Radomski, Thomas2023-09-192023-09-192023-04https://hdl.handle.net/11299/257073University of Minnesota Ph.D. dissertation. April 2023. Major: Ecology, Evolution and Behavior. Advisor: Kenneth Kozak. 1 computer file (PDF); viii, 150 pages.No species is distributed everywhere on Earth, accordingly “what drives the limitation of species' geographic ranges?” has become a fundamental question in ecology and evolution. Numerous hypotheses have been proposed to explain geographic range limits, with varying empirical support. For my doctoral dissertation, I examined the geographic range of the South Mountains Gray-cheeked Salamander (Plethodon meridianus) because it possesses a small geographic distribution, making it a pragmatic study system. In the first two chapters of my dissertation I focus on the natural history of P. meridianus, a narrowing-ranging mountaintop endemic that is restricted the the South Mountains in North Carolina: Chapter 1 focuses on the origin of P. meridianus and the processes that may underlie its allopatric isolation from closely related species distributed to the west and north of its current distribution (and thus the genesis of a geographic range). In Chapter 1, I used a combination of molecular genetics, morphology, and habitat modeling to evaluate the taxonomy of P. meridianus with reference to closely related and ecologically similar species. I found that P. meridianus merits species status based on phylogenetic tests, though results were not unanimous across methods. Environmental data suggest habitat similarity among closely related species and that niche conservatism underlies the current allopatric isolation of P. meridianus, and possibly its origin as well. Morphological data indicated subtle, but statistically significant, differences among closely related species. In Chapter 2, I used survey data and species distribution modeling to better delineate the geographic range of P. meridianus. This also served as an opportunity to compare species distribution model techniques for small datasets; I found that Maxent with model selection outperformed other techniques, especially the most sophisticated ones. The geographic distribution was primarily predicted by a combination of elevation and near-stream habitat. Not every species distribution modeling method outperformed a baseline model using geographic distance alone (an ecologically neutral model). In the latter two chapters, I focused on testing range-limit hypotheses. In Chapter 3, I studied occupancy and abundance of P. meridianus along a transect running from its range center to range edge, with the goal of testing the hypothesis whether abiotic factors or competition with the low-elevation species, P. cylindraceus, explains the geographic range limit of P. meridianus. Plethodon cylindraceus was rarely encountered, during field surveys; therefore, it is unlikely that it interactions with this species has a major impact on the distribution of P. meridianus. Occupancy and abundance decline toward the range edge of P. meridianus; detection probability may also decline toward the range edge, but this remains unclear. In chapter 4, I implemented a mechanistic niche model from organismal traits to test if the distribution of P. meridianus can be predicted from its Grinnellian niche alone (i.e., habitat tolerance). I measured several ecophysiological traits related to water balance, and I gathered metabolic data previously collected for P. meridianus by other researchers. I tested the ability of the mechanistic niche model to predict the distribution of P. meridianus. The mechanistic model suggests that the geographic distribution of P. meridianus is determined largely by its climatic tolerances and the geographic distribution of climatically-suitable habitats (i.e. the Grinnellian niche). Many geographically peripheral populations lie in unsuitable habitat, suggesting they are at high risk of local extinction. Moreover, unsuitable habitats lie in between fragments of suitable habitat that are adjacent to the range of P. meridanius, suggesting it is unlikely that this species will be able to shift its geographic range to other suitable habitats in the mountains to the west and north in response to climate change.enThesis or Dissertation