Biogeography of montane mammals in the Great Basin and American Southwest: review and perspectives for the future

Abstract The distributions of mammals on mountaintops in western North America have long fascinated mammalogists and the systems have served as natural laboratories for the development of biogeographic theory. The equilibrium theory of island biogeography (ETIB) proposed that insular faunas are products of opposing rates of colonization and local extinction with colonization dictated by isolation and extirpation dictated by area of an island. To the contrary, seminal work in the Great Basin found a strong species–area relationship but absence of a species–isolation relationship and therefore concluded that montane mammal distributions were not in equilibrium due to a lack of intermountain colonization and thus a product of only extirpations. Subsequent work based on improved occurrence data would reverse this conclusion and rather posit that the distributions of all but a few boreal species were products of both colonization and extirpation. Meanwhile, in the American Southwest the advancement of biogeographic thought originally stemmed from deeper historical perspectives based on range expansions during Pleistocene glacial advances and subsequent vicariance during warmer interglacials. However, ecological studies based on influence of area and isolation tended to support an equilibrium-based pattern of distribution. The most recent analysis though, which also included island age and explicitly tested alternative hypotheses, concluded that the relatively rich assemblage of boreal mammals in the American Southwest was a product of historical vicariance with no appreciable contemporary colonization. Although the mechanistic approach of the ETIB discounts species identity, a species' history, habitat, and traits strongly influence biogeographic patterns. Improved technology and emerging approaches related to individual species may spark future insights into the processes that shape montane mammal distributions. Finally, climate change and other stressors are causing montane mammal distributions to contract and populations to be extirpated. The biogeographic patterns and processes in these sky island systems provide insight into these conservation issues.