Abstract
AbstractEcosystems around the world, and the wildlife species that rely on them, have been impacted by changes in the structure and composition of vegetation. Multiple factors may contribute to habitat degradation, but two common factors are invasion by non-native plants and the expansion of native plants beyond their historical ranges. Habitat degradation, defined as a reduction in habitat quality, occurs when altered habitat lowers an individual’s probability of occupying and reproducing in a habitat, either due to increased risk of predation or reduced resources such as food. Therefore, understanding how changes to habitat structure affect distributions of both predator and prey species is an important part of assessing the impact of structural changes on habitat quality.
The sagebrush ecosystem of the western United States is one such ecosystem that has been impacted by invasive and expanding plants. Invasions of non-native annual grasses, most notably cheatgrass (Bromus tectorum), and the expansion of pinyon (Pinus spp.) and juniper (Juniperus spp.) into habitat once dominated by sagebrush have impacted wildlife. Changes to plant structure and composition in sagebrush habitat may impact wildlife by altering the availably of resources such as food and cover, leading to changes in the composition of the wildlife community.
One species that has been impacted by habitat degradation in the sagebrush steppe is the greater sage-grouse (Centrocercus uropahsianus). Populations of this sagebrush obligate have declined across their range, and conservation efforts, including conifer removal, have been undertaken to support remaining populations. Other groups of species associated with sagebrush cover have received less study, but community dynamics for these species are likely to be affected by altered habitat structure as well. Because the conifer woodland and sagebrush communities feature little overlap in composition, the final outcomes of a stable state transition from sagebrush to conifer woodland are well understood for many species. However, the potential effects of early-stage conifer establishment in sagebrush habitat are under-examined for sagebrush-associatesother than sage-grouse.
Beginning in 2019, the Bruneau-Owyhee Sage-grouse Habitat (BOSH) project initiated the removal of conifer within an area of up to 676,000-ha of multiuse land managed by the Bureau of Land Management (BLM) in southwest Idaho. The goal of this management is to improve habitat for sage-grouse populations. However, the BOSH project also presents a unique opportunity to examine the ways in which the presence of conifer in a sagebrush landscape potentially affects population processes for a suite of functional groups. My study examined the dynamics of the songbird and avian predator communities both before and after the removal of ~15,000 ha of juniper. My study also investigated the relationship between small mammals and habitat characteristics, including conifer and shrub structure. Understanding the ways that habitat mediates species distributions and interactions, as well as the consequences of these interactions for the wildlife community, will allow managers to better predict the results of management actions for both sagebrush and juniper woodland-associated species.
Avian predators
In chapter two of this dissertation, I examine the relationship between habitat characteristics and occupancy of two generalist avian predators, common ravens (Corvus corax) and red-tailed hawks (Buteo jamaicensis). Increased abundances of these two predators have been attributed to human development in sagebrush habitat, and increased area of conifer woodlands have been implicated as a factor that may influence habitat use for avian predators through the addition of nest and perch sites. However, empirical evidence for a causal relationship between conifer expansion in sagebrush habitats and increased abundances of avian predators is lacking. Anecdotal evidence suggests that conifer expansion may affect predator-prey dynamics because survival rates for sage-grouse that use habitat featuring conifers are lower than those for sage-grouse in areas with no conifers. However, structural resources such as trees are not the only factor that may influence habitat use for avian predators. Prey resources may also be an important aspect of habitat quality that influences habitat use. To investigate the importance of structural and prey resources on habitat use by avian predators, we used a Bayesian occupancy analysis to test the effects of habitat characteristics on habitat use by avian predators. Increased conifer cover, especially cover > 20%, was associated with increased occupancy probability for common ravens but not red-tailed hawks. For red-tailed hawks, distance to the nearest cliff was the most influential factor for habitat use. As the distance to the nearest cliff increased, occupancy probability for red-tailed hawks decreased. We did not find support for an effect of prey abundance on habitat use for either species.
Small mammals
In chapter three of this dissertation, I examined the effects of habitat structure along an invasion gradient for small mammals. Small nocturnal mammals, including mice and voles, are an important component of the ecosystem because they serve as both predator and prey and alter vegetation structure and composition through herbivory and caching behavior. Therefore, changes to species distributions and interactions among species within the small mammal community may have implications for a broad suite of ecosystem functions. We examined small mammal density, survival, and home-range size along a gradient of cheatgrass and conifer establishment for the deer mouse (Peromyscus maniculatus), a generalist omnivore, and the Great Basin pocket mouse (Perognathus parvus), a specialized herbivore. Neither density nor survival for either species was affected by cheatgrass cover. However, the home-range size of deer mice was 2.3 times smaller in areas with high cover of cheatgrass compared to areas with no or low cheatgrass cover. Conifer cover was an important predictor of density for deer mice, as well as survival for both deer mice and pocket mice. Deer mouse density was highest in habitat with 10% conifer cover, but deer mouse density decreased as conifer cover increased beyond 10%. Survival of deer mice decreased as conifer stem density increased, while survival of pocket mice increased as conifer stem density increased. We found evidence of intraspecific effects of density on home-range size for pocket mice. Home-range size for pocket mice was two times smaller in areas with the highest densities of deer mice compared to areas with low deer mice densities. These results suggest that the removal of conifers as part of wildlife management efforts will likely affect community dynamics for small mammals.
Songbirds
In chapter four of this dissertation, I examined the effect of conifer and shrub structure in sagebrush habitat on the density of shrub-nesting songbirds, as well as the short-term response of shrub-nesting and conifer-nesting songbirds to conifer removal. Studies of the effectiveness of conifer removal to support populations of songbirds associated with sagebrush habitat have yielded weak or unclear results. However, the spatial scale of the response of individual species to conifer in sagebrush habitat has rarely been considered. Further, shrub structure is a critical feature of habitat for shrub-nesting songbirds, so conifer removal alone may not increase the density of shrub-nesting songbirds if existing shrub structure is not adequate. Shrub-nesting songbirds responded to conifer cover at different spatial scales. Brewer’s sparrow (Spizella breweri) density was negatively associated with conifer cover within 500 m, while sage thrasher (Oreoscoptes montanus) density was strongly negatively associated with conifer cover > 5% within 100 m. Conversely, green-tailed towhee (Pipilo chlorurus) density increased as conifer cover within 1000 m increased. Brewer’s sparrow and green-tailed towhee, but not sage thrasher, were positively associated with greater density, volume, and mean height of sagebrush. Following conifer removal, mean density of shrub-nesting species increased compared to the three years prior to conifer removal. Locations where the highest amount of conifer was removed exhibited smaller increases in shrub-nesting songbird density compared to locations where less conifer was removed, suggesting removal of dense conifer may not immediately lead to increased abundance of sagebrush songbirds. The density of conifer-nesting songbirds, which was strongly associated with conifer cover > 20%, increased or was unchanged in areas of remaining conifer woodlands.