Open Access
Amburgey, Staci
Graduate Program:
Doctor of Philosophy
Document Type:
Date of Defense:
January 30, 2019
Committee Members:
  • David Miller, Dissertation Advisor
  • David Andrew Miller, Committee Chair
  • Matthew R. Marshall, Committee Member
  • Tyler Wagner, Committee Member
  • Tracy Langkilde, Outside Member
  • Evan Grant, Special Member
  • species distribution modeling
  • range dynamics
  • bioclimatic envelope model
  • species occupancy model
  • amphibian
  • reptile
  • mammal
  • climate change
  • interspecific competition
  • multispecies
  • habitat fragmentation
Species’ distributions change over time, contracting (e.g., through habitat loss, declining suitability from changing climate, negative interspecific interactions) or expanding (e.g., through restoration efforts, colonization of newly suitable habitats from changing climate, newly available niches due to turnover in species communities). A pressing need in conservation ecology is the identification of factors contributing to patterns of species occurrence and range limits. Conservation decisions rely on understanding ecological patterns and relationships between species and their environment. My dissertation aims to improve capacity for conservation by elucidating factors contributing to the formation of species distributions. Our understanding of the causes of species range limits is complicated by scale-dependency in species’ responses to natural or anthropogenic pressures. For example, climate, species interactions, and habitat fragmentation can act at different scales to explain patterns of species occurrence across space and time. Climate is considered one of the most important, broad-scale filters of species occurrence as it acts on all components of an ecosystem. On the other hand, species interactions are considered more important in explaining finer, local-scale occurrence patterns. Additionally, the impact of anthropogenic changes such as urbanization can alter the distribution of species and their biological communities and interact with broad- and fine-scale pressures to explain species occurrence. Robust methods, such as quantitative population modeling, are necessary to understand the role of these factors in shaping species distributions while accounting for sampling error and process variation. In the following dissertation chapters, I advance our understanding of species distributions and the formation of range limits at multiple scales using three different systems and a suite of quantitative population modeling approaches. In my first chapter, I assessed the importance of range position in explaining the climate sensitivity of wood frog (Lithobates sylvaticus) populations. The wood frog is a widely distributed, pond-breeding amphibian species that occurs across much of the northeastern United States and into Canada and Alaska. I developed a state-space model to quantify the effect that annual climate anomalies have on population growth rates relative to the regional climate of a site at its position in the range. I hypothesized that climate sensitivity would be highest at the climate extremes of the wood frog range. Increased sensitivity would translate into reduced population growth rates during years with climate anomalies approaching those extremes (i.e., climate envelope hypothesis). By understanding the influence of climate on demographic rates in different regional climates, I was better able to understand how climate contributes to the formation of range limits and hindcast wood frog population trajectories over the last thirty years. Some models supported the climate envelope hypothesis, where decreased population growth rates were associated with hotter years in warmer regions and more water availability was associated with increased recruitment in drier regions. Spatial and temporal variation in responses to climate highlight the importance of using dynamic models that focus on demographic responses to understand population trajectories. In my second chapter, I focused on species interactions between the Shenandoah salamander (Plethodon shenandoah) and the red-backed salamander (P. cinereus) to understand fine-scale drivers of species’ range limits. In sympatry, the widely distributed red-backed salamander is thought to competitively exclude the range-restricted Shenandoah salamander from deeper soil habitats, and it is largely considered responsible for setting the lower range limit of Shenandoah salamanders. Using repeat surveys to account for imperfect detection, I fit a suite of occupancy models to see if single-species or two-species conditional models better estimated the range boundary. Two-species models condition the occupancy and detection probabilities of one species on the occupancy and detection probabilities of a second species, accounting for potential species interactions to explain patterns in occupancy. I also included conditional autoregressive random effects that incorporate spatial information that may improve estimation of the range boundary. I found that accounting for imperfect detection prevented underestimation of the range boundary and area of co-occurrence of these two species. Spatially explicit single-species models also performed the best given the data, indicating that species interactions were not necessary for predicting the range boundary at this scale. I found that the co-occurrence zone of these two species is wider than previously thought, highlighting the importance of methods that incorporate detection probability and spatial autocorrelation in understanding range boundaries. For my third chapter, I build off the second chapter’s findings to investigate trait, behavior, and population process differences between Shenandoah and red-backed salamanders that may facilitate this broader than expected area of co-occurrence. When species with strong competitive interactions co-occur, theory predicts that co-occurrence can be facilitated by the differentiation of traits or behaviors in a way that reduces the strength of competition and therefore also limits the niche overlap of the species. In the case of the Shenandoah salamander, I predicted that if strong competitive exclusion was the primary cause of the range boundary between these two species, individuals in sympatry would have different physical characteristics, behaviors, or demography than in areas of allopatry. Using range boundaries identified in Chapter Two, I found little support for character displacement in individual trait or behavioral measures. While differentiation existed, traits showed species-level differences or similarly varied in both species by location on the transect. Differentiation in demography in the co-occurrence zone did support competitive processes potentially altering recruitment or dispersal in the co-occurrence zone. Additionally, microhabitat behavioral selection indicated some level of differentiation in cover object use that may help facilitate broader areas of co-occurrence. Species interactions may help structure species range limits through their influence on population-level processes, but traits may not show morphological differentiation rather than demographic differences when range limits occur over such a fine-scale gradient. Lastly, my fourth chapter investigates the way intermediate-scale pressures such as habitat fragmentation can shape the occurrence of species across a landscape. Using multispecies occupancy models, I was able to estimate occupancy probabilities of 45 different species in small vertebrate communities across the California Floristic Province. This landscape was largely modified by urbanization in the last several decades, impacting the patterns of species richness in remaining habitat fragments. I investigated if species-specific variation in occupancy at the level of the overall patch and at sites within the patch could be explained by taxonomy, species life history, and range information. Specifically, I estimated species commonness (relative prevalence) and sensitivity (response to patch size) at the patch and site. Taxonomic differences in species commonness explained patterns of biogeography in this region while species differences in fecundity and responses to extreme climate were correlated to sensitivity to fragmentation. Amphibian and mammalian sensitivity to patch size depended on whether patches occurred in more extreme portions of their climate range while reptiles did not differ in their response. By understanding biogeographic patterns in species occurrence and how fragmentation may impact species persistence in habitat patches, conservation measures can be targeted to those species most sensitive to the compounding effects of these broad-scale pressures. Conservation decisions are based on an understanding of ecological patterns and relationships that are scale-dependent. I investigated broad-scale, intermediate, and local-level factors that contribute to population responses and shape species distributions. I identified key findings in each system that can be used by practitioners in conservation and management efforts. I also provided methodologies that allow for the quantification of these processes on species distributions, improving our understanding of species distribution modeling.