Comparing the population structure of the specialist Butler’s Gartersnake (Thamnophis butleri) and the generalist Eastern Gartersnake (Thamnophis sirtalis sirtalis) in Ontario (Canada) and Michigan (USA)

Species differing in life-history attributes vary in their responses to features within a shared landscape. We evaluated genetic structure of sympatric gartersnake species in southwestern Ontario (Canada) and southeastern Michigan (USA), where habitat fragmentation is high due to agriculture and urbanization. We surveyed genetic structure of a habitat specialist, Butler’s Gartersnake (Thamnophis butleri (Cope, 1889)), and a habitat generalist, Eastern Gartersnake (Thamnophis sirtalis sirtalis (Linnaeus, 1758)), using DNA microsatellites. Bayesian clustering, discriminant analysis of principal components, and pairwise population comparisons revealed genetic differentiation among three major regional clusters of Butler’s Gartersnake with evidence of further division within one. Genetic clustering of Butler’s Gartersnake suggest that inhospitable habitat limits dispersal. Eastern Gartersnakes showed less structure, with assignment tests implying a single genetic cluster. We found positive significant Mantel’s r for both species in the smallest distance class (<15 km), but significant isolation by distance for Butler’s Gartersnake only. These findings together imply that connectivity for Eastern Gartersnakes is less impacted by habitat loss and fragmentation or that we were less able to detect their effects. Our study shows the value of multispecies comparisons in studies seeking to understand the underlying causes of genetic structure in natural populations.