Pleistocene-Holocene vicariance not Anthropocene landscape change,
explains the genetic structure of American black bear (Ursus americanus)
populations in the American Southwest and northern Mexico
Abstract
The phylogeography of the American black bear (Ursus americanus) is
characterized by isolation into glacial refugia, followed by population
expansion and genetic admixture. Anthropogenic activities, including
overharvest, habitat loss, and transportation infrastructure, have also
influenced their landscape genetic structure. We describe the
phylogeography of the American black bear in the American Southwest and
northern Mexico and investigate how prehistoric and contemporary forces
shaped genetic structure and influenced gene flow. Using a suite of
microsatellites and a sample of 550 bears, we identified 14
subpopulations organized hierarchically following the distribution of
ecoregions and mountain ranges containing black bear habitat. The
pattern of subdivision we observed is more likely a product of
postglacial habitat fragmentation during the Pleistocene and Holocene,
rather than a consequence of contemporary anthropogenic barriers to
movement during the Anthropocene. We used linear mixed-effects models to
predict genetic distance among individuals, which indicated that both
isolation by resistance and geographic distance govern gene flow. Gene
flow was highest among subpopulations occupying large tracts of
contiguous habitat, was reduced among subpopulations in the Madrean Sky
Island Archipelago, where montane habitat exists within a lowland matrix
of arid lands, and was essentially nonexistent between two isolated
subpopulations. We found significant asymmetric gene flow supporting the
hypothesis that bears expanded northward from a Pleistocene refugium
located in the American Southwest and northern Mexico and that major
highways were not yet affecting gene flow. The potential vulnerability
of the species to climate change, transportation infrastructure, and the
U.S.-Mexico border wall highlights conservation challenges and
opportunities for binational collaboration.