The influence of habitat characteristics on grassland community composition and avian productivity in southern Illinois
Date of Award
Doctor of Philosophy
Grassland birds are the most rapidly declining bird guild in North America, due in large part to extensive loss and fragmentation of grassland habitat resulting from the spread of agriculture and other human-dominated landscapes. Over the past several decades, grassland birds have increasingly become a guild of high conservation and management interest as their populations continue to decline and suitable grassland habitat becomes continually scarcer. Although studies investigating grassland bird responses to management actions and habitat structure are common, few studies are concerned with clarifying the mechanisms through which habitat structure may affect grassland birds. Filling this knowledge gap is critical for increasing our understanding of grassland bird ecology and improving the effectiveness of management and restoration actions for grassland birds. To address this knowledge gap, I took a uniquely holistic approach to traditional grassland bird-habitat studies by concurrently gathering data on multiple wildlife taxa that may interact with birds to examine how these different taxa respond to habitat characteristics across multiple spatial scales, and how those responses may in turn impact grassland birds. Research was conducted on 10 grassland sites at Burning Star State Wildlife Management Area in northeast Jackson County, Illinois, during the bird breeding season (May-July) from 2018 to 2021. My first five objectives were to determine the grassland habitat characteristics that were most highly associated with the following taxa: arthropods, an important food source for adult and nestling grassland birds; small mammals, which are occasional nest predators and an alternate prey source for more prolific nest predators; snakes; raccoons; and grassland birds. My final objective was to estimate the extent to which grassland bird habitat associations were mediated through nest predator abundance, alternate prey abundance, and food availability. In Chapter 2, I examined associations between grassland arthropod communities and habitat characteristics representing three spatial scales: local (within-patch vegetation structure and composition), patch (size, shape, edge composition), and landscape (landcover composition within a 400 m buffer). In addition to their relevance for grassland birds, arthropods play important functional roles in grasslands and are useful indicators of grassland health. I collected arthropod samples using pan traps in grassland patches at Burning Star, and used generalized linear mixed models to relate variation in arthropod biomass and diversity to habitat predictor variables. I found that arthropod biomass increased with vegetation height at the local scale and proportion of forest/shrub edge at the patch scale, while arthropod diversity responded only to local-scale variables, including a negative association with vegetation height and woody vegetation cover, and a positive association with forb cover. I conclude that local vegetation structure and composition are the main drivers shaping arthropod communities at Burning Star, and that limiting woody encroachment and increasing forb cover and variation in vegetation height within grassland patches may encourage arthropod abundance and diversity in tallgrass prairies. In Chapter 3, I estimated associations between small mammal abundance and habitat variables, again representing three spatial scales. I surveyed small mammal communities using a grid of 100 Sherman traps set out for three nights at each study site. I identified all captured individuals to genus, individually marked them with ear tags, and estimated abundance using a combination of Huggins P and C models in Program Mark and generalized linear mixed models in Program R. I found that small mammal abundance was positively related to vegetation density and negatively related to plant diversity, though variation in plant diversity affected Microtus voles more strongly than Peromyscus mice. At the landscape scale, small mammal abundance was positively associated with the amount of water surrounding a patch, and negatively associated with the amount of grassland surrounding a patch. Variation in small mammal community composition (proportions of Microtus vs Peromyscus) was mostly governed by differences in habitat structure at the landscape scale, rather than differences in vegetation structure at the local scale. I suggest that managers interested in influencing small mammal abundance in grasslands encourage dense vegetation growth by limiting disturbance if increased small mammal abundance is desired, or reduce vegetation density by increasing disturbance frequency to reduce small mammal abundance. Additionally, increasing plant diversity by sowing a high diversity of seeds may be an effective way to control Microtus vole populations. In Chapter 4, I estimated the habitat associations of snakes at Burning Star, focusing on the relative abundance of snakes among different grassland sites, as well as snake diversity and species-specific occupancy. Although snakes are prolific nest predators of grassland birds, they are also integral components of grassland systems, and there may be instances where managers and decision-makers wish to increase, rather than decrease, their abundance in grasslands. I found that snake community metrics were strongly and positively related to an increase in woody plant cover at the local (within-patch) scale. Snake relative abundance was also positively related to an increase in grass cover and a decrease in forb cover, though my occupancy results suggest that this was primarily driven by an increase in black kingsnakes (Lampropeltis nigra). At the patch scale, snake relative abundance and diversity were both positively related to the proportion of patch edge composed of roads. Habitat structure at the landscape scale had the smallest impact on snakes in this study, though the proportion of trees in the landscape was positively related to snake diversity. I suggested that managers and conservationists interested in manipulating snake abundance in grasslands focus on within-patch vegetation structure and composition. Decreasing woody cover in grasslands, or increasing the ratio of forbs to grasses, may reduce the presence of snakes, while maintaining a woody component could encourage both snake abundance and diversity. In Chapter 5, I estimated the habitat characteristics that were most strongly associated with raccoon abundance estimates in grassland patches at Burning Star. Raccoons have become increasingly important avian nest predators in midwestern grasslands due to rampant habitat fragmentation. I estimated raccoon abundance using an occurrence index from a series of baited trail cameras located in grassland sites. I found no convincing evidence of raccoon abundance being influenced by local scale habitat structure, beyond a weak association with vegetation height. At the patch scale, raccoon abundance was positively related to the proportion of patch edge composed of roads. At the landscape scale, raccoon abundance was negatively related to grassland, and positively related to water, within 400 meters of a grassland patch. I recommended that managers concerned with minimizing the presence of raccoons in grasslands should limit roads along grassland perimeters, maximize the proportion of grassland in the landscape surrounding a grassland patch, and avoid planning a grassland restoration in close proximity to open water if possible. In Chapter 6, I estimated the habitat characteristics that were most strongly associated with daily nest survival, nest density, and abundance of Dickcissels (Spiza americana), Field Sparrows (Spizella pusilla), and Common Yellowthroats (Geothlypis trichas), but also considered responses of all grassland bird species combined. I considered habitat characteristics representing four spatial scales: nest site, within-patch, patch, and landscape, though the nest site scale was only considered for nest survival analyses. I found that Dickcissels, an obligate grassland species, exhibited the strongest response to fire, as nest density drastically improved after previously undisturbed grasslands were burned. Dickcissel abundance was positively related to agriculture at the landscape scale and negatively related to woody cover. Field sparrows demonstrated a preference for woody cover and proximity to forests and shrublands, and Common Yellowthroats were positively associated with forb cover. Both Field Sparrow and Common Yellowthroat nest survival increased with greater distance from an edge, though no edge effect was detected for Dickcissel nest survival. All bird species benefitted from increased plant diversity and greater patch size. All species also responded negatively to vegetation height or litter depth, suggesting that fire, which reduces vegetation biomass and litter, may indirectly benefit the facultative grassland birds of Burning Star in addition to Dickcissels. In Chapter 7, I used structural equation models and data gathered in the previous five chapters to estimate whether the effects of habitat structure on breeding Field Sparrows is mediated through changes in predator (snake and raccoon) abundance, alternate prey availability, or arthropod biomass. I used Field Sparrows as the focal species for this chapter because they were the most common grassland bird in my dataset. I found no evidence of nest survival or nest density of Field Sparrows being directly influenced by nest predator abundance, alternate prey, or arthropod biomass, although habitat characteristics associated with increased nest survival were also associated with greater arthropod biomass and reduced predator abundance. I suggested that habitat structure at Burning Star may primarily impact breeding Field Sparrows through direct means, such as influencing nest concealment or foraging efficiency. These results also suggest that nest success and nest density are decoupled in this study area, so Field Sparrows may be preferentially selecting nest sites with structural characteristics that do not increase nest survival. Ultimately, my findings from this study indicate that while predator avoidance and food provisioning likely play an important role in determining nest survival for grassland birds, predator abundance and arthropod biomass may not necessarily predict predation risk and foraging efficiency to the extent that is often assumed.
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