Landscape genetics of the camas pocket gopher (Thomomys bulbivorus), an endemic mammal of Oregon's Willamette Valley

Animal species living in small populations with small ranges may be particularly vulnerable to habitat loss and fragmentation. We investigated genetic population structure of the camas pocket gopher (Thomomys bul-bivorus), a species endemic to the Willamette Valley, Oregon, an area strongly affected by human development. Pocket gophers collected across much of the range of the species were analyzed for genetic structure, diversity, and influences of landscape on gene flow using microsatellite markers identified in closely related gopher species. We used k-means clustering to group individuals into genetically similar clusters based on their locations along principal component axes, and we evaluated the strength of evidence for this clustering using discriminant analysis of principal components (DAPC). We used mixed-effects modeling to evaluate the influence of rivers and hills (represented by slopes) as explana-tory variables for pairwise individual genetic distances along principal component axes. Results supported a clinal, isola-tion-by-distance or isolation-by-resistance model, with greater genetic diversity near the central portion of the range. A model with both rivers and slopes was best supported, compared to either variable alone or a simple distance model. The Willamette River, which divides the range of the camas pocket gopher, appeared to have some restrictive effect on gene flow but was not a complete barrier. In population-level analyses, we observed strong genetic differentiation as well as persistent Wahlund effects, suggesting that complex genetic structure exists within a population connected across gradients of distance, rivers, and hills. The camas pocket gopher has persisted despite persecution as an agricul-tural pest and intensive habitat fragmentation due to agriculture, but the future may hold new conservation challenges as human population and urban development increase in the limited range of this endemic Oregon species.