. At these intensities,
grazing is selective, promoting habitat heterogeneity and thus, to some extent, mimicking
historical bison grazing (Hartnett et al. 1997).
2.10 Research and management needs for grassland bird conservation
2.10.1 Multiple-scale studies and ecosystem management
Choosing appropriate spatial scales of study is critical, particularly if the results are to be
applied towards management decisions. Results often differ across spatial scales because
relationships among ecosystem components vary with scale (Wiens 1989a); therefore,
extrapolating from one scale to another can lead to erroneous conclusions about the
community being studied and, consequently, the implementation of inappropriate
management practices. Furthermore, management decisions regarding large tracts of
land are often based on small-scale studies, yet results from small-scale studies may not
reflect the dynamics of the ecological community at larger scales (Winter et al. 2005).
Spatial scales of study should capture the spatial dynamics of the species, communities,
and processes that are of interest (Wiens 1981, Gordon et al. 1997). Multiple-scale
38
studies are particularly valuable because they have greater potential to capture
community dynamics that may be overlooked in single-scale studies, and because they
can capture scale-dependent changes in relationships among ecosystem variables.
Conducting multiple-scale studies is important in grassland ecosystems because both the
effects of disturbances on grassland communities and the disturbances within themselves
vary across scales (WallisDeVries et al. 1999, Vallentine 2001). Further, responses to such
disturbances by grassland communities also vary across scales (Glenn et al. 1992,
Fuhlendorf and Smeins 1999). The need for multiple-scale studies is particularly urgent for
prairie birds because they are a high conservation priority, yet our understanding of how
grazing affects prairie bird diversity is limited.
2.10.2 Habitat restoration and conservation
While protecting portions of grasslands in parks and wilderness areas is one positive
approach to grassland bird conservation, it is impossible to secure all the land within a
species‘ range to ensure its survival (Haslem and Bennett 2008). Sustainable
management practices on private lands are also required for long-term viability of some
species. Applying an ecosystem approach focused on habitat restoration and
conservation may be sufficient for the conservation of many grassland species (Holroyd
1995). The mixed-grass prairie is classified as an endangered ecosystem; however, there
is potential for restoration of marginal agricultural lands (Ricketts et al. 1999). In the
United States, the Conservation Reserve Program, which restores agricultural fields to
perennial grasslands, has reduced population declines for several species (Johnson and
Schwartz 1993, Herkert 1998, Niemuth et al. 2007) and even reversed regional
population trends for some species (Vickery and Herkert 2001). Similarly, in Canada, the
39
Permanent Cover Program has resulted in the resettlement of revegetated marginal lands
by many grassland species (Vickery et al. 1999). Rangelands, in particular, have strong
potential for recovery because livestock grazing imparts relatively little damage to the
landscape when stocking rates are low to moderate (Iwaasa and Schellenberg 2004).
Furthermore, rangelands are the most attractive form of agricultural land for most
grassland birds (Temple et al. 1999). Because a significant portion of mixed-grass prairie
is managed as rangelands (Ricketts et al. 1999), the opportunity exists to apply
sustainable, ecosystem-based grazing practices to these lands. However, given our
limited understanding of the ecological patterns and processes in grassland ecosystems,
the challenge remains to develop grazing systems that preserve the heterogeneity of the
landscape needed to support a variety of grassland bird species (Redmann 1995, Ricketts
et al. 1999).
40
3.0 METHODS
3.1 Study area
The study area was situated in the East Block of Grasslands National Park of Canada
(GNPC) and adjacent community pastures in southern Saskatchewan (Figure 1). As of
2007, the park protected 50,227 hectares of mixed-grass prairie between its two blocks,
with a goal of protecting 92,074 hectares once all lands were purchased (Parks Canada
2007). The lands within the East Block of GNPC had been ungrazed since their
acquisition, in some cases up to 25 years (Parks Canada 2002). The neighbouring
community pastures were owned by the provincial government and conventionally
grazed annually following a season-long grazing system.
The study area consisted of open, rolling upland prairie interspersed with riparian
lowlands and creeks. Vegetation was dominated by short to medium-tall grasses
characteristic of northern mixed-grass prairie (Bragg and Steuter 1996). Common
grasses included needlegrasses (Stipa spp.), blue grama (Bouteloua gracilis), western
wheatgrass (Pascopyrum smithii), northern wheatgrass (Elymus lanceolatus), and
bluegrasses (Poa spp.). In lowland areas, salt grass (Distichlis sricta), sedges, and reeds
were common. Forbs and shrubs were scattered across the landscape. Sagebrush
(Artemisia cana) was the most common shrub in upland areas. Western snowberry
(Symphoricarpos occidentalis) and greasewood (Sarcobatus vermiculatus) were common
in lowland areas. There was minimal invasion of exotic species (e.g., crested wheatgrass,
Agropyron cristatum, alfalfa, Medicago sativa, leafy spurge, Euphorbia esula).
Passerine species characteristic of this area included five of the six species endemic to
North American grasslands, as well as seven secondary species and a number of other
41
wide-ranging species (see Appendix I). Passerines were the dominant avian group, but
raptors, shorebirds, game birds, and waterfowl were also present.
3.2 Study design
This study was part of the first phase of a large-scale, long-term experimental study at
GNPC (see Henderson 2006 for details) following a modified Before-After-ControlImpact design (―Beyond BACI;‖ Underwood 1994). Phase 1 involved the collection of
baseline (Before/Control) data on avian and plant diversity. Nine ~300-ha pastures were
established in the East Block of GNPC, each with a similar proportion and spatial
arrangement of riparian, lowland, and upland habitat. Four conventionally grazed
pastures of similar size and topography were established in the community pastures
bordering the park (Figure 2). The community pastures were stocked with cattle for a
target 50% utilization rate, though actual utilization rates were estimated to be somewhat
lower, resulting in a light-moderate grazing pressure in these pastures. Within each
pasture, ten 100-m-fixed-radius point-count plots (Hutto et al. 1986) were set up, six in
upland habitat and four in lowland habitat, totalling 130 plots (13 pastures x 10 plots).
Plot centres were at a minimum distance of 250 m from adjacent plot centres to prevent
duplicate observations of individual birds. Within each point-count plot, eight 1-m x 0.5m habitat sampling frames were arranged so that two frames radiated from the centre of
the point-count plot in each cardinal direction, one at 25 m and the other at 75 m from
centre.
42
Figure 1. Location of the study area. Reprinted with permission from Parks Canada (2002).
43
Figure 2. Layout of the study area. Pastures 1-9 were situated in Grasslands National Park
(ungrazed). Pastures 10-13 were situated in the Mankota Community Pastures (grazed).
The ten points within each pasture indicate the approximate location of each 100-mradius point-count plot (n=130). Axis values are easting and northing coordinates (UTM
Zone 13U; NAD83). Each square on the grid represents 500 m2. Reprinted with
permission from Parks Canada: Grasslands National Park.
44
3.3 Avian surveys
Three observers conducted three rounds of point counts from 27 May to 10 June 2008. The
third round did not include six of the GNPC pastures because cattle had already been
introduced into these pastures as part of Phase 2 of the BACI project. We began
observations at sunrise and finished by 10:00 a.m. CST, when avian activity tended to
decline (Hutto et al. 1986). Also, wind speed tended to increase dramatically around 10:00
a.m., making it difficult to hear the birds. We recorded all passerines seen or heard within
the plot within a five-minute period. Only birds that made visible use of a plot were
included in calculations. This included swallows because these species forage in flight.
Observations were cancelled during rain and when wind speed reached 16-20 km/h.
3.4 Habitat surveys
Habitat surveys were conducted from 20 May to 23 May 2008 to coincide with the period
when most birds were seeking out suitable breeding habitat, establishing territories, and
building nests (Wiens 1969), though some species, such as horned lark and western
meadowlark, had already initiated clutches by this point (Maher 1979). Four individuals
worked independently to gather data. Within each habitat sampling frame, we visually
estimated percent cover of litter, dead standing vegetation, exposed moss/lichen, bare
ground, grasses, forbs, cacti, and shrubs relative to the entire area of the frame following
the methods outlined by Daubenmire (1959). Vegetation was left undisturbed during
percent cover estimation so that cover values represented the habitat in its natural state as
a bird would see it during territory and nest-site selection. We also measured vegetation
height-density, maximum vegetation height, mean vegetation height, and litter depth.
Vegetation height-density (vertical density) was measured as the height of visual
45
obstruction of a Robel pole marked at 5-cm intervals at a distance of 4 m and elevation of
1 m from the pole (Robel et al. 1970). Maximum vegetation height was measured from
the tallest plant within the frame. Mean vegetation height was determined using a
method adapted from Grant-Hoffman and Detling (2006) in which a 20-cm x 50-cm
piece of light Styrofoam was placed over the vegetation in the centre of the frame. Mean
vegetation height was then measured from a metre-stick inserted through a slit in the
centre of the piece of Styrofoam. Litter depth was measured from a metre-stick placed in
the centre of the frame.
3.5 Statistical analysis
I selected the eight most common species for relative abundance analyses. All eight
species occurred in >40% of plots and >90% of pastures. I also selected Brown-headed
Cowbird and Red-winged Blackbird for relative abundance analyses because these
species are of management interest due to their detrimental impacts on the reproductive
success of host species (i.e., Brown-headed Cowbird [Goguen and Mathews 2001]) and
on agricultural production (i.e., Red-winged Blackbird [Forcey et al. 2008]). I assessed
abundance of these two species in lowland habitat (52 plots) only because 95% and 100%
of individuals, respectively, were observed there. Both species occurred in >45% of
lowland plots and >85% of pastures. I also analyzed the abundance of Brewer‘s Sparrow
and McCown‘s Longspur, both of which were relatively rare in the study area, but are of
conservation interest. For each species, I defined relative abundance as the number of
individuals/3.2 hectares (the area of a point-count plot) averaged across the 2 or 3 rounds
of point counts for that plot. I also evaluated total bird abundance (total number of
individuals of all species/3.2 hectares averaged across rounds), species richness (number
46
of species/plot), and species diversity. Species diversity was calculated using the
Shannon Diversity Index (H′). I chose this index because it is sensitive to rare species
(Wiens 1989b), which were of interest in this study.
Plot-scale structure and heterogeneity for each habitat variable were measured as the
mean and standard deviation (SD) for that variable across the eight habitat sampling
frames within each point-count plot. Pasture-scale structure was calculated as the
average of the 10 plot mean values for each habitat variable. Pasture-scale heterogeneity
was measured by calculating the standard deviation across the 10 plot mean values for
each habitat variable. I also used the coefficient of variation (CV) as a heterogeneity
measure at both spatial scales, calculated by dividing the standard deviation by its
respective mean value for each habitat variable at each scale. I chose to use both SD and
CV for the purpose of comparing their value as heterogeneity measures in grassland
avian community studies.
Of the 12 measured habitat variables, I selected seven to be used in bird analyses.
This reduction was necessary because habitat variables tended to be correlated to varying
degrees; entering correlated variables into statistical models could lead to confounded
results, making interpretation difficult. Habitat variables were selected based on
correlation values (no greater than r =│0.6│), as well as on a review of the existing
literature on habitat associations of grassland passerines and on personal observations of
habitat use by birds in the field.
In preliminary analyses, I also added northing to statistical models as a suspect
nuisance variable representing subtle latitudinal differences in topography and moisture
47
levels within the study area; however, results indicated that northing did not significantly
influence either habitat or bird variables, so I did not include it in my final analyses.
Plot-scale relationships among avian diversity and abundance, habitat structure and
heterogeneity, and grazing were analyzed using mixed-effects models. Mixed-effects
models were particularly suitable for this study for a few reasons. Firstly, they permit
analyses at multiple scales and within nested sampling units (Aragón et al. 2007). Subsample units (in my study, point-count plots) are retained as replicates in the analysis
instead of averaging their values across the larger spatial units (in my study, pastures);
however, degrees of freedom are calculated separately for each nested spatial scale, so
they are not artificially increased at the larger scale. This feature of mixed-effects models
enabled me to analyze a hierarchical data set and maximize my power of detection, while
avoiding pseudoreplication (Bell and Grunwald 2004). Secondly, both categorical and
continuous effects (variables) can be entered simultaneously into mixed-effects models,
whereas traditional analytical approaches such as analysis of variance (ANOVA) and
linear regression permit entry of only categorical or continuous variables, respectively.
Categorizing continuous variables to fit into ANOVAs reduces statistical power of
detection and introduces artificial breaks in the data that have no real biological meaning.
In this study, both categorical (grazing treatment) and continuous (habitat characteristics)
variables were of interest as predictor variables. Thirdly, mixed-effects models include
both fixed effects (i.e., where all levels of an effect are represented; in this study, this
included grazing treatment, habitat variables, and avian variables) and random effects
(i.e., where levels of an effect are assumed random and not fully represented; in this
study, this included pasture as a random grouping variable) in the statistical model (Bell
48
and Grunwald 2004). Including random effects in statistical analyses allows for a
broader range of applicability and inference from the results gathered (Winer 1971).
Whereas conclusions from fixed effects treatments can only be applied to differences
among those treatments addressed in the study, conclusions from random effects
treatments can be extrapolated to beyond the scope of the study (Sahai and Ageel 2000).
Thus, by treating pastures as random treatments, I was able to apply my conclusions to
beyond the study area and make broader generalizations about avian-habitat-grazing
relationships within the northern mixed-grass prairie.
The effects of grazing on habitat structure and heterogeneity at plot scale were
analyzed using linear mixed-effects models (LMEs). Several variables were logtransformed to better meet the linear assumptions of the model and are noted as such in
the results. LMEs were also used for the analysis of grazing treatment, habitat structure,
and habitat heterogeneity on avian richness, diversity, and abundance of all but the two
rare species at plot scale. Abundance values for Clay-colored Sparrow and Western
Meadowlark were first square-root-transformed to better approximate normality prior to
running the model. I selected five of the seven habitat variables for analyses of Brownheaded Cowbird and Red-winged Blackbird to avoid overparameterization, as I only used
the 52 lowland plots for these species. See Appendix II for a sample model and
interpretation. Generalized linear mixed-effects models (GLMEs) with a binomial
distribution were used to analyze the effects of grazing on the occurrence of Brewer‘s
Sparrow and McCown‘s Longspur, as their abundances were too low to evaluate effects
of grazing on relative abundance. GLMEs are an extension of LMEs that permit the
analysis of data for which error does not follow a normal distribution (McCulloch et al.
49
2008). Abundance data for Brewer‘s Sparrow and McCown‘s Longspur was transformed
into binary data; however, since both species occurred in low abundance (mostly one,
maximum two individuals in a plot), binary data was similar to observed abundance
values.
Personal observations in the field suggested that habitat and avian community
characteristics varied between upland and lowland habitat. Exploratory analyses of the
effects of habitat type (upland/lowland) on avian and habitat variables using mixed
models confirmed that the majority of response variables did differ between habitat types,
so I also analyzed the effects of grazing on the avian community and habitat variables in
each habitat type separately to determine if grazing influence differed between the two
habitat types. Brown-headed Cowbird and Red-winged Blackbird were excluded from
habitat analyses since these species were almost exclusively found in lowland habitat
only. McCown‘s Longspur was also excluded from the analyses because this species was
observed in upland habitat only.
Pasture-scale relationships among avian diversity and abundance, habitat structure and
heterogeneity, and grazing were analyzed using generalized linear models (GLMs). Most
habitat and avian response variables fit the normal distribution model; however, several
variables were log-transformed to better approximate normality prior to running the
model and are noted as such in the results. To avoid overparameterization at the pasture
scale, I only used vegetation height-density, bare ground, and shrub cover to analyze the
effects of habitat structure and heterogeneity on birds at this scale. I chose these three
variables because they were the most structurally distinct and because I felt they were the
most biologically relevant for grassland passerines based on what was known of avian-
50
habitat associations from the literature (e.g., Wiens 1969, Wiens 1974, Roth 1976,
Madden et al. 2000, Harrell and Fuhlendorf 2002, Coppedge et al. 2008).
The effects of scale on avian species richness, diversity, and habitat heterogeneity
were assessed using Wilcoxon rank sum tests. These tests are the nonparametric
equivalent of independent Student‘s t-tests and thus were appropriate for these analyses
because most response variables did not follow normal distributions (Wilcoxon 1945).
Plot-scale heterogeneity values were averaged for each pasture to meet the criteria of the
test (equal sample sizes) and to permit the use of standard deviation as a heterogeneity
measure in these analyses. Standard deviation is strongly influenced by sample size in
that larger samples will generally have higher standard deviation values than smaller
samples. Thus perceived differences in heterogeneity between the scales could be
artefacts of unequal sample sizes if plot-scale values were not averaged within pastures. I
separated the response variables into grazed and ungrazed treatments prior to analysis so
that any differences in scale effects between treatments (i.e., if heterogeneity differed
between scales in one treatment, but not the other) could be detected.
I used an alpha value of 0.1 for all statistical analyses. I chose this value to reduce the
risk of making Type II errors (i.e., falsely concluding there is no effect of a treatment).
Reducing the risk of making Type II errors comes at the expense of increasing the risk of
making Type I errors (i.e., falsely concluding a treatment effect exists); however, Type II
errors are more serious in conservation biology and when management decisions depend
on scientific research. For example, failing to detect a negative influence of grazing on a
threatened bird species is much more dangerous than incorrectly concluding that grazing
negatively influenced the species, when in reality grazing had no effect on the species.
51
4.0 RESULTS
4.1 Avian community structure
Twenty-three passerine species were recorded within the study area (Appendix I). The
number of species within one pasture ranged from 10-16 and within one plot from 2-12.
Baird‘s Sparrow was by far the most abundant and ubiquitous species, with 308
individuals recorded across all 130 plots. Sprague‘s Pipit was also fairly abundant, with
182 individuals recorded across 118 plots. Chestnut-collared Longspur, Horned Lark,
and Savannah Sparrow each occurred in approximately 60% of plots. Clay-colored
Sparrow, Vesper Sparrow, Western Meadowlark, and Brown-headed Cowbird each
occurred in approximately 40-45% of plots. Ten species occurred in fewer than 10% of
plots, four of which were single observations.
4.2 Influence of grazing on the avian community
Most species occurred in both grazed and ungrazed pastures. Chipping Sparrow, Lark
Sparrow, Cliff Swallow, and American Goldfinch were recorded in ungrazed habitat only
and Barn Swallow was recorded in grazed habitat only; however, because these were
predominantly single sightings, no conclusions were drawn about their preferences for
grazed versus ungrazed habitat.
Species richness and diversity were similar between the grazing treatments at both
scales; however, total bird abundance was on average 1.16 times higher in the grazed
treatment (Table 1). Baird‘s Sparrow and Red-winged Blackbird were on average 1.22
and 5.47 times more abundant in ungrazed pastures, respectively. Chestnut-collared
Longspur and Horned Lark were on average 2.89 and 2.08 times more abundant in grazed
52
Table 1. Influence of grazing treatment on avian community structure and species abundance at plot (n=130) and pasture (n=13) scales
at Grasslands National Park (ungrazed) and adjacent community pastures (grazed) in 2008. Species diversity values were calculated
using the Shannon Diversity Index (H′). Abundance was measured as the number of individuals/3.2 ha. LME = linear mixed-effects
model; GLME = generalized linear mixed-effects model; GLM = generalized linear model.
Plot Scale
Model
β (SE)
P
Species Richness
LME
-0.84 (0.51) 0.133
Species Diversity
LME
-0.24 (0.14) 0.120
Total Bird Abundance
LME
-1.48 (0.81) 0.095
Baird's Sparrow
LME
0.47 (0.19)
0.029
Sprague's Pipit
LME
0.10 (0.14)
0.509
Chestnut-collared Longspur LME
-1.32 (0.26) <.001
Horned Lark
LME1
-0.25 (0.05) <.001
Savannah Sparrow
LME1
0.12 (0.08)
0.160
Clay-colored Sparrow
LME1
0.02 (0.06)
0.729
1
Vesper Sparrow
LME
0.06 (0.05)
0.261
Western Meadowlark
LME1
0.03 (0.06)
0.583
Brown-headed Cowbird4
LME
-0.47 (0.34) 0.198
Red-winged Blackbird4
LME
0.85 (0.26)
0.008
2
Brewer's Sparrow
GLME
1.63 (1.81)
0.429
McCown's Longspur
GLME2,3
—
—
1
Square root + 1 transformation of response variable
2
Family = binomial; β (SE) value converted to odds ratio
3
Model did not converge
4
Lowland habitat only
Pasture Scale
Ungrazed
Mean (SE)
5.99 (0.22)
2.25 (0.06)
9.53 (0.29)
2.51 (0.07)
1.43 (0.08)
0.70 (0.09)
0.62 (0.07)
0.82 (0.07)
0.66 (0.09)
0.51 (0.07)
0.53 (0.08)
1.53 (0.20)
1.04 (0.17)
0.19 (0.04)
0.04 (0.02)
Grazed
Mean (SE)
6.82 (0.34)
2.49 (0.07)
11.01 (0.37)
2.05 (0.13)
1.33 (0.12)
2.02 (0.21)
1.29 (0.11)
0.52 (0.11)
0.60 (0.12)
0.66 (0.11)
0.60 (0.11)
2.00 (0.23)
0.19 (0.10)
0.12 (0.05)
0.31 (0.10)
53
Model
GLM
GLM
GLM
GLM
GLM
GLM
GLM
GLM
GLM
GLM
GLM
GLM
GLM
GLM
GLM
β (SE)
-0.58 (1.29)
-0.06 (0.12)
-1.48 (0.87)
0.47 (0.20)
0.10 (0.12)
-1.32 (0.28)
-0.67 (0.10)
0.30 (0.22)
0.06 (0.13)
-0.15 (0.10)
-0.07 (0.17)
-0.47 (0.37)
0.85 (0.22)
0.06 (0.08)
-0.27 (0.08)
P
0.651
0.630
0.091
0.020
0.434
<.0001
<.0001
0.178
0.656
0.133
0.670
0.199
0.0001
0.448
0.001
Ungrazed
Mean (SE)
13.67 (0.69)
3.26 (0.07)
9.53 (0.54)
2.51 (0.11)
1.43 (0.08)
0.70 (0.17)
0.62 (0.05)
0.82 (0.11)
0.66 (0.08)
0.51 (0.05)
0.53 (0.10)
1.53 (0.22)
1.04 (0.14)
0.19 (0.05)
0.04 (0.02)
Grazed
Mean (SE)
14.25 (1.18)
3.32 (0.09)
11.01 (0.42)
2.05 (0.16)
1.33 (0.05)
2.02 (0.18)
1.29 (0.12)
0.52 (0.23)
0.60 (0.07)
0.66 (0.09)
0.60 (0.12)
2.00 (0.23)
0.19 (0.12)
0.12 (0.06)
0.31 (0.12)
pastures, respectively. Results for McCown‘s Longspur could not be generated for plot
scale, as the model did not converge, but at pasture scale, this species was on average
7.43 times more abundant in grazed pastures.
Species richness, diversity, total bird abundance, and abundance of all species except
Savannah Sparrow differed significantly between upland and lowland habitat (Table 2).
Baird‘s Sparrow, Sprague‘s Pipit, and Chestnut-collared Longspur were more abundant in
upland habitat, whereas species richness, diversity, total bird abundance, and relative
abundance of Horned Lark, Clay-colored Sparrow, Vesper Sparrow, Brewer‘s Sparrow,
and Western Meadowlark were higher in lowland habitat. Species richness and diversity
were similar between grazing treatments in both habitat types (Table 3). Total bird
abundance was higher in grazed than in ungrazed upland, but was similar between
treatments in lowland habitat. Baird‘s Sparrow abundance was higher in the ungrazed
treatment in both habitat types and was altogether highest in ungrazed uplands.
Conversely, Chestnut-collared Longspur abundance was higher in the grazed treatment in
both habitat types and was altogether highest in grazed uplands. Grazing had a particularly
strong influence on this species in upland habitat, where abundance of Chestnut-collared
Longspur was on average 2.75 times higher in the grazed treatment. Horned Lark
abundance was higher in the grazed treatment in both habitat types and was altogether
highest in grazed lowland. Savannah Sparrow abundance was similar between treatments
in upland habitat, but was higher in ungrazed lowland. Vesper Sparrow was more
abundant in grazed lowland. Results could not be generated for this species in upland
habitat, possibly due to low sample size in that habitat, but mean abundance values suggest
that grazing did not have a significant influence on this species in upland habitat.
54
Table 2. Influence of habitat type (upland/lowland) on avian community structure and
species abundance in upland (n=78) and lowland (n=52) plots at Grasslands National
Park and adjacent community pastures in 2008. Species diversity values were calculated
using the Shannon Diversity Index (H′). Abundance was measured as the number of
individuals/3.2 ha. LME = linear mixed-effects model; GLME = generalized linear
mixed-effects model.
Model
β (SE)
Species Richness
LME
-2.99 (0.24)
Species Diversity
LME
-0.68 (0.06)
Total Bird Abundance
LME
-3.05 (0.33)
Baird‘s Sparrow
LME
0.73 (0.11)
Sprague‘s Pipit
LME
1.06 (0.10)
Chestnut-Collared Longspur
LME1
0.38 (0.04)
Horned Lark
LME
-0.46 (0.12)
Savannah Sparrow
LME
-0.09 (0.11)
Clay-colored Sparrow
LME
-1.07 (0.11)
Vesper Sparrow
LME1
-0.32 (0.04)
Western Meadowlark
LME1
-0.11 (0.05)
Brewer‘s Sparrow
GLME2
0.24 (1.62)
1
Square root + 1 transformation of response variable
2
Family = binomial; β (SE) value converted to odds ratio
55
P
<.0001
<.0001
<.0001
<.0001
<.0001
<.0001
<.001
0.431
<.0001
<.0001
0.020
0.003
Upland
Mean (SE)
5.05 (0.16)
2.06 (0.05)
8.76 (0.23)
2.66 (0.08)
1.55 (0.14)
1.83 (0.06)
0.64 (0.08)
0.69 (0.08)
0.22 (0.06)
0.22 (0.05)
0.44 (0.08)
0.09 (0.03)
Lowland
Mean (SE)
8.04 (0.26)
2.74 (0.06)
11.81 (0.36)
1.93 (0.09)
0.44 (0.09)
0.76 (0.07)
1.10 (0.11)
0.78 (0.10)
1.28 (0.10)
1.05 (0.10)
0.72 (0.10)
0.29 (0.06)
Table 3. Influence of grazing treatment on avian community structure and species abundance in upland (n=78) and lowland (n=52)
plots at Grasslands National Park (ungrazed) and adjacent community pastures (grazed) in 2008. Species diversity values were
calculated using the Shannon Diversity Index (H′). Abundance was measured as the number of individuals/3.2 ha. LME = linear
mixed-effects model; GLME = generalized linear mixed-effects model.
Upland Habitat
Model
β (SE)
P
Species Richness
LME
-0.71 (0.44) 0.139
Species Diversity
LME
-0.21 (0.15) 0.200
Total Bird Abundance
LME1
-0.30 (0.11) 0.022
Baird's Sparrow
LME
0.45 (0.22)
0.065
Sprague's Pipit
LME
0.15 (0.14)
0.317
Chestnut-collared Longspur LME
-1.78 (0.31) 0.0001
Horned Lark
LME
-0.64 (0.15) 0.002
Savannah Sparrow
LME1
0.04 (0.10)
0.676
Clay-colored Sparrow
GLME2
3.86 (4.18)
0.363
2,3
Vesper Sparrow
GLME
—
—
Western Meadowlark
LME
0.04 (0.17)
0.830
Brewer's Sparrow
—4
—
—
1
Square root + 1 transformation of response variable
2
Family = binomial; β (SE) value converted to odds ratio
3
Model did not converge
4
Insufficient data
Ungrazed
Mean (SE)
4.83 (0.20)
1.99 (0.07)
8.18 (0.25)
2.80 (0.08)
1.87 (0.07)
1.01 (0.12)
0.44 (0.08)
0.72 (0.09)
0.28 (0.08)
0.23 (0.07)
0.45 (0.10)
0.13 (0.05)
Lowland Habitat
Grazed
Mean (SE)
5.54 (0.22)
2.20 (0.06)
10.08 (0.40)
2.35 (0.18)
1.72 (0.14)
2.78 (0.22)
1.08 (0.14)
0.62 (0.16)
0.08 (0.06)
0.21 (0.08)
0.42 (0.12)
0
56
Model
LME
LME
LME
LME
LME
LME
LME
LME
LME
LME
LME1
LME
β (SE)
-1.03 (0.84)
-0.28 (.16)
-0.84 (1.28)
0.49 (0.19)
0.02 (0.21)
-0.64 (0.23)
-0.71 (0.22)
0.60 (0.20)
-0.14 (0.29)
-0.42 (0.20)
-0.09 (0.10)
-0.03 (0.18)
P
0.248
0.113
0.524
0.025
0.933
0.018
0.009
0.014
0.624
0.063
0.390
0.849
Ungrazed
Mean (SE)
7.72 (0.29)
2.65 (0.07)
11.56 (0.46)
2.08 (0.12)
0.77 (0.09)
0.24 (0.09)
0.88 (0.12)
0.96 (0.11)
1.24 (0.13)
0.93 (0.12)
0.65 (0.13)
0.28 (0.08)
Grazed
Mean (SE)
8.75 (0.48)
2.94 (0.08)
12.40 (0.55)
1.59 (0.12)
0.75 (0.11)
0.88 (0.17)
1.59 (0.17)
0.36 (0.14)
1.38 (0.13)
1.34 (0.12)
0.88 (0.18)
0.31 (0.12)
4.3 Influence of grazing on habitat structure and heterogeneity
Grazing strongly influenced habitat structure; all habitat variables except percent grass
cover differed significantly between grazing treatments (Table 4). On average, grazed
pastures had shorter, sparser vegetation, less litter, more bare and exposed ground, more
forbs, and fewer shrubs. Habitat type also had a strong influence on habitat structure;
five of the seven habitat variables differed between upland and lowland habitat (Table 5).
On average, lowland habitat had taller, denser vegetation, more bare ground, less exposed
moss/lichen, more grass and shrubs, and fewer forbs, but litter cover was similar between
the habitat types. Separate analyses of upland and lowland habitat structure indicated that
grazing influenced habitat structure primarily in upland habitat (Table 6). All significant
plot-scale relationships between grazing treatment and habitat structure were also
significant in upland habitat; however, only one-third of these relationships were apparent
in lowland habitat. Percent bare ground, exposed moss/lichen, and forb cover were
significantly higher in grazed upland only, and percent shrub cover was significantly
higher in ungrazed upland only.
Grazing also influenced SD-derived habitat heterogeneity, but trends were less
apparent. Patchiness of four habitat variables differed between treatments at the plot
scale (Table 4). Patchiness of bare ground, exposed moss/lichen, and forbs was greater in
the grazed treatment. Conversely, patchiness of shrub cover was greater in the ungrazed
treatment. The influence of grazing on patchiness carried over to the pasture scale for
exposed moss/lichen and shrub cover. Patchiness of grass cover was greater in the
grazed treatment at pasture scale, though patchiness was similar between treatments at
plot scale. Overall, no obvious trends existed based on SD-derived heterogeneity values;
57
Table 4. Influence of grazing treatment on habitat structure and heterogeneity at plot (n=130) and pasture (n=13) scales at Grasslands
National Park (ungrazed) and adjacent community pastures (grazed) in 2008. Heterogeneity was measured both from standard
deviation (SD) and coefficient of variation (CV) values for comparative purposes. LME = linear mixed-effects model; GLM =
generalized linear model.
Plot Scale
LME
Pasture Scale
GLM
β (SE)
P
Ungrazed
Mean (SE)
Structure (Mean)
Vegetation Height-Density (cm)
% Litter Cover
% Exposed Moss/Lichen
% Bare Ground
% Grass
% Forbs
% Shrubs
0.25 (0.05)1
15.65 (5.44)
-10.63 (3.08)
-0.21 (0.09)1
-2.34 (3.01)
-2.41 (0.99)
0.18 (0.09)1
<.001
0.015
0.005
0.048
0.452
0.033
0.069
7.44 (0.31)
60.88 (1.82)
16.65 (1.39)
4.70 (0.79)
15.55 (0.67)
4.53 (0.31)
6.19 (0.81)
3.96 (0.37)
45.23 (2.66)
27.28 (2.91)
8.38 (1.61)
17.89 (1.30)
6.94 (0.61)
3.64 (0.97)
3.48 (0.77)
15.65 (5.87)
-10.63 (3.32)
-3.68 (1.40)
-2.34 (3.24)
-2.52 (1.07)
2.68 (1.06)
<.0001
0.008
0.001
0.009
0.468
0.019
0.011
7.44 (0.42)
60.88 (3.54)
16.65 (2.03)
4.70 (0.40)
15.55 (1.69)
4.42 (0.44)
6.19 (0.58)
3.96 (0.68)
45.23 (3.50)
27.28 (1.83)
8.38 (2.01)
17.89 (3.10)
6.94 (1.32)
3.64 (0.91)
Heterogeneity (SD)
Vegetation Height-Density (cm)
% Litter Cover
% Exposed Moss/Lichen
% Bare Ground
% Grass
% Forbs
% Shrubs
0.05 (0.03)1
-0.63 (1.45)
-7.68 (2.73)
-0.20 (0.10)1
-2.11 (1.26)
-0.14 (0.07)1
0.21 (0.10)1
0.167
0.670
0.017
0.078
0.121
0.068
0.063
5.19 (0.26)
26.92 (0.83)
17.91 (1.18)
7.99 (1.14)
9.75 (0.49)
5.48 (0.40)
8.84 (1.04)
4.51 (0.36)
27.55 (1.02)
25.60 (1.88)
12.76 (2.15)
11.86 (0.85)
8.44 (0.94)
6.07 (1.38)
0.60 (0.55)
-2.04 (2.13)
-6.50 (1.75)
-2.24 (1.98)
-1.90 (1.06)
-0.49 (0.56)
2.40 (1.36)
0.277
0.338
0.002
0.257
0.074
0.386
0.077
2.62 (0.32)
14.20 (1.34)
12.16 (0.96)
7.14 (1.04)
4.18 (0.51)
2.57 (0.29)
7.74 (0.55)
2.01 (0.38)
16.24 (0.88)
18.66 (1.48)
9.38 (1.85)
6.08 (1.13)
3.06 (0.54)
5.35 (1.68)
0.0001
0.036
0.128
0.521
0.386
0.481
0.219
0.77 (0.04)
0.51 (0.03)
1.42 (0.06)
1.85 (0.09)
0.64 (0.02)
1.33 (0.06)
1.65 (0.10)
1.33 (0.08)
0.70 (0.05)
1.21 (0.10)
1.74 (0.12)
0.72 (0.05)
1.23 (0.07)
1.40 (0.17)
-0.16 (0.06)
-0.12 (0.05)
0.07 (0.11)
0.30 (0.20)
-0.06 (0.07)
0.15 (0.11)
-0.05 (0.06)3
0.008
0.020
0.493
0.139
0.368
0.179
0.427
0.35 (0.03)
0.24 (0.03)
0.77 (0.06)
1.48 (0.12)
0.29 (0.04)
0.60 (0.07)
1.32 (0.13)
0.51 (0.05)
0.37 (0.04)
0.69 (0.07)
1.18 (0.10)
0.35 (0.07)
0.45 (0.07)
1.45 (0.12)
Heterogeneity (CV)
Vegetation Height-Density
-0.25 (0.04)2
% Litter Cover
-0.16 (0.07)2
% Exposed Moss/Lichen
0.21 (0.12)
% Bare Ground
0.12 (0.16)
% Grass
-0.04 (0.04)2
% Forbs
0.10 (0.14)
% Shrubs
0.25 (0.19)
1
Log10 + 1 transformation of response variable
2
Log10 transformation of response variable
3
Log10 + 0.001 transformation of response variable
Grazed
Mean (SE)
β (SE)
P
Ungrazed
Mean (SE)
Grazed
Mean (SE)
58
Table 5. Influence of habitat type (upland/lowland) on habitat structure and heterogeneity
at Grasslands National Park and adjacent community pastures in 2008. Heterogeneity
was measured both from standard deviation (SD) and coefficient of variation (CV) values
for comparative purposes. LME = linear mixed-effects model.
Model
β (SE)
P
Upland
Mean (SE)
Lowland
Mean (SE)
Habitat Structure (Mean)
Vegetation Height-Density (cm)
% Litter Cover
% Exposed Moss/Lichen
% Bare Ground
% Grass
% Forbs
% Shrubs
LME
LME
LME
LME1
LME
LME
LME
-0.95 (0.46)
-0.06 (2.74)
16.85 (2.14)
-0.53 (0.07)
-1.78 (0.91)
1.87 (0.49)
-9.55 (0.99)
0.041
0.982
<.0001
<.0001
0.052
<.001
<.0001
5.99 (0.31)
56.04 (2.15)
26.66 (1.64)
2.43 (0.55)
15.56 (0.68)
6.02 (0.37)
1.58 (0.29)
6.93 (0.52)
56.10 (2.50)
9.81 (1.61)
10.93 (1.41)
17.34 (1.15)
4.15 (0.46)
11.13 (1.16)
Habitat Heterogeneity (SD)
Vegetation Height-Density (cm)
% Litter Cover
% Exposed Moss/Lichen
% Bare Ground
% Grass
% Forbs
% Shrubs
LME1
LME
LME
LME1
LME
LME1
LME1
-0.08 (0.02)
-1.33 (1.33)
10.75 (1.71)
-0.57 (0.08)
-2.38 (0.79)
0.10 (0.04)
-0.59 (0.08)
0.001
0.320
<.0001
<.0001
0.003
0.025
<.0001
4.42 (0.19)
26.58 (0.89)
24.58 (0.95)
4.42 (0.87)
9.45 (0.50)
6.82 (0.52)
3.76 (0.69)
5.83 (0.42)
27.91 (0.94)
13.82 (1.87)
17.01 (1.82)
11.83 (0.73)
5.75 (0.67)
14.32 (1.43)
-0.06 (0.03)
-0.02 (0.04)
-0.69 (0.09)
0.15 (0.14)
-0.06 (0.02)
-0.38 (0.08)
0.41 (0.18)
0.074
0.594
<.0001
0.295
0.010
<.0001
0.025
0.89 (0.05)
0.56 (0.04)
1.08 (0.04)
1.87 (0.10)
0.64 (0.03)
1.15 (0.04)
1.74 (0.14)
1.02 (0.08)
0.57 (0.04)
1.77 (0.10)
1.72 (0.09)
0.71 (0.03)
1.53 (0.08)
1.33 (0.08)
Habitat Heterogeneity (CV)
Vegetation Height-Density
LME2
% Litter Cover
LME2
% Exposed Moss/Lichen
LME
% Bare Ground
LME
% Grass
LME2
% Forbs
LME
% Shrubs
LME
1
Log10 + 1 transformation of response variable
2
Log10 transformation of response variable
59
Table 6. . Influence of grazing treatment on habitat structure and heterogeneity in upland (n=78) and lowland (n=52) plots at
Grasslands National Park (ungrazed) and adjacent community pastures (grazed) in 2008. Heterogeneity was measured both from
standard deviation (SD) and coefficient of variation (CV) values for comparative purposes. LME = linear mixed-effects model.
Upland Habitat
LME
Lowland Habitat
LME
β (SE)
P
Ungrazed
Mean (SE)
Grazed
Mean (SE)
β (SE)
P
Ungrazed
Mean (SE)
Grazed
Mean (SE)
Structure (Mean)
Vegetation Height-Density (cm)
% Litter Cover
% Exposed Moss/Lichen
% Bare Ground
% Grass
% Forbs
% Shrubs
3.78 (0.74)
16.15 (6.67)
-14.93 (3.56)
-0.19 (0.09)1
-1.86 (2.70)
-3.04 (1.05)
0.23 (0.09)1
<.001
0.034
0.001
0.052
0.504
0.014
0.021
7.15 (0.30)
61.01 (2.30)
22.07 (1.60)
1.67 (0.50)
14.48 (0.79)
4.91 (0.35)
2.05 (0.39)
3.37 (0.37)
44.86 (3.87)
37.00 (3.03)
4.12 (1.33)
16.85 (1.36)
8.12 (0.74)
0.52 (0.23)
3.02 (1.18)
14.91 (5.84)
-4.18 (4.96)
-0.22 (0.15)1
-3.07 (4.20)
-1.47 (1.34)
4.07 (2.83)
0.027
0.027
0.417
0.158
0.480
0.297
0.178
7.86 (0.62)
60.69 (3.00)
8.53 (1.83)
9.23 (1.54)
16.39 (1.22)
3.69 (0.52)
12.39 (1.42)
4.84 (0.71)
45.78 (3.39)
12.71 (3.18)
14.76 (2.85)
19.47 (2.53)
5.16 (0.89)
8.31 (1.88)
Heterogeneity (SD)
Vegetation Height-Density (cm)
% Litter Cover
% Exposed Moss/Lichen
% Bare Ground
% Grass
% Forbs
% Shrubs
0.07 (0.03)1
-0.40 (2.69)
-8.31 (2.13)
-0.21 (0.11)1
-2.41 (1.15)
-4.23 (1.40)
0.34 (0.11)1
0.049
0.885
0.002
0.076
0.060
0.012
0.010
4.70 (0.25)
26.46 (1.11)
22.02 (1.12)
3.21 (0.83)
8.71 (0.53)
5.51 (0.40)
4.81 (0.93)
3.77 (0.20)
26.86 (1.49)
30.33 (1.13)
7.14 (2.05)
11.12 (1.06)
9.75 (1.27)
1.40 (0.63)
0.02 (0.06)1
-0.99 (2.05)
-6.72 (5.91)
-6.03 (3.89)
-1.67 (2.58)
-0.07 (0.12)
1.82 (3.72)
0.744
0.640
0.280
0.150
0.531
0.544
0.635
5.92 (0.51)
27.60 (1.25)
11.76 (2.06)
15.16 (2.07)
11.32 (0.86)
5.43 (0.80)
14.88 (1.76)
5.62 (0.77)
28.59 (1.21)
18.47 (3.79)
21.18 (3.52)
12.99 (1.39)
6.47 (1.24)
13.06 (2.46)
Heterogeneity (CV)
Vegetation Height-Density
-0.61 (0.08)
% Litter Cover
-0.17 (0.09)1
% Exposed Moss/Lichen
0.16 (0.12)
% Bare Ground
-0.01 (0.23)
% Grass
-0.05 (0.06)1
% Forbs
0.003 (0.15)
% Shrubs
0.76 (0.29)
1
Log10 transformation of response variable
<.0001
0.107
0.223
0.969
0.367
0.985
0.024
0.70 (0.04)
0.50 (0.03)
1.13 (0.05)
1.87 (0.12)
0.60 (0.03)
1.15 (0.05)
1.97 (0.15)
1.30 (0.09)
0.71 (0.08)
0.97 (0.09)
1.88 (0.18)
0.72 (0.08)
1.15 (0.09)
1.21 (0.28)
-0.21 (0.08)1
-0.15 (0.07)1
0.28 (0.23)
0.28 (0.26)
-0.03 (0.07)
0.26 (0.24)
-0.50 (0.16)
0.016
0.043
0.255
0.297
0.732
0.313
0.010
0.86 (0.08)
0.53 (0.04)
1.86 (0.11)
1.81 (0.11)
0.71 (0.03)
1.60 (0.10)
1.17 (0.10)
1.37 (0.15)
0.67 (0.05)
1.57 (0.19)
1.53 (0.15)
0.73 (0.06)
1.35 (0.13)
1.67 (0.09)
60
grazing appeared to both positively and negatively influence patchiness at both spatial
scales.
Coefficient of variation-derived heterogeneity of two habitat variables differed
between treatments at both spatial scales (Table 4). Patchiness of vegetation heightdensity and litter cover was greater in the grazed treatment. Because only two significant
relationships existed, trends in the influence of grazing on CV-derived habitat
heterogeneity could not be generalized. Comparatively, the two measures of
heterogeneity yielded very different results.
Habitat type (upland/lowland) had a strong influence on habitat heterogeneity, but
relationships varied between the two heterogeneity measures (Table 5). Vegetation
height-density and grass cover were consistently patchier in lowland habitat; however,
exposed moss/lichen cover and forb cover were patchier in upland habitat based on SD
values, but patchier in lowland habitat based on CV values. Shrub cover was patchier in
lowland habitat based on SD values, but patchier in upland habitat based on CV values.
Bare ground was patchier in lowland habitat based on SD values, but patchiness was
similar between habitats based on CV values. Only litter cover appeared altogether
unaffected by habitat type.
Separate analyses of upland and lowland habitat heterogeneity indicated that grazing
influenced habitat heterogeneity primarily in upland habitat when SD was used as the
measure of heterogeneity (Table 6). No relationships between grazing and SD-derived
heterogeneity existed in lowland habitat. Patchiness of vegetation height-density and
patchiness of grass cover were negatively and positively influenced by grazing,
respectively, in upland habitat, whereas patchiness of these variables was similar between
61
grazing treatments at the overall plot scale (both habitats included), though patchiness of
grass cover was significantly greater in the grazed treatment at pasture scale.
In contrast to SD results, CV values indicated that grazing influenced habitat
heterogeneity similarly in upland and lowland habitat, with two and three significant
relationships, respectively (Table 6). Patchiness of litter cover was greater in the grazed
treatment in lowland only. Patchiness of shrubs was greater in ungrazed upland, but
greater in grazed lowland, whereas plot-scale patchiness of shrubs did not appear
significantly influenced by grazing.
4.4 Relationships among birds, habitat, and grazing
4.4.1 Plot-scale relationships
Habitat structure and composition had a strong influence on the avian community. The
most influential habitat variables were shrub cover, bare ground, and exposed
moss/lichen cover, which had significant relationships with 10, six, and six of the 15
avian response variables, respectively (Table 7). The other four habitat variables
influenced two to four avian variables each. Species richness and diversity increased as
bare ground and shrub cover increased, while diversity decreased as vegetation heightdensity increased. Total bird abundance increased as bare ground and shrub cover
increased, but decreased as exposed moss/lichen cover increased. Species diversity and
total bird abundance also increased as forb cover increased. Bird-habitat relationships
varied widely among species and were not always consistent with habitat preferences that
would be predicted from grazing preferences. For example, Baird‘s Sparrow preferred
ungrazed habitat, yet was negatively associated with shrub cover, which was greater in
ungrazed habitat. In contrast, horned lark was positively associated with shrub cover, but
62
Table 7. Influence of grazing treatment, habitat structure, and habitat heterogeneity on avian community structure and species
abundance at plot (n=130) scale at Grasslands National Park and adjacent community pastures in 2008. Species diversity values were
calculated using the Shannon Diversity Index (H′). Abundance was measured as the number of individuals/3.2 ha. Heterogeneity was
measured both from standard deviation (SD) and coefficient of variation (CV) values for comparative purposes. Linear mixed-effects
models (LMEs) were used for statistical analysis unless otherwise noted.
Species Richness
β (SE)
P
Species Diversity
β (SE)
P
Total Bird Abundance
β (SE)
P
Baird's Sparrow
β (SE)
P
Structure (Mean)
Grazing Treatment
Vegetation Height-Density (cm)
% Litter Cover
% Exposed Moss/Lichen
% Bare Ground
% Grass
% Forbs
% Shrubs
-0.702 (0.717)
-0.096 (0.073)
0.008 (0.015)
-0.022 (0.015)
0.080 (0.027)
0.011 (0.027)
0.084 (0.054)
0.141 (0.025)
0.348
0.193
0.590
0.163
0.004
0.690
0.120
<.0001
-0.168 (0.184)
-0.042 (0.018)
0.002 (0.004)
-0.006 (0.004)
0.017 (0.007)
-0.002 (0.007)
0.023 (0.014)
0.035 (0.006)
0.380
0.026
0.584
0.129
0.012
0.738
0.095
<.0001
-1.226 (1.009)
-0.102 (0.091)
0.001 (0.019)
-0.036 (0.019)
0.067 (0.034)
0.052 (0.034)
0.128 (0.067)
0.149 (0.031)
0.250
0.264
0.942
0.068
0.048
0.129
0.060
<.0001
0.556 (0.205)
<-.0001 (0.031)
-0.003 (0.006)
-0.004 (0.007)
-0.022 (0.011)
0.005 (0.011)
0.032 (0.022)
-0.031 (0.011)
0.020
0.998
0.617
0.506
0.046
0.664
0.141
0.005
Heterogeneity (SD)
Grazing Treatment
Vegetation Height-Density (cm)
% Litter Cover
% Exposed Moss/Lichen
% Bare Ground
% Grass
% Forbs
% Shrubs
-0.652 (0.645)
-0.020 (0.078)
0.005 (0.021)
-0.011 (0.015)
0.063 (0.013)
0.036 (0.032)
0.054 (0.034)
0.102 (0.021)
0.334
0.795
0.831
0.467
<.0001
0.263
0.112
<.0001
-0.241 (0.171)
-0.015 (0.020)
0.004 (0.005)
-0.005 (0.003)
0.014 (0.003)
-0.001 (0.008)
0.013 (0.009)
0.024 (0.005)
0.188
0.437
0.441
0.070
<.0001
0.905
0.146
<.0001
-1.404 (0.905)
0.001 (0.096)
-0.015 (0.026)
-0.032 (0.013)
0.056 (0.016)
0.073 (0.040)
0.084 (0.042)
0.089 (0.026)
0.149
0.990
0.559
0.017
0.001
0.072
0.051
0.001
0.432 (0.214)
0.039 (0.033)
-0.001 (0.009)
-0.001 (0.005)
-0.017 (0.006)
-0.006 (0.014)
0.007 (0.014)
-0.026 (0.009)
0.068
0.235
0.943
0.896
0.003
0.639
0.622
0.005
Heterogeneity (CV)
Grazing Treatment
Vegetation Height-Density
% Litter Cover
% Exposed Moss/Lichen
% Bare Ground
% Grass
% Forbs
% Shrubs
-0.398 (0.527)
0.946 (0.459)
0.283 (0.711)
1.242 (0.292)
-0.303 (0.207)
1.446 (0.683)
0.460 (0.358)
-0.025 (0.163)
0.465
0.042
0.691
<.0001
0.145
0.037
0.202
0.880
-0.116 (0.150)
0.420 (0.114)
0.221 (0.184)
-0.018 (0.003)
-0.064 (0.049)
0.266 (0.163)
0.006 (0.0910)
-0.020 (0.038)
0.457
<.001
0.234
<.0001
0.196
0.106
0.946
0.602
1.232 (0.848)
1.418 (0.565)
0.763 (0.920)
-0.078 (0.016)
-0.261 (0.242)
1.472 (0.806)
0.455 (0.451)
-0.133 (0.188)
0.174
0.014
0.409
<.0001
0.283
0.071
0.316
0.481
0.253 (0.197)
-0.556 (0.18)
-0.226 (0.286)
0.017 (0.005)
0.016 (0.079)
-0.261 (0.258)
0.095 (0.143)
0.011 (0.062)
0.225
0.003
0.430
0.002
0.836
0.313
0.510
0.855
63
Table 7. (continued).
Sprague's Pipit
β (SE)
P
Chestnut-collared Longspur
β (SE)
P
Horned Lark
β (SE)
P
Savannah Sparrow
β (SE)
P
Structure (Mean)
Grazing Treatment
Vegetation Height-Density (cm)
% Litter Cover
% Exposed Moss/Lichen
% Bare Ground
% Grass
% Forbs
% Shrubs
0.173 (0.149)
0.022 (0.028)
0.006 (0.005)
0.011 (0.006)
-0.013 (0.009)
0.012 (0.009)
0.013 (0.019)
-0.045 (0.010)
0.270
0.445
0.289
0.084
0.170
0.193
0.492
<.0001
-0.835 (0.279)
-0.036 (0.032)
-0.007 (0.007)
0.020 (0.007)
-0.018 (0.012)
-0.029 (0.012)
0.056 (0.024)
-0.020 (0.011)
0.012
0.266
0.323
0.005
0.123
0.015
0.019
0.074
-0.308 (0.141)
-0.072 (0.027)
-0.001 (0.005)
0.001 (0.006)
0.032 (0.009)
-0.002 (0.008)
0.014 (0.018)
0.022 (0.010)
0.052
0.008
0.907
0.849
0.001
0.828
0.450
0.028
0.147 (0.243)
0.016 (0.030)
0.006 (0.006)
-0.002 (0.006)
0.007 (0.011)
-0.004 (0.011)
0.005 (0.022)
0.004 (0.010)
0.559
0.607
0.352
0.740
0.535
0.743
0.821
0.697
Heterogeneity (SD)
Grazing Treatment
Vegetation Height-Density (cm)
% Litter Cover
% Exposed Moss/Lichen
% Bare Ground
% Grass
% Forbs
% Shrubs
0.155 (0.150)
-0.002 (0.031)
-0.007 (0.009)
0.008 (0.004)
-0.018 (0.005)
-0.008 (0.013)
0.004 (0.013)
-0.024 (0.009)
0.323
0.953
0.394
0.060
0.001
0.538
0.770
0.009
-0.942 (0.263)
-0.014 (0.035)
-0.005 (0.009)
0.029 (0.005)
-0.009 (0.006)
-0.016 (0.015)
0.030 (0.015)
-0.019 (0.010)
0.004
0.691
0.588
<.0001
0.148
0.283
0.053
0.050
-0.436 (0.138)
-0.029 (0.030)
0.006 (0.008)
0.006 (0.004)
0.027 (0.005)
0.012 (0.012)
0.007 (0.012)
0.009 (0.009)
0.009
0.333
0.460
0.181
<.0001
0.333
0.596
0.319
0.153 (0.220)
0.078 (0.031)
-0.011 (0.008)
-0.008 (0.004)
0.001 (0.005)
-0.004 (0.013)
-0.009 (0.014)
-0.011 (0.009)
0.503
0.014
0.200
0.072
0.863
0.741
0.503
0.200
Heterogeneity (CV)
Grazing Treatment
Vegetation Height-Density
% Litter Cover
% Exposed Moss/Lichen
% Bare Ground
% Grass
% Forbs
% Shrubs
-0.068 (0.159)
-0.538 (0.163)
-0.500 (0.255)
0.027 (0.005)
0.019 (0.073)
-0.244 (0.234)
0.014 (0.13)
0.116 (0.057)
0.678
0.001
0.053
<.0001
0.790
0.300
0.914
0.045
-0.950 (0.280)
-0.022 (0.198)
-0.384 (0.322)
0.041 (0.006)
0.071 (0.085)
-0.072 (0.283)
-0.052 (0.158)
-0.124 (0.066)
0.006
0.910
0.235
<.0001
0.404
0.800
0.742
0.065
-0.324 (0.139)
0.555 (0.151)
0.524 (0.234)
-0.008 (0.005)
-0.113 (0.068)
0.065 (0.217)
0.152 (0.120)
-0.048 (0.054)
0.040
<.001
0.027
0.093
0.099
0.764
0.211
0.373
0.252 (0.239)
0.119 (0.169)
-0.329 (0.275)
-0.006 (0.005)
0.039 (0.073)
0.050 (0.242)
0.010 (0.135)
-0.053 (0.057)
0.314
0.484
0.234
0.245
0.593
0.838
0.941
0.352
64
Table 7. (continued 2).
Clay-colored Sparrow1
β (SE)
P
Vesper Sparrow
β (SE)
P
Western Meadowlark1
β (SE)
P
Brown-headed Cowbird2
β (SE)
P
Structure (Mean)
Grazing Treatment
Vegetation Height-Density (cm)
% Litter Cover
% Exposed Moss/Lichen
% Bare Ground
% Grass
% Forbs
% Shrubs
-0.066 (0.066)
-0.004 (0.010)
0.0004 (0.002)
-0.006 (0.002)
0.005 (0.004)
0.002 (0.004)
-0.004 (0.007)
0.018 (0.004)
0.336
0.729
0.821
0.012
0.186
0.627
0.630
<.0001
0.054 (0.137)
-0.034 (0.026)
-0.009 (0.005)
-0.006 (0.006)
0.024 (0.009)
-0.014 (0.008)
0.014 (0.017)
0.031 (0.009)
0.703
0.188
0.065
0.314
0.006
0.083
0.433
0.001
-0.099 (0.066)
0.027 (0.012)
-0.002 (0.002)
-0.005 (0.003)
0.004 (0.004)
0.001 (0.004)
0.006 (0.008)
-0.007 (0.004)
0.165
0.025
0.274
0.032
0.371
0.704
0.466
0.125
-0.590 (0.402)
-0.037 (0.067)
-0.005 (0.012)
—
-0.017 (0.022)
—
-0.038 (0.054)
0.038 (0.026)
0.170
0.578
0.707
—
0.456
—
0.489
0.155
Heterogeneity (SD)
Grazing Treatment
Vegetation Height-Density (cm)
% Litter Cover
% Exposed Moss/Lichen
% Bare Ground
% Grass
% Forbs
% Shrubs
-0.039 (0.060)
-0.004 (0.011)
-0.002 (0.003)
-0.006 (0.001)
0.004 (0.002)
0.007 (0.004)
0.001 (0.005)
0.013 (0.003)
0.533
0.703
0.490
<.001
0.015
0.115
0.763
<.001
-0.025 (0.142)
-0.005 (0.028)
-0.002 (0.008)
0.004 (0.004)
0.029 (0.005)
-0.003 (0.012)
0.005 (0.012)
0.022 (0.008)
0.864
0.860
0.749
0.336
<.0001
0.767
0.676
0.007
-0.062 (0.062)
0.012 (0.013)
0.002 (0.004)
-0.005 (0.002)
0.001 (0.002)
0.006 (0.005)
0.003 (0.005)
-0.002 (0.004)
0.343
0.367
0.642
0.004
0.639
0.225
0.613
0.501
-0.602 (0.351)
-0.098 (0.067)
0.035 (0.029)
—
-0.015 (0.014)
—
-0.017 (0.035)
0.049 (0.021)
0.115
0.150
0.232
—
0.313
—
0.624
0.030
0.886
0.005
0.562
<.0001
0.091
0.053
0.960
0.380
0.070 (0.147)
0.440 (0.159)
0.615 (0.247)
-0.016 (0.005)
-0.062 (0.072)
0.237 (0.230)
-0.006 (0.128)
0.005 (0.057)
0.642
0.007
0.015
0.002
0.391
0.304
0.965
0.931
-0.098 (0.064)
-0.075 (0.065)
0.136 (0.103)
-0.006 (0.002)
-0.041 (0.029)
0.128 (0.094)
-0.030 (0.052)
0.016 (0.023)
0.155
0.256
0.187
0.004
0.159
0.175
0.572
0.495
-0.523 (0.459)
-0.418 (0.381)
0.625 (0.826)
—
-0.011 (0.274)
—
0.010 (0.344)
0.134 (0.319)
0.278
0.280
0.455
—
0.967
—
0.977
0.677
Heterogeneity (CV)
Grazing Treatment
0.009 (0.058)
Vegetation Height-Density
0.173 (0.060)
% Litter Cover
0.055 (0.094)
% Exposed Moss/Lichen
-0.013 (0.002)
% Bare Ground
-0.046 (0.027)
% Grass
0.169 (0.086)
% Forbs
0.002 (0.048)
% Shrubs
0.019 (0.021)
1
Square root + 1 transformation of response variable
2
Lowland habitat only
65
Table 7. (continued 3).
Red-winged Blackbird2
β (SE)
P
Brewer‘s Sparrow3,4
β (SE)
P
McCown‘s Longspur3
β (SE)
P
Structure (Mean)
Grazing Treatment
Vegetation Height-Density (cm)
% Litter Cover
% Exposed Moss/Lichen
% Bare Ground
% Grass
% Forbs
% Shrubs
0.433 (0.293)
0.028 (0.049)
0.015 (0.009)
—
0.002 (0.016)
—
-0.014 (0.039)
0.023 (0.019)
0.167
0.566
0.093
—
0.890
—
0.730
0.236
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
0.023 (5.228)
3.083 (1.350)
1.008 (1.039)
1.226 (1.054)
1.070 (1.063)
0.839 (1.073)
1.161 (1.131)
0.523 (1.307)
0.043
<.001
0.838
<.001
0.265
0.013
0.227
0.017
Heterogeneity (SD)
Grazing Treatment
Vegetation Height-Density (cm)
% Litter Cover
% Exposed Moss/Lichen
% Bare Ground
% Grass
% Forbs
% Shrubs
0.741 (0.266)
-0.020 (0.051)
-0.014 (0.022)
—
-0.012 (0.011)
—
0.009 (0.026)
0.022 (0.016)
0.018
0.697
0.531
—
0.286
—
0.738
0.192
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
0.311 (2.430)
1.062 (1.245)
0.989 (1.042)
1.091 (1.021)
0.987 (1.028)
0.982 (1.059)
1.024 (1.06)
0.915 (1.069)
0.215
0.786
0.784
<.001
0.633
0.754
0.681
0.186
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
0.075 (3.435)
0.078 (3.130)
0.725 (3.261)
1.154 (1.036)
1.476 (1.464)
2.627 (2.300)
0.216 (2.184)
1.154 (1.236)
0.059
0.027
0.786
<.001
0.310
0.249
0.052
0.501
Heterogeneity (CV)
Grazing Treatment
0.593 (0.331)
0.101
Vegetation Height-Density
0.079 (0.280)
0.779
% Litter Cover
-1.185 (0.617)
0.063
% Exposed Moss/Lichen
—
—
% Bare Ground
0.153 (0.203)
0.456
% Grass
—
—
% Forbs
0.246 (0.253)
0.338
% Shrubs
-0.032 (0.238)
0.895
2
Lowland habitat only
3
GLME with binomial distribution; β (SE) converted to odds ratio
4
Models did not converge
66
showed preference for grazed habitat. Most species were related to one to four of the
seven habitat variables. Chestnut-collared Longspur, Vesper Sparrow, and McCown‘s
Longspur held the most relationships with habitat structure. Savannah Sparrow and
Brown-headed Cowbird were not related to any of the habitat variables.
Shrub cover, bare ground, and exposed moss/lichen cover were also the most
influential habitat variables on the avian community when habitat heterogeneity was
measured as SD. Patchiness of these habitat variables was significantly related to nine,
eight, and eight bird variables, respectively (Table 7). Patchiness of vegetation heightdensity, grass cover, and forb cover had limited influence on the avian community, with
only one to two significant associations existing for each habitat variable. Patchiness of
litter cover did not influence any bird variable. Species richness and diversity increased
as patchiness of bare ground and shrub cover increased. Total bird abundance increased
as patchiness of bare ground, grass, forb, and shrub cover increased. Species diversity
and total bird abundance decreased as patchiness of exposed moss/lichen increased. As
with bird-habitat associations, bird-heterogeneity associations varied widely among
species. Most species were related to one to three of the seven heterogeneity variables.
Red-winged Blackbird was not related to any of the heterogeneity variables. Of the 29
significant associations, 19 were positive and 10 were negative. Avian relationships to
patchiness of bare ground and shrub cover were generally positive, whereas relationships
with patchiness of exposed moss/lichen were mixed. Since patchiness of shrub cover was
greater in the ungrazed treatment, avian relationships with this variable also represented
an indirect negative relationship with grazing. For example, Clay-colored Sparrow
abundance increased as patchiness of shrub cover increased. Patchiness of shrub cover
67
was greater in the ungrazed treatment, thus this species was indirectly negatively related
to grazing because of its positive association with the patchiness of a habitat variable that
was negatively related to grazing. Overall avian community relationships with SDderived heterogeneity appeared positive; however, no clear patterns in the relationships
between bird-heterogeneity associations and grazing were apparent.
Coefficient of variation measurements of heterogeneity generated very different
results from those based on SD values. Vegetation height-density and exposed
moss/lichen cover were the most influential habitat variables on the avian community
when habitat heterogeneity was measured as CV. Patchiness of these habitat variables
was significantly related to eleven and nine bird variables, respectively (Table 7). Bare
ground and shrub cover, which were most influential in SD measurements, had only two
significant associations each. Litter cover, which had no influence when measured using
SD, had four significant associations. Species richness, diversity, and total bird
abundance increased as patchiness of vegetation height-density increased. Species
richness and total bird abundance also increased as patchiness of grass cover increased,
and species richness increased as patchiness of exposed moss/lichen cover increased. In
contrast, species diversity and total bird abundance decreased as patchiness of exposed
moss/lichen increased. As was the case with SD results, bird-heterogeneity associations
based on CV values varied widely among species. Most species were related to one to
four of the seven heterogeneity variables. Savannah Sparrow and Brown-headed
Cowbird were not related to any of the heterogeneity variables. Of the 32 significant
associations, 17 were positive and 15 were negative. Avian relationships with patchiness
of vegetation height-density and exposed moss/lichen cover were not clearly directional;
68
multiple positive and negative associations existed for each heterogeneity variable.
Consequently, no clear patterns in the relationships between bird-heterogeneity
associations and grazing were apparent. Most bird-heterogeneity associations were
unique to either SD or CV (Table 7). Nine associations were consistent between the two
measures, seven of which were with exposed moss/lichen heterogeneity, and three
associations conflicted: abundance of Horned Lark and Clay-colored Sparrow increased
as SD-derived patchiness of bare ground increased, but decreased as CV-derived
patchiness of bare ground increased, and abundance of Sprague‘s Pipit decreased as SDderived patchiness of shrubs increased, but increased as CV-derived patchiness of shrubs
increased.
4.4.2 Pasture-scale relationships
Pasture-scale relationships between the avian community and habitat were analyzed using
vegetation height-density, bare ground, and shrub cover only, due to the smaller sample
size at this scale. Of the 20 significant associations between birds and habitat structure
that existed across these three habitat variables at plot scale, only one carried over to
pasture scale, that being a positive relationship between Horned Lark and bare ground
(Table 8). Five bird-habitat relationships existed at pasture scale that were not apparent
at plot scale. Species richness, diversity, and total bird abundance showed no habitat
associations at pasture scale. Seven species were not related to any of the three habitat
variables. Brown-headed Cowbird and Savannah Sparrow, which showed no significant
habitat associations with these three habitat variables at plot scale, were associated with
two and one habitat variable(s), respectively, at pasture scale.
69
Of the 18 significant associations between the avian community and SD-derived
heterogeneity that existed across the three habitat variables at plot scale, none carried
over to pasture scale (Table 8). Four bird-heterogeneity relationships existed at pasture
scale that were not apparent at plot scale. Species richness, diversity, total bird
abundance, and the abundance of seven species showed no associations with SD-derived
heterogeneity at pasture scale. Brewer‘s Sparrow was positively associated with
patchiness of vegetation height-density and negatively associated with patchiness of
shrub cover; however, since no results could be generated for this species at the plot
scale, it is unknown if and how these relationships differed between scales.
Of the 13 significant associations between the avian community and CV-derived
heterogeneity that existed across the three habitat variables at plot scale, one carried over
to pasture scale, that being a positive relationship between Sprague‘s Pipit and patchiness
of vegetation height-density (Table 8). Chestnut-collared Longspur was negatively
associated with patchiness of shrub cover at plot scale, but this relationship was reversed
at pasture scale. Species richness showed no associations with CV-derived heterogeneity
at the pasture scale; however, species diversity was positively associated with patchiness
of bare ground. Total bird abundance was positively associated with patchiness of shrub
cover. Patchiness of shrub cover had the most influence on the avian community, with
five associations, four of which were positive. Seven species were not related to
heterogeneity of any of the three habitat variables. Brewer‘s Sparrow was most related to
CV-derived heterogeneity, with two significant associations; however, no results could be
generated for this species at plot scale, so it is unknown if and how these relationships
differed between scales.
70
Table 8. Influence of grazing treatment, habitat structure, and habitat heterogeneity on avian community structure and species
abundance at pasture (n=13) scale at Grasslands National Park and adjacent community pastures in 2008. Species diversity values
were calculated using the Shannon Diversity Index (H′). Abundance was measured as the number of individuals/3.2 ha.
Heterogeneity was measured both from standard deviation (SD) and coefficient of variation (CV) values for comparative purposes.
Generalized linear models (GLMs) were used for statistical analysis.
Species Richness
β (SE)
P
Species Diversity
β (SE)
P
Total Bird Abundance
β (SE)
P
Baird's Sparrow
β (SE)
P
Structure (Mean)
Grazing Treatment
Vegetation Height-Density (cm)
% Bare Ground
% Shrubs
-0.141 (2.525)
0.487 (0.885)
0.102 (0.312)
-0.656 (0.634)
0.956
0.582
0.743
0.300
-0.155 (0.243)
0.068 (0.085)
0.005 (0.030)
-0.045 (0.061)
0.523
0.427
0.880
0.459
-1.910 (1.658)
0.683 (0.581)
0.143 (0.205)
-0.529 (0.416)
0.250
0.240
0.485
0.204
0.249 (0.411)
0.101 (0.144)
0.003 (0.051)
-0.044 (0.103)
0.545
0.483
0.946
0.666
Heterogeneity (SD)
Grazing Treatment
Vegetation Height-Density (cm)
% Bare Ground
% Shrubs
0.683 (1.825)
-0.062 (1.156)
0.287 (0.247)
-0.243 (0.461)
0.708
0.957
0.244
0.597
0.001 (0.169)
0.048 (0.107)
0.021 (0.023)
-0.016 (0.043)
0.997
0.653
0.349
0.700
-0.887 (1.167)
-0.074 (0.739)
0.233 (0.158)
-0.011 (0.295)
0.447
0.921
0.140
0.970
0.389 (0.306)
0.020 (0.194)
-0.009 (0.041)
0.020 (0.077)
0.204
0.919
0.837
0.799
Heterogeneity (CV)
Grazing Treatment
Vegetation Height-Density
% Bare Ground
% Shrubs
-1.405 (1.911)
-0.595 (6.577)
3.029 (2.086)
1.408 (1.825)
0.462
0.928
0.147
0.441
-0.173 (0.150)
-0.021 (0.517)
0.406 (0.164)
-0.069 (0.143)
0.248
0.968
0.013
0.629
-1.545 (1.169)
0.291 (4.021)
1.274 (1.275)
2.115 (1.116)
0.186
0.942
0.318
0.058
0.548 (0.354)
-0.043 (1.217)
-0.218 (0.386)
0.180 (0.338)
0.121
0.972
0.573
0.594
71
Table 8. (continued).
Sprague's Pipit
β (SE)
P
Chestnut-collared Longspur1
β (SE)
P
Horned Lark1
β (SE)
P
Savannah Sparrow1
β (SE)
P
Structure (Mean)
Grazing Treatment
Vegetation Height-Density (cm)
% Bare Ground
% Shrubs
0.058 (0.229)
0.048 (0.080)
-0.019 (0.028)
-0.073 (0.057)
0.800
0.551
0.500
0.203
-0.175 (0.105)
0.016 (0.037)
0.023 (0.013)
-0.022 (0.026)
0.096
0.655
0.073
0.399
-0.089 (0.028)
0.003 (0.010)
0.013 (0.003)
-0.009 (0.007)
0.002
0.765
<.001
0.217
-0.015 (0.091)
0.048 (0.032)
-0.010 (0.011)
-0.039 (0.023)
0.874
0.136
0.364
0.091
Heterogeneity (SD)
Grazing Treatment
Vegetation Height-Density (cm)
% Bare Ground
% Shrubs
0.114 (0.140)
-0.205 (0.089)
-0.005 (0.019)
0.040 (0.035)
0.414
0.020
0.798
0.253
-0.186 (0.063)
-0.014 (0.040)
0.025 (0.009)
-0.005 (0.016)
0.003
0.734
0.003
0.762
-0.105 (0.022)
-0.015 (0.014)
0.011 (0.003)
-0.004 (0.006)
<.0001
0.284
<.001
0.436
0.108 (0.074)
0.063 (0.047)
-0.002 (0.010)
-0.027 (0.019)
0.143
0.178
0.871
0.150
Heterogeneity (CV)
Grazing Treatment
-0.067 (0.174)
Vegetation Height-Density
-1.224 (0.599)
% Bare Ground
-0.069 (0.190)
% Shrubs
0.119 (0.166)
1
Log10 + 1 transformation of response variable
0.702
0.041
0.716
0.476
-0.191 (0.070)
0.288 (0.239)
<.001 (0.076)
0.189 (0.066)
0.006
0.229
0.995
0.004
-0.119 (0.038)
0.095 (0.131)
-0.031 (0.042)
0.046 (0.036)
0.002
0.470
0.452
0.212
0.034 (0.092)
-0.172 (0.315)
0.092 (0.100)
0.036 (0.087)
0.710
0.585
0.359
0.684
72
Table 8. (continued 2).
Clay-colored Sparrow
β (SE)
P
Vesper Sparrow1
β (SE)
P
Western Meadowlark
β (SE)
P
Brown-headed Cowbird2
β (SE)
P
Structure (Mean)
Grazing Treatment
Vegetation Height-Density (cm)
% Bare Ground
% Shrubs
-0.228 (0.229)
0.155 (0.080)
0.011 (0.028)
-0.080 (0.057)
0.319
0.053
0.709
0.163
-0.035 (0.051)
0.009 (0.018)
-0.001 (0.006)
-0.016 (0.013)
0.496
0.600
0.924
0.220
-0.450 (0.303)
0.160 (0.106)
-0.007 (0.037)
-0.076 (0.076)
0.137
0.131
0.849
0.318
0.059 (0.410)
-0.357 (0.125)
-0.006 (0.030)
0.127 (0.052)
0.886
0.004
0.852
0.014
Heterogeneity (SD)
Grazing Treatment
Vegetation Height-Density (cm)
% Bare Ground
% Shrubs
0.086 (0.201)
0.012 (0.127)
0.009 (0.027)
-0.006 (0.051)
0.670
0.923
0.742
0.906
-0.023 (0.040)
0.004 (0.025)
0.004 (0.005)
-0.006 (0.010)
0.569
0.888
0.510
0.581
-0.167 (0.219)
-0.164 (0.139)
0.009 (0.030)
0.089 (0.055)
0.447
0.238
0.770
0.107
-0.218 (0.644)
-0.038 (0.201)
0.036 (0.068)
-0.076 (0.110)
0.735
0.850
0.598
0.488
Heterogeneity (CV)
Grazing Treatment
-0.127 (0.217)
Vegetation Height-Density
-0.532 (0.747)
% Bare Ground
0.303 (0.237)
% Shrubs
-0.065 (0.207)
1
Log10 + 1 transformation of response variable
2
Lowland habitat only
0.559
0.476
0.201
0.753
-0.037 (0.037)
-0.036 (0.127)
-0.002 (0.040)
0.085 (0.035)
0.322
0.777
0.963
0.017
-0.371 (0.233)
-1.267 (0.802)
0.407 (0.254)
0.227 (0.223)
0.112
0.114
0.110
0.308
-0.568 (0.454)
1.051 (1.144)
-0.273 (0.797)
-0.262 (0.487)
0.212
0.358
0.732
0.591
73
Table 8. (continued 3).
Red-winged Blackbird2
β (SE)
P
Brewer‘s Sparrow
β (SE)
P
McCown‘s Longspur1
β (SE)
P
Structure (Mean)
Grazing Treatment
Vegetation Height-Density (cm)
% Bare Ground
% Shrubs
0.818 (0.309)
-0.069 (0.094)
-0.002 (0.022)
0.057 (0.039)
0.008
0.462
0.939
0.147
-0.149 (0.150)
0.054 (0.053)
-0.006 (0.019)
0.001 (0.038)
0.320
0.306
0.740
0.974
-0.077 (0.054)
-0.005 (0.019)
0.006 (0.007)
0.009 (0.013)
0.152
0.788
0.334
0.516
Heterogeneity (SD)
Grazing Treatment
Vegetation Height-Density (cm)
% Bare Ground
% Shrubs
0.733 (0.382)
0.098 (0.119)
-0.009 (0.041)
-0.028 (0.065)
0.055
0.411
0.817
0.664
0.040 (0.074)
0.186 (0.047)
-0.006 (0.010)
-0.043 (0.019)
0.591
<.001
0.541
0.024
-0.110 (0.030)
-0.019 (0.019)
0.005 (0.004)
0.015 (0.008)
<.001
0.318
0.233
0.040
Heterogeneity (CV)
Grazing Treatment
0.827 (0.274)
Vegetation Height-Density
0.309 (0.690)
% Bare Ground
0.168 (0.480)
% Shrubs
-0.267 (0.294)
1
Log10 + 1 transformation of response variable
2
Lowland habitat only
0.003
0.654
0.727
0.363
0.005 (0.084)
0.339 (0.287)
0.292 (0.091)
-0.212 (0.080)
0.949
0.238
0.001
0.008
-0.057 (0.037)
0.162 (0.126)
-0.008 (0.040)
0.073 (0.035)
0.121
0.199
0.840
0.037
74
Most bird-heterogeneity associations at pasture scale were unique to one or the other
heterogeneity measure (Table 8). Three associations were consistent between the two
measures: Sprague‘s Pipit was negatively associated with patchiness of vegetation heightdensity, Brewer‘s Sparrow was negatively associated with patchiness of shrubs, and
McCown‘s Longspur was positively associated with patchiness of shrubs. No
associations conflicted between the two measures (i.e., a positive association with one
measure and a negative association with the other).
4.4.3 Grazing-habitat relationships
Relationships between grazing and habitat variables as they pertained to birds were
implicit in the changes in significance of grazing on avian community structure and
species abundance as different habitat variables were entered into statistical models. For
example, when analyzed independently, grazing had a significant influence on Baird‘s
Sparrow abundance; however, when analyzed concurrently with CV-derived habitat
heterogeneity at plot scale, grazing was no longer significant (Table 7). This suggests
that the influence of grazing was, in fact, driven by habitat associations. At pasture scale,
grazing was not significant when analyzed concurrently with any of the habitat models
for this species (Table 8). Similar trends existed for Red-winged Blackbird and
McCown‘s Longspur. Only Chestnut-collared Longspur and Horned Lark consistently
showed significant relationships with grazing.
75
4.5 Scale effects
Species richness and diversity were both significantly higher at pasture scale (Figure 3).
Mean values for richness at pasture scale were more than double those at plot scale in
both grazing treatments. Mean species diversity was 25-30% greater at pasture scale.
Habitat heterogeneity was generally lower at pasture scale for both SD and CV
measures. Patchiness of exposed moss/lichen cover in the grazed treatment and bare
ground and shrub cover in both treatments remained similar between spatial scales based
on SD values (Figure 4). In comparison, only patchiness of shrub cover in the grazed
treatment remained similar between spatial scales based on CV values (Figure 5). No
habitat variables had significantly greater patchiness at pasture scale.
76
18
Plot Scale
4.0
Pasture Scale
Plot Scale
14
*
*
12
10
8
6
4
Pasture Scale
3.5
Species Diversity (H' )
Species Richness (S)
16
3.0
*
*
2.5
2.0
1.5
1.0
0.5
2
0
0.0
Ungrazed
Grazed
Ungrazed
Grazed
Figure 3. Effects of spatial scale on avian community structure at Grasslands National
Park (ungrazed) and adjacent community pastures (grazed) in 2008. Species diversity
values were calculated using the Shannon Diversity Index (H′). Error bars represent
standard error. Scale effects were analyzed using Wilcoxon rank sum tests. Asterisk (*)
denotes a significant effect of spatial scale at α-value of 0.1.
77
30.0
*
*
Ungrazed Plot
Ungrazed Pasture
Grazed Plot
Grazed Pasture
Heterogeneity (SD)
25.0
*
20.0
15.0
*
*
10.0
5.0
*
*
*
*
0.0
Vegetation
Height-Density
Litter Cover
Exposed
Moss/Lichen
Bare Ground
Grass
Forbs
Shrubs
Figure 4. Effects of spatial scale on habitat heterogeneity measured from standard deviation
(SD) at Grasslands National Park (ungrazed) and adjacent community pastures (grazed) in
2008. Heterogeneity values were calculated at plot (n=130) and pasture (n=13) scales.
Error bars represent standard error. Scale effects were analyzed using Wilcoxon rank sum
tests. Asterisk (*) denotes a significant effect of spatial scale at α-value of 0.1.
78
2.0
*
Ungrazed Plot
Ungrazed Pasture
Grazed Plot
Grazed Pasture
Heterogeneity (CV)
1.5
*
*
*
*
*
*
*
1.0
*
*
*
*
*
0.5
0.0
Vegetation
Height-Density
Litter Cover
Exposed
Moss/Lichen
Bare Ground
Grass
Forbs
Shrubs
Figure 5. Effects of spatial scale on habitat heterogeneity measured from coefficient of
variation (CV) at Grasslands National Park (ungrazed) and adjacent community pastures
(grazed) in 2008. Heterogeneity values were calculated at plot (n=130) and pasture (n=13)
scales. Error bars represent standard error. Scale effects were analyzed using Wilcoxon rank
sum tests. Asterisk (*) denotes a significant effect of spatial scale at α-value of 0.1.
79
5.0 DISCUSSION
5.1 Avian relationships with grazing, habitat structure, and habitat type
The presence of most species in both treatments suggests that some habitat heterogeneity
existed in the ungrazed treatment, so that species associated with both short and tall
grasses were present, and grazing by cattle in the grazed treatment was sufficiently
patchy to leave enough habitat for those species that prefer taller, denser vegetation. A
possible reason why no significant differences in species richness or diversity were
observed between the treatments was because grazing pressure was fairly low in the
grazed treatment. Grazing pressure estimates from 2007 indicated that grazing intensity
in the community pastures was at approximately 24% forage utilization. In semi-arid
grasslands, moderate intensity ranges from 35-45% utilization (Holecheck et al. 1999).
Another consideration is that temperate grasslands are relatively depauperate in species
(Mengel 1970, Cody 1985b), which limits the range of potential diversity values in these
systems (Wiens 1974). A larger sample size may be required to detect effects of lightmoderate-intensity grazing on avian richness and diversity in this environment.
Although grazing did not have an effect on overall species richness or diversity, some
species were influenced by grazing. This suggests that the lack of grazing effects on
richness and diversity may have been in part due to these species-specific relationships
being averaged out at the community level. For example, species richness values do not
indicate which species are present in a given area, only how many, so an equal number of
species may be present in two treatments, yet the communities are different. Similarly,
one species may be more abundant in one treatment, while a second species is more
abundant in another treatment, such that diversity is still calculated to be similar between
80
the treatments, even though there is a treatment effect on the individual species.
Similarity indices could provide additional information about variability in community
structure among pastures and grazing treatments in this system.
Lowland habitat contained more species than upland habitat within the same
treatment. This is not surprising as lowland habitat had a greater variety of microhabitats
containing both grassland and mesic vegetation. This supported not only the obligate
grassland species, but riparian species such as Common Yellowthroat and Eastern
Kingbird as well. Moreover, species such as Brewer‘s Sparrow and Clay-colored
Sparrow use shrubs and tall vegetation as perches and for nesting (Wiens 1973, Kantrud
1981, Knapton 1994, Rotenberry et al. 1999), so they were more frequently encountered
in low-lying areas where wetter conditions allowed for taller, denser vegetation and
brush.
The lack of differences in species richness and diversity between grazed and ungrazed
lowland plots was somewhat surprising. Cattle tend to congregate more frequently
around water sources (Vallentine 2001), so one would expect the associated trampling of
vegetation and intensified grazing to have an impact on bird populations within the
grazed lowland plots. One possible explanation for this lack of grazing effect is that
cattle congregated only in some lowland plots, particularly those which were closest to
water and perhaps most accessible, so that within-treatment effects of cattle grazing and
trampling varied too much to permit detection of between-treatment effects. Personal
observations of cattle repeatedly frequenting the same plots throughout the season while
avoiding others support this explanation.
81
Baird‘s Sparrow was more abundant in ungrazed habitat, positively associated with
vegetation height-density, and negatively associated with bare ground. These results are
consistent with those of other studies, which have also found Baird‘s Sparrow to be
associated with taller, denser vegetation with less bare ground in ungrazed to moderately
grazed mixed-grass prairie (Sutter and Brigham 1998, Davis et al. 1999, Winter 1999,
Madden et al. 2000). While the overall population of this species is declining at dramatic
rates across North America (most recent North American Breeding Bird Survey results
estimate an average annual 2.7% population decline (P=0.008) from 1966-2007; Sauer et
al. 2008), it is still fairly common in some areas, particularly in parts of North Dakota,
southern Saskatchewan, and southeastern Alberta (reviewed in Dechant et al 1998a;
Winter 1999). Population decline in Canada is estimated at 1.2% from 1966-2007, but
this decline is not significant (P>0.1) (Sauer et al. 2008). Despite its preference for
ungrazed habitat in this study, Baird‘s Sparrow was, nevertheless, the most abundant and
ubiquitous species in the grazed treatment, with at least one individual recorded in every
grazed plot. Thirty percent of individuals (82 birds) were observed in grazed plots and
mean abundance in the ungrazed treatment was only 20% greater than in the grazed
treatment. Thus, it does not appear that grazing was strongly detrimental to this species.
Baird‘s Sparrow was also negatively associated with shrub cover. This was not
consistent with this species‘ grazing preference, since shrub cover was greater in
ungrazed habitat; however, Baird‘s Sparrow abundance was greater in upland habitat,
which had significantly lower shrub cover than lowland habitat, so the negative
association with shrub cover may have been attributed more to this species‘ habitat
preferences (upland/lowland) than to its grazing preferences. Other studies have also
82
found this species to be negatively associated with shrubs (Madden et al. 2000, Davis
2004).
Horned Lark, McCown‘s Longspur, and Chestnut-collared Longspur were all
positively influenced by grazing and were positively associated with bare or exposed
(moss/lichen) ground. Other studies have also found these species to be associated with
grazed habitat characterized by short, sparse vegetation (Wiens 1973, Kantrud 1981,
Davis 2004, Fritcher et al. 2004). Horned Lark abundance increased as shrub cover
increased; however, shrub cover was greater in the ungrazed treatment, so this habitat
association conflicted with the grazing preference of this species. Further, this species is
typically associated with habitat containing little to no woody vegetation (Beason 1994).
Again, one possible reason for this may be that habitat preference (upland/lowland)
overshadowed specific habitat associations and grazing preference. Horned Lark
abundance was greater in lowland habitat, which generally had more shrub cover
irrespective of grazing treatment. Lowland habitat also contained more bare ground, so
there may have been a trade-off between shrub cover and bare ground, with bare ground
being the more important habitat characteristic for this species. McCown‘s Longspur
was positively associated with vegetation height-density, which was at odds with this
species‘ habitat and grazing preferences since vegetation height-density was greater in
lowland habitat and in the ungrazed treatment, whereas this species occurred almost
exclusively in grazed upland habitat. The biological significance of this relationship is
unclear.
Sprague‘s Pipit abundance was similar between grazing treatments. This species is
tolerant of a range of grazing intensities (Kantrud 1981, Knopf 1996). Other studies have
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found Sprague‘s Pipit to be associated with taller vegetation (Davis 2004) or, conversely,
with moderate to heavy grazing (Kantrud 1981). Sprague‘s Pipit was also negatively
associated with shrub cover and more abundant in upland habitat, which is consistent
with this species‘ preference for well-drained habitat with little to no woody vegetation
(Robbins and Dale 1999).
Savannah Sparrow and Vesper Sparrow were similarly abundant between grazing
treatments, but Savannah Sparrow was more abundant in ungrazed lowland and Vesper
Sparrow was more abundant in grazed lowland. Savannah Sparrow showed no
significant associations with habitat characteristics, suggesting that this species is more of
a habitat-generalist, which is consistent with the literature (Knopf 1996, Davis et al.
1999), though some studies have found this species to be associated with taller vegetation
(reviewed by Kantrud 1981, Davis and Duncan 1999), which is consistent with this
species‘ preference for ungrazed lowland habitat in this study. An association between
Savannah Sparrow and lowland habitat appears to be an indirect result of higher moisture
levels in lowlands permitting the growth of dense vegetation, which is generally favoured
by this species (Wiens 1969), though no association between Savannah Sparrow
abundance and vegetation height-density was apparent in this study. Vesper Sparrow
was positively associated with bare ground and shrub cover, both of which were greater
in lowland habitat. Other studies have found this species to be associated with shorter
vegetation (Davis and Duncan 1999), as well as moderate to heavy grazing (Kantrud and
Kologiski 1983), which indirectly results in greater exposure of bare ground (Hartnett et
al. 1997). Although Vesper Sparrow is reported to prefer drier habitat, studies in this
regard have focused either on moister, tall-grass prairie or compared lowland tall-grass to
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upland mixed-grass sites (reviewed in Dechant et al. 2000). Many lowland plots in this
study area remained dry throughout most of the breeding season.
Clay-colored Sparrow and Brewer‘s Sparrow were not influenced by grazing, but both
species were more abundant in lowland habitat. Both species are strongly associated with
shrub cover (Knapton 1994, Rotenberry et al. 1999), which was higher in lowland
habitat. Clay-colored Sparrow was significantly associated with shrub cover, but
Brewer‘s Sparrow was not. However, results for this species could only be generated at
the pasture scale, which overall showed fewer bird-habitat relationships (see Section 5.5
for further discussion on scale). Clay-colored Sparrow was also not associated with
shrub cover at the pasture scale. Both species are tolerant of light-moderate grazing
(Wiens 1973, Kantrud 1981); among conservative intensities of grazing, shrub cover is
likely a better indicator of these species‘ presence than grazing treatment (Dechant et al
1998c).
Western Meadowlark abundance was similar between grazing treatments in both
habitat types, but was higher in lowland habitat as a whole. This species is more of a
habitat generalist; it is tolerant of a wide range of grazing intensities and occurs in a
variety of habitats (Kantrud 1981, Lanyon 1994). As with Vesper Sparrow, studies of
Western Meadowlark that have found it to prefer upland habitat have compared lowland
tall-grass to upland mixed-grass sites (e.g., Bock et al. 1999). Further, habitat
preferences of this species are highly variable and region-specific (e.g., mesic shrubsteppe, xeric tallgrass prairie, etc.; reviewed in Dechant et al. 1999b).
Red-winged Blackbird was more abundant in ungrazed lowland. Only 7% of
individuals (three birds) were observed in the grazed treatment. This species is tolerant
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of light grazing (Kantrud 1981); thus, its general absence from grazed lowland plots may
not have been related to grazing treatment so much as to pre-existing differences in the
landscape. The ungrazed pastures contained several relatively large, permanent creeks.
In contrast, most of the creeks in the grazed pastures were small and ephemeral.
Individuals may have selected ungrazed habitat because it contained more desirable
breeding habitat irrespective of grazing treatment. Alternatively, abundance of this
species may have been indirectly influenced by grazing due to the associated trampling
by cattle of the vegetation surrounding the few larger, more permanent creeks in the
grazed pastures. From a conservation standpoint, Red-winged Blackbird is not a
grassland obligate and is abundant across most of its range (Forcey et al 2008), reaching
maximum breeding densities in the Great Plains (Yasukawa and Searcy 1995). Further,
this species is frequently viewed as an agricultural pest because large flocks will raid seed
crops (Forcey et al 2008), so management strategies are often aimed at reducing its
reproductive output, not increasing it (e.g., Moulton et al. 2008). Thus, from a
management perspective, a reduction in Red-winged Blackbird abundance resulting from
the introduction of grazing into this area may not necessarily be of conservation concern.
Brown-headed Cowbird abundance was similar between grazing treatments and was
not associated with any habitat structural variables. This species is found in a wide
variety of habitats (Shaffer et al. 2003), so the lack of associations between Brownheaded Cowbird abundance and habitat structure in this study likely reflects this species‘
flexibility in selecting breeding habitat. Furthermore, since Brown-headed Cowbirds do
not construct their own nests, but instead lay their eggs in a number of other species‘
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nests, each of which has unique breeding habitat preferences, they may not be as selective
regarding habitat structure in their territories as those species that build their own nests.
The influence of grazing on birds was less apparent when analyzed concurrently with
habitat variables. This suggests that grazing was indirectly influencing avian distribution
and abundance through its direct impact on habitat structure and heterogeneity.
However, since grazing did remain significant in the habitat models for some species
(e.g., Chestnut-collared Longspur), other environmental factors beyond the habitat
variables analyzed in this study contributed to the influence grazing was having on the
avian community.
5.2 Measures of heterogeneity
The two measures of heterogeneity produced very different results. When heterogeneity
was calculated from SD values, avian relationships with habitat heterogeneity were
frequently consistent with avian relationships with habitat structure for a given habitat
variable. A total of 35 bird-heterogeneity (SD) relationships existed between the spatial
scales, of which 24 (69%) mirrored the bird-habitat structure relationship. For example,
species richness was positively associated with both quantity and patchiness of bare
ground cover; Baird‘s sparrow was negatively associated with both quantity and
patchiness of shrub cover; and so forth. Only 11 (31%) bird-heterogeneity relationships
existed where no bird-structure relationship existed for that habitat variable. In
comparison, when heterogeneity was calculated from CV values, bird-heterogeneity
relationships were frequently opposite of the bird-structure relationships for a given
habitat variable. Forty bird-heterogeneity variables existed between the scales, of which
only seven (17.5%) mirrored the bird-structure relationship. Seven were the reverse and
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26 (65%) were unique. These differences in results between the measures can be
attributed to their mathematical properties. Since CV is a standardized value that controls
for differences among mean values, it is expected that correlations between mean and CV
will be weaker than between SD and mean. This was indeed the case in this study.
Standard deviation-derived heterogeneity values were highly correlated with their
structural mean counterparts (plot: r >0.95; pasture: r >0.80). In comparison, CV-derived
heterogeneity values showed weaker, reversed correlations with their structural mean
counterparts (plot: r <-0.70; pasture: r <-0.50). This pattern of negative correlations
between CV and mean values is consistent with other studies that have used CV as a
measure of heterogeneity (e.g., Harrell and Fuhlendorf 2002, Lueders et al. 2006) and
results from the mean entering the CV equation in the denominator, which inverts its
effects on a variable (Sørensen 2002). Harrell and Fuhlendorf (2002) argued that CV
values are, therefore, nevertheless dependent on their respective means and may be of
limited use as a measure of heterogeneity. Thus, since both SD and CV are correlated to
mean, SD may be the more useful measure of heterogeneity because it is a true measure
of variation, whereas CV is a mean-variance ratio. Since both mean and SD potentially
have an effect on a given variable, and these effects may differ (e.g., one having a
positive effect and the other a negative effect), combining the two into a single measure
(CV) can lead to ambiguous results or, worse, may not depict true patterns in the system
(Sørensen 2002). If there is a negative effect of the mean value, no effect of SD, and a
positive effect of CV on a subject, it is uncertain if the positive effect of CV is a real
phenomenon or if it is an artefact of the inverted negative effect of the mean (Sørensen
2002). For example, in this study, Horned Lark abundance decreased as vegetation
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height-density increased, but appeared unaffected by SD-derived patchiness of this
variable (Table 7), so the positive relationship between Horned Lark abundance and CVderived patchiness of vegetation height-density is questionable. Another example of this
ambiguity is found in the relationships between grazing and habitat heterogeneity in
lowland habitat (Table 6). Grazing had no influence on SD-derived habitat patchiness,
but appeared to positively influence CV-derived patchiness of vegetation height-density
and litter cover. Grazing negatively influenced the mean values of these two habitat
variables, so once again it is uncertain if the CV-derived results represented real
relationships or resulted from an inverse correlation with their respective means.
Three important criteria to consider when choosing a heterogeneity measure are the
identity of the study subjects, the location of the study, and the scale(s) of study.
Coefficient of variation is the more commonly used measure of heterogeneity in
ecological studies and is frequently applied to studies of grassland plant (e.g., Fuhlendorf
and Smeins 1999) and bird (e.g., Wiens 1974, Roth 1976, Rotenberry and Wiens 1980,
Sutter and Brigham 1998, Lueders 2006) communities. While CV may be suitable for
plant community studies, it may not adequately depict ecological patterns in bird
communities because of the perceptive abilities of birds that are absent in plants. For
example, two sites with similar proportional or relative variability in a given habitat
characteristic, but with very different mean values of that characteristic, may be
perceived very differently by a bird seeking suitable breeding habitat, but CV values will
not capture this difference. Cody (1985b) reported that habitat selection appears more
precise in tallgrass than in shortgrass species. This suggests that measurable or absolute
differences (versus proportional or relative differences) in habitat structure serve as
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important cues in avian habitat selection because mean and, by association, SD values of
habitat characteristics (e.g., vegetation height, litter depth) are generally higher in tall
than in short grasslands.
The ability of CV to equalize marked differences in habitat structure may also pose a
problem in studies of diverse habitats. Habitat structure is expected to vary less among
sites within a single habitat type (e.g., two grassland sites) than between different habitat
types (e.g., grassland versus shrub-steppe). In the latter scenario, variability in shrub
cover may be proportionally similar between the two habitats, but measurably much
higher in the shrub-steppe habitat. To a Brewer‘s Sparrow, a 1% range of shrub cover
values in grassland versus a 10% range in shrub-steppe may be significantly different, but
this difference may not be captured by CV.
Results of this study suggest that upland and lowland habitat were fundamentally
different in their structural composition, possibly due to hydrological or other
environmental factors. Plant community composition was notably different between the
two habitat types, which may have led to some of the observed structural differences.
For example, greasewood was present only in lowland habitat and western snowberry
was much more abundant in lowland habitat. Shrub cover was on average seven times
higher in lowland habitat. Moister conditions in lowland habitat may have permitted the
growth of species absent from uplands or encouraged more vigorous growth of species
present in both habitats but limited by water availability in uplands. The avian
community was significantly influenced by habitat type; only Savannah Sparrow was
similarly abundant in upland and lowland pastures. This suggests that species response to
habitat cues differed between the habitats. Avian relationships with habitat heterogeneity
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were not analyzed between the two habitats; however, initial exploratory analyses did
suggest that some species relationships with heterogeneity differed between upland and
lowland habitat.
A final consideration is the potential for CV to negate scale effects in multiple-scale
studies, such as when two sites with similar proportional variability, but different
measurable variability, are grouped together into a larger experimental unit. Whereas SD
will be averaged into an intermediate value in this scenario, CV will remain the same.
Results of this study do not suggest that this occurred, as scale had a greater influence on
CV-derived than SD-derived heterogeneity; however, it is impossible to be certain.
Based on the three selection criteria, it appears that SD was the most biologically
relevant measure of heterogeneity in this study for the following reasons: (1) the study
subjects were grassland birds, which arguably discriminate between measurable
differences in habitat structure that may be lost when CV is used as the measure of
heterogeneity; (2) the study area consisted of two fundamentally different habitat types,
so using CV values ran the risk of misinterpreting significant differences in habitat
structure as being similar; and (3) the study was conducted at multiple spatial scales, so
using CV values ran the risk of negating scaling effects within the system. All
subsequent discussion of heterogeneity will focus on SD-derived results only. Although
both measures were used in this study for the purpose of comparing their value as
heterogeneity measures in grassland avian community studies, it is recommended that it
is decided prior to analysis which measure will be used in a study in order to avoid
confusion.
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5.3 Influence of grazing, habitat type, and scale on habitat structure and heterogeneity
Relationships between grazing and habitat structure were generally consistent with those
found in the literature. Grazed pastures generally had shorter, sparser vegetation, less
litter, more bare and exposed ground, more forbs, and fewer shrubs. Grazing affects
habitat structure by reducing plant biomass. More plant material consumed by grazers
translates into less litter accumulation (Bragg and Steuter 1996, Temple et al. 1999),
which results in greater exposure of bare ground (Hartnett et al. 1997). The associated
trampling of vegetation by foraging cattle also increases bare ground and reduces shrub
cover (Hartnett et al. 1997). Reduced litter and cover of preferentially-foraged grass
species also results in increased cover of some forbs, which would otherwise be at a
competitive disadvantage (Stohlgren et al. 1999). Lowland habitat generally had taller,
denser vegetation, more bare ground, less exposed moss/lichen, more grass and shrubs,
and fewer forbs. Although cattle spend a disproportionate amount of time in riparian
areas (Bailey et al. 2008), which could result in more exposed bare ground, these
differences in habitat structure between the habitat types were significant regardless of
grazing treatment. It is more likely that differences in habitat structure were related to
differences in moisture levels between the two habitats.
I predicted habitat heterogeneity would be greater in the grazed treatment; however,
no clear trends emerged as grazing both positively and negatively influenced habitat
heterogeneity. Grazing can both increase and decrease habitat heterogeneity in
grasslands depending on soil type, moisture level, and primary productivity (Olff and
Ritchie 1998, Willms et al. 2002); however, decreases in heterogeneity due to grazing are
more typical of shortgrass prairie, which has a long evolutionary history of heavy grazing
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by bison, has relatively low net primary productivity due to the arid conditions, and is
dominated by grasses that respond positively (i.e., increased growth) to grazing, thereby
restricting the growth of other plant species and creating a more uniform landscape
(Milchunas et al. 1988, 1998). In contrast, grazing generally increases heterogeneity in
mixed-grass prairie, where grazing pressure was historically lower, net primary
productivity is higher, and the plant community is more varied, containing grasses that
respond positively and negatively to grazing (Milchunas et al. 1988, Olff and Ritchie
1998).
Vegetation height-density (upland habitat) and shrub cover (upland habitat and both
scales) were both patchier in the ungrazed treatment. One possible explanation for this
pattern is that mean values of these two variables were significantly lower in the grazed
treatment, restricting the range of potential heterogeneity values. For example, shrub cover
was very low or non-existent in many grazed upland plots. Similarly, vegetation heightdensity was significantly lower in grazed upland plots. Thus, one would expect the range
of available heterogeneity values for these habitat variables to be limited as well.
Despite significant directional trends between grazing and habitat heterogeneity,
negative β values for most habitat variables (Tables 4 and 6) suggested a potential trend
towards increased heterogeneity in the grazed treatment had grazing pressure been
stronger. Only patchiness of vegetation height-density and shrub cover appeared
negatively influenced by grazing. This is pure speculation at this time as most
relationships were not statistically significant; however, it would be insightful to learn
how a stronger grazing pressure more closely representing moderate grazing intensity
influences habitat heterogeneity in this system. Moderate grazing in mixed-grass prairie
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is generally believed to optimize habitat heterogeneity because it permits grazers to
forage selectively, consuming only desirable plant species and thereby creating structural
patches in the landscape (Hartnett et al. 1997). In contrast, heavy grazing forces grazers
to forage uniformly, consuming even less desirable plant species, in order to meet their
caloric needs, and light grazing produces few structural patches in the landscape (Collins
and Barber 1985, Milchunas et al. 1988).
Lowland habitat was generally patchier than upland habitat, suggesting that lowland
habitat was inherently variable irrespective of grazing treatment. This was further
suggested by the lack of any significant relationships between grazing and habitat
heterogeneity in lowland habitat. Some plots were predominantly riparian, with
extensive shrub cover and permanent creeks running through them, while other plots
consisted of dried mudflats and sparse vegetation cover. Thus, any effects of grazing that
existed may have been more difficult to detect due to this inherent variability. The lack
of significant relationships in lowland habitat also suggests that relationships between
grazing and habitat heterogeneity in upland habitat were driving plot-scale patterns, such
that the patterns of significant relationships when the two habitat types were analyzed
together were similar to those patterns when upland habitat was analyzed alone.
The existence of positive associations between grazing and habitat heterogeneity
suggests that cattle were grazing selectively. Cattle make foraging decisions at six spatial
scales: home range (landscape), camp (pasture), feeding site (particular area within
pasture foraged for a few hours), patch, feeding station (group of plants within immediate
reach), and bite (plants ingested) (Vallentine 2001). Of these, patch and camp scales best
correspond to plot and pasture scales in this study. Habitat heterogeneity was
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significantly influenced by grazing at both spatial scales, suggesting that cattle were
foraging selectively both among plots within a pasture (i.e., favouring some plots over
others) and within the plots themselves (i.e., favouring some plants over others). Other
studies have found selectivity in foraging to increase with spatial scale (Wallace et al.
1995, WallisDeVries et al. 1999, Barnes et al. 2008). For example, Wallace et al. (1995)
found that bison grazed randomly within patches and only became selective at the larger
feeding site scale. Cattle were selective at patch scale, but grazed randomly within
feeding stations (WallisDeVries et al. 1999). Bison are less selective than cattle (Hartnett
et al. 1997), so are more likely to graze randomly at smaller spatial scales. An important
factor regarding cattle foraging decisions is water availability, which can strongly
influence grazing patterns in the landscape (Briske et al. 2008). Artificial water supplies
were provided in uplands to encourage forage utilization in these areas and to reduce
cattle damage to riparian areas, which almost certainly altered the natural movements of
cattle at the larger feeding site and camp (pasture) scales. In the absence of these water
supplies, cattle may have spent more time in lowlands, resulting in greater variability
among plots within pastures.
Pasture-scale heterogeneity was generally lower than plot-scale heterogeneity in both
grazed and ungrazed treatments, suggesting that heterogeneity was inherent in the habitat
irrespective of grazing treatment. Habitat heterogeneity exists in the absence of grazing
due to various abiotic and biotic factors, including topography, moisture, soil type, small
mammal disturbance, and insect grazing (Cid et al. 1991, Stohlgren et al. 1999,
Vallentine 2001, Harrison et al. 2003, Joern 2005). Fuhlendorf and Smeins (1999) also
found that habitat heterogeneity was highest at the smallest spatial scale in their study of
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grazing and scale effects on Sonoran grasslands. Further, their results suggested the
existence of interactive effects between grazing disturbance and spatial scale. My results
similarly suggest that grazing may alter the effects of spatial scale on habitat
heterogeneity. While habitat heterogeneity decreased as spatial scale increased in both
grazing treatments, the difference between plot- and pasture-scale heterogeneity within
the grazed treatment appeared larger for some habitat variables. This suggests that
grazing was altering the effect of scale on these variables, so that relationships between
grazing and heterogeneity did not follow a similar pattern between the treatments as scale
increased. Grazed plots were generally more heterogeneous than ungrazed plots, likely
due to selective foraging by cattle. Thus, greater heterogeneity at the smaller plot scale in
grazed pastures may have resulted in less variability among the pastures themselves since
more variability was captured within the pastures in the grazed treatment.
5.4 Effects of spatial scale on bird-habitat relationships
I hypothesised that both species richness and diversity and habitat heterogeneity would
increase with spatial scale; however, while species richness and diversity increased with
scale, habitat heterogeneity decreased or was averaged out among plots within pastures.
The increase in species richness from plot to pasture scale is not surprising given the
well-documented species-area relationship known to exist in ecology in which larger
areas are expected to hold more species (MacArthur and Wilson 1967). Habitat
heterogeneity may already have been relatively high at plot scale, such that the larger
pasture scale did not capture any additional variability in habitat structure. Thus,
increased avian richness and diversity at pasture scale could not be attributed to greater
availability of microhabitats. Possibly intra- and interspecific interactions restricted
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species distributions at the plot scale. For example, plot-scale competition may have
excluded species or individuals from plots that would otherwise be suitable. Interspecific
territoriality (―interference competition‖ [Cody 1985b]) has been reported for some
species, including McCown‘s Longspur (With 1994a), Chestnut-collared Longspur (Hill
and Gould 1997), Red-winged Blackbird (Yasukawa and Searcy 1995), and between
Clay-colored and Brewer‘s Sparrows (Knapton 1994). Alternatively, conspecific
attractions may have led multiple individuals of a particular species to establish territories
in one plot or in a group of neighbouring plots, displacing other species (e.g., Baird‘s
Sparrow [Ahlering et al. 2006]). Similar outcomes could occur when semi-colonial
species establish territories adjacent to one another (e.g., McCown‘s Longspur [Greer and
Anderson 1989]). Since there was only so much space available in a plot, saturation may
have added to these interactions, i.e., no more individuals could fit within a given plot, so
that species richness and diversity at plot scale became dependent on and limited by
which species were first to establish territories within the plots. Such indirect
interspecific interaction (―exploitation competition‖) is difficult to document, but has
been demonstrated with Savannah Sparrows, which were absent from suitable sites in one
study when Brewer‘s and Vesper Sparrows were present, despite a lack of overt
aggression among the species (reviewed in Cody 1985b).
Most bird-habitat relationships existed at the plot scale. Few relationships existed at
the pasture scale and fewer still carried over from plot to pasture scale. This suggests that
habitat characteristics are important predictors of avian habitat use at small spatial scales,
but become less important as spatial scale increases. Instead, other factors, such as
landscape variables (e.g., patch size, distance to edge), may be more important predictors
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of habitat use at larger spatial scales. Wiens (1989b) suggested that habitat quality
should be measured from factors influencing individual reproductive success (e.g.,
habitat structure and composition around a prospective nest site or within a prospective
foraging area). Indeed, Koper and Schmiegelow (2006) found small-scale habitat
characteristics were the strongest predictors of avian densities and nest success in a
multiple-scale analysis of mixed-grass prairie. Wiens (1989b) also cautioned against
averaging small-scale habitat characteristics over areas much larger than individual bird
territories at risk of losing their biological meaningfulness to birds. My results support
his contention as it appears that habitat characteristics, when averaged at the pasture
scale, lost their significance to birds, suggesting that birds were not factoring localized
habitat characteristics into habitat selection decisions at the larger scale. A move from
habitat characteristics at small scales to landscape characteristics at large scales as habitat
use predictors has been documented for grassland birds (Wiens 1986) and other avian
communities (Böhning-Gaese 1997, Thogmartin and Knutson 2007). In all likelihood,
birds make habitat selection decisions following a cascading approach, beginning with
landscape variables at large scales and ending with local habitat characteristics at small
scales. Thus, a bird seeking suitable breeding habitat may begin by selecting a patch of
habitat that meets its area requirements, then focusing in on a location in the patch that is
sufficiently far from the edge of the patch, and finally establishing a territory in a spot
that contains all the necessary microhabitats to meet its breeding-season needs. Support
for this argument comes from a Mediterranean example, in which the authors concluded
that habitat descriptors (habitat structure and composition, patch size, patch number) at
multiple spatial scales (foraging, territory, landscape) and interactions between these
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scales best explained the relationships observed between avian community structure and
the surrounding landscape (Coreau and Martin 2007).
5.5 Influence of habitat heterogeneity on grassland birds
The habitat heterogeneity hypothesis predicts that species diversity is positively
correlated with habitat heterogeneity (MacArthur and MacArthur 1961). In this study,
both species richness and diversity generally increased as habitat heterogeneity increased
where relationships were significant. Total bird abundance also increased as habitat
heterogeneity increased, suggesting that more heterogeneous habitats contained more
microhabitats for more species and individuals to occupy. These results are consistent
with the niche concept. A niche describes how a species ‗fits‘ in its environment, both in
terms of how it uses resources and how it interacts with other species. As more
microhabitats exist, there is a greater variety of resources (e.g., nest sites, foraging sites,
food) available for the community to exploit. This increased dimensionality in resources
and space permits more species, each within its defined niche, to occupy a given area
than would be possible if that same area contained only one type of habitat.
Baird‘s Sparrow and Sprague‘s Pipit were negatively associated with both quantity
and patchiness of bare ground and shrub cover. Thus, it appears that these species were
negatively influenced by these habitat variables regardless of their spatial distribution.
Both species are frequently associated with idle or lightly-grazed prairie where vegetation
is relatively tall and dense (Davis et al. 1999, Green et al. 2002, Davis 2004) and thus
may have a preference for more homogenous habitats. Sutter and Brigham (1998) found
Baird‘s Sparrow to be negatively associated with patchiness of litter cover, which also
suggests this species may prefer more uniformly dense habitat cover.
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Horned Lark, McCown‘s Longspur, and Chestnut-collared Longspur were all
positively associated with patchiness of some element of exposed ground: Horned Lark
with bare ground and the two longspurs with exposed moss/lichen cover. Differences in
these associations likely reflect differences in these species‘ distributions, with Horned
Lark more abundant in lowland, which had significantly more bare ground, and the two
longspurs more abundant in upland, which had significantly more exposed moss/lichen
cover. All three species typically forage on the ground in short, sparse vegetation (With
1994a, Beason 1995, Hill and Gould 1997). Both longspur species build their nests in
sparse vegetation, but will frequently place them next to a clump of grass, forb, shrub,
cactus, dung, etc. (Mickey 1943, Hill and Gould 1997). Thus, an association with patchy
exposed moss/lichen cover may reflect these species‘ preference for sparse habitat
interspersed with these structural elements. However, both longspur species were also
negatively associated with patchiness of shrub cover, suggesting that, while both species
will build their nests beside a shrub (With 1994b, Dechant et al. 1998b), shrubs are not
required to meet either species‘ nesting needs as both will readily use other structures for
this purpose (Mickey 1943, Hill and Gould 1997). Both longspurs were also negatively
associated with quantity of shrub cover, so it appears that these species were negatively
influenced by shrub cover regardless of its spatial distribution. Horned Lark prefers to
build its nest on bare ground in the open, with no concealing vegetation (With and Webb
1993). This species is also typically associated with large expanses of bare ground
(Beason 1995), so it is unclear why it was positively associated with patchiness of bare
ground in this study. Horned Lark was also positively associated with quantity of bare
ground, so it may be that this species was selecting for large patches of bare ground.
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Horned Lark and Chestnut-collared Longspur were both positively associated with
patchiness of bare ground at the pasture scale, suggesting that patchiness in the habitat
was important to these species beyond their territorial limits. Horned Lark is an early
nester, with fledglings from the first nest beginning to form flocks as early as mid-May
(Beason 1995), so availability of suitable foraging habitat within the larger area may be
sought out in anticipation of these flocks. Chestnut-collared Longspur is a semi-colonial
species which clumps its breeding territories into large aggregations (Hill and Gould
1997), so large-scale associations with habitat characteristics for this species may be a
result of multiple individuals making small-scale habitat selection decisions. Chestnutcollared Longspur will also forage outside of its territory (Hill and Gould 1997), so
availability of suitable foraging habitat may be sought out within the larger area.
Clay-colored Sparrow and Brewer‘s Sparrow both typically nest in shrubs (Knapton
1994, Rotenberry et al. 1999), but showed opposite associations with patchiness of shrub
cover. Brewer‘s Sparrow is more of a shrub-steppe species than a grassland species
(Walker 2004). The study area was located at the northeastern limits of this species‘
range and was not representative of its typical habitat, which is characterized by
extensive shrub cover. Thus, a negative association between Brewer‘s Sparrow and
patchiness of shrub cover likely reflects its preference for habitat with a more uniform
distribution of shrubs. In contrast, Clay-colored Sparrow prefers habitats that have both
grassland and woody elements (Knapton 1994), which may explain this species‘ positive
association with patchiness of shrub cover in this study. Territories of this species are
small (0.1-0.4 ha) compared to the area of a point-count plot (3.2 ha) because they are
used only for nesting (Knapton 1994). Further, territories are frequently located next to
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foraging sites, which are typically open areas containing short, sparse vegetation
(Dechant et al. 1998c). Clay-colored Sparrow was also positively associated with
patchiness of bare ground. Thus, it is highly plausible that Clay-colored Sparrow selects
for patchy habitat in order to meet both its nesting and foraging needs. Surprisingly, this
species was negatively associated with patchiness of exposed moss/lichen despite its
foraging habitat preferences; however, this association may be a spurious result of this
species being almost six times more abundant in lowland habitat, which had significantly
lower cover of exposed moss/lichen than upland habitat (9.81% versus 26.66% mean
cover).
Vesper Sparrow was positively associated with patchiness of bare ground and shrub
cover. While this species‘ overall preference is for sparsely vegetated habitat, it is noted
to occur in patchy habitat (Dechant et al. 2000). Vesper Sparrow is commonly observed
foraging on bare ground (Hales 1927), but uses shrubs and other taller structures as
singing perches and nest cover (Dechant et al. 2000). Thus, this species may select for
heterogeneous habitat containing a combination of these habitat characteristics.
Savannah Sparrow was positively associated with patchiness of vegetation heightdensity, which may reflect its general habitat preference for intermediate vegetation
density and low to intermediate vegetation height (Swanson 1998), but taller vegetation
around nest sites (Wheelwright and Rising 1993, but see Wiens 1969). For all of these
preferences to be met, a combination of tall and short vegetation is required.
Red-winged Blackbird, Western Meadowlark, and Brown-headed Cowbird were
poorly related to habitat heterogeneity. Red-winged Blackbird abundance was not
associated with any heterogeneity variable and the latter two species each had one
102
significant relationship. However, these species were also only weakly associated with
habitat structure, with at most two significant relationships (i.e., Western Meadowlark).
All three species are considered to be more habitat generalists, occurring in a wide range
of habitats (Yasukawa and Searcy 1995, Madden et al. 2000, Shaffer et al. 2003), which
may explain the scarcity of habitat associations. Western Meadowlark exhibits the most
restricted range of the three species; however, even this species occurs in a relatively
wide range of open habitats and tolerates a moderate range of grazing intensities
(Kantrud 1981, Lanyon 1994). Red-winged Blackbird will nest in a variety of substrates
(Yasukawa and Searcy 1995) and Brown-headed Cowbird does not build its own nest,
further reducing these species‘ dependence on specific habitat characteristics. Brownheaded Cowbird was positively associated with patchiness of shrub cover, which, in
combination with its positive association with quantity of shrub cover, reflects this
species‘ preference for open habitats interspersed with woody vegetation (Lowther 1993).
Western Meadowlark was negatively associated with patchiness of exposed moss/lichen,
which may be a result of this species‘ preference for dense vegetation in which to conceal
its nest (Lanyon 1994). Wiens (1974) found Western Meadowlark to occur along a wide
gradient of vegetation height-density (i.e., vertical) heterogeneity values.
In conclusion, species relationships with habitat heterogeneity were complex with no
clear directional trends. For those species with multiple heterogeneity associations,
simultaneous positive and negative relationships with patchiness were typical, thus it is
uncertain whether individual species were selecting for homogenous patches of
microhabitat contained within larger, heterogeneous habitats, or whether they were
selecting for heterogeneous habitats specifically. For example, a female Brewer‘s
103
Sparrow may seek out habitat dominated by shrubs to build her nest, whereas a female
McCown‘s Longspur may seek out habitat that includes some combination of short
vegetation, low litter, patchy grass, and perhaps a small shrub. Most grassland species
have distinct habitat preferences for nest sites and foraging sites, and these often differ
from one another (Wiens 1974); thus, selecting heterogeneous habitats containing
structural characteristics suitable for both activities would seem logical. Nevertheless, it
may be that both habitat selection methods are used by different species within the
community. Both methods of habitat selection have been documented concurrently in
other avian communities, such as among shrubland birds in the Mediterranean (Coreau
and Martin 2007). In this study, simultaneous positive and negative relationships with
patchiness for some species suggests that individual species require both patchy and
uniform structural components in their breeding habitat and the configuration of these
components depends on each species‘ unique habitat needs. The lack of directional
trends in bird-heterogeneity associations in this study is not surprising given the amount
of variability in habitat associations alone documented for most of these species in the
literature. Nevertheless, my results clearly indicate that habitat heterogeneity has a
significant influence on mixed-grass prairie bird communities, as all but one (Red-winged
Blackbird) of the focal species in this study were related to the patchiness of at least one
habitat variable. More importantly, my results are consistent with early reports by
Rotenberry and Wiens (1980) suggesting that habitat heterogeneity not only plays a role in
avian community assemblage and dynamics, but also influences the distribution and
activities of individual species.
104
6.0 CONCLUDING REMARKS
My results clearly show that significant relationships exist between grassland bird
communities and habitat heterogeneity. However, because this study was based on one
year of data, these relationships cannot be confidently attributed to grazing treatment.
Also, I conducted a large number of individual tests, so some of the relationships I found
may exist by chance alone or may reflect other, unexplored relationships. Furthermore,
correlations between grassland birds and habitat heterogeneity do not imply causation
(i.e., that associations are a result of habitat selection decisions). Similarly, relationships
between birds and grazing and between habitat attributes and grazing may or may not
represent direct responses of birds and habitat attributes to grazing treatment. Additional
years of study are needed to conclude that the relationships observed in this study
represent direct responses by the grassland community to grazing. Despite these caveats,
the number of significant relationships among birds, habitat features, and grazing suggest
that birds are responding to these factors regulating their environments. Further research
into bird-heterogeneity relationships warrants immediate attention given the continued
population declines of many grassland species. Studies of ultimate factors regulating
habitat selection (e.g., nest site and food availability, nest success) in the context of
heterogeneous habitats would help determine if birds use habitat patchiness as a
proximate cue of habitat selection.
105
7.0 MANAGEMENT IMPLICATIONS
My results demonstrate the high variability in relationships among grassland birds, their
habitats, and grazing. Further, they highlight the importance of conducting local studies
as many associations among grassland birds, habitat structure and heterogeneity, and
grazing are region-specific as indicated by the wide array of associations, sometimes
conflicting, documented in the literature. Even within one ecosystem, such as mixed-grass
prairie, variations in soil and moisture can lead to differences in community patterns and
how processes such as grazing impact those communities, making it impossible to develop
a universally-applicable management plan (Askins et al. 2008). Instead, a synthesis of
regional studies into general guidelines of acceptable ranges of habitat attributes for each
species would permit managers to select grazing regimes that would create those desired
attributes within a given area (Madden et al. 2000).
Spatial scale had a strong influence on the outcome of this study. Associations
between birds and habitat heterogeneity existed predominantly at the smaller plot scale.
An analysis conducted at pasture scale alone would have missed these associations.
Thus, my results show the limitations of extrapolating from one scale to another and
emphasize the importance of choosing spatial scales that are biologically relevant to the
community in focus (i.e., that capture the patterns and processes of interest), particularly
if the results are to be applied towards management decisions. Extrapolating from one
scale to another can lead to erroneous conclusions about the community being studied
and, consequently, the implementation of ineffective management practices.
Fuhlendorf and Engle (2001) proposed a paradigm promoting heterogeneity on
rangelands for both biodiversity and sustainable livestock production. In this study,
106
grazing intensity was estimated to be approximately 24% in 2007, which is a lightmoderate level of grazing in this semi-arid system (Holechek 1999), so it is possible that
grazing impact on habitat structure and heterogeneity was not strong enough to have a
clear influence on the avian community. Nevertheless, some species appeared to benefit
from grazing even at this light-moderate intensity. Most importantly, grazing did not
have a significant negative influence on the avian community. Only two species, Baird‘s
Sparrow and Red-winged Blackbird, were negatively associated with light-moderate
grazing, the former being the most abundant species in the study area and the latter being
a habitat generalist widespread and abundant across North America (Yasukawa and
Searcy 1995). These results suggest that cattle grazing may be a useful management tool
for grassland bird conservation when implemented at sustainable intensities.
107
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9.0 APPENDICES
9.1 Appendix I ―Passerine species observed in the study area during the breeding
season between 2006-2008
Endemic
*Sprague‘s Pipit Anthus spragueii
*Baird‘s Sparrow Ammodramus bairdii
*Lark Bunting Calamospiza melanocorys
*McCown‘s Longspur Calcarius mccownii
*Chestnut-collared Longspur Calcarius ornatus
Secondary endemic
*Horned Lark Eremophila alpestris
*Clay-colored Sparrow Spizella pallida
*Vesper Sparrow Pooecetes gramineus
*Lark Sparrow Chondestes grammacus
*Savannah Sparrow Passerculus sandwichensis
*Grasshopper Sparrow Ammodramus savannarum
*Western Meadowlark Sturnella neglecta
Other
Western Kingbird Tyrannus verticalis
*Eastern Kingbird Tyrannus tyrannus
Loggerhead Shrike Lanius ludovicianus
*Northern Rough-winged Swallow Stelgidopteryx serripennis
*Barn Swallow Hirundo rustica
*Cliff Swallow Petrochelidon pyrrhonota
Yellow Warbler Dendroica petechia
*Common Yellowthroat Geothlypis trichas
*Chipping Sparrow Spizella passerine
*Brewer‘s Sparrow Spizella brewerii
Le Conte‘s Sparrow Ammodramus leconteii
Nelson‘s Sharp-tailed Sparrow Ammodramus nelsoni
Song Sparrow Melospiza melodia
Bobolink Dolichonyx oryzivorus
*Red-winged Blackbird Agelaius phoeniceus
Yellow-headed Blackbird Xanthocephalus xanthocephalus
*Brewer‘s Blackbird Euphagus cyanocephalus
*Brown-headed Cowbird Molothrus ater
*American Goldfinch Carduelis tristis
*Recorded during point count surveys in 2008
123
9.2 Appendix II ―Example of a mixed-effects model run in S-Plus Version 8.0.4*
Linear mixed-effects model fit by maximum likelihood
Data: X2008.DATA..May.
AIC
BIC
logLik
245.3802 276.9231 -111.6901
Random effects:
Formula: ~ 1 | PASTURE
(Intercept) Residual
StdDev: 0.00003459105 0.5713325
Fixed effects: HOLA ~ REGIME + HT.DNS..x. + .LIT.CV..x. + .EX.M.L..x. +
.BR.GRD..x. + .GRSS..x. + .FORB..x. + .ALL.SHRB..x.
(Intercept)
REGIME
HT.DNS..x.
.LIT.CV..x.
.EX.M.L..x.
.BR.GRD..x.
.GRSS..x.
.FORB..x.
.ALL.SHRB..x.
Value
1.161686
-0.307606
-0.071547
-0.000573
0.001095
0.031665
-0.001821
0.013564
0.021544
Std.Error
0.4989677
0.1410602
0.0265523
0.0049027
0.0057175
0.0089393
0.0083465
0.0178967
0.0096788
DF
110
11
110
110
110
110
110
110
110
t-value p-value
2.328178 0.0217
-2.180672 0.0518
-2.694585 0.0082
-0.116865 0.9072
0.191501 0.8485
3.542153 0.0006
-0.218211 0.8277
0.757893 0.4501
2.225900 0.0281
Standardized Within-Group Residuals:
Min
Q1
Med
Q3
Max
-2.01573 -0.7145302 -0.2106575 0.7201776 3.02632
Number of Observations: 130
Number of Groups: 13
Value = Beta (β) value
Beta (β) values are interpreted as follows:
Horned Lark (HOLA) abundance was 0.31 times higher in the grazed treatment
(REGIME) than in the ungrazed treatment (i.e., a negative β value denotes a positive
influence of grazing);
As vegetation height-density (HT.DNS) increased by 1 cm, Horned Lark abundance
decreased by 0.07 individuals/3.2 ha (the area of a point-count plot);
As shrub cover (ALL.SHRB) increased by 1%, Horned Lark abundance increased by
0.03 individuals/3.2 ha.
*Insightful Corporation (2007)
124