Using landscape genetics to understand connectivity of an island population of eastern massasauga rattlesnakes (Sistrurus catenatus)

Populations that experience low levels of gene flow commonly display increased levels of inbreeding, lower genetic diversity, and reduced adaptive potential. Landscape genetics allows for spatial and genetic information to be analyzed simultaneously to better understand how the landscape influences gene flow. The eastern massasauga rattlesnake (Sistrurus catenatus) is a federally threatened viper found in wetlands throughout the Great Lakes region. Many remaining populations are small and isolated due primarily to habitat loss. Atypical from a range-wide perspective, eastern massasaugas on Bois Blanc Island (BBI), Michigan live in a relatively undisturbed landscape with a potential for high connectivity across the 88 km2 island. We used landscape genetics to examine the influence of landscape features on gene flow across BBI to determine if eastern massasaugas have detectable gene flow across a well-connected landscape with high abundance. We genotyped 102 snakes at 15 microsatellite loci and measured pairwise genetic distances using the proportion of shared alleles (Dps). We created resistance surfaces for land cover, roads, distance from roads, and compound topographic index. The R package ResistanceGA was used to optimize resistance surfaces and calculate pairwise effective resistance among individuals. We calculated an average observed heterozygosity of 0.574 and found no well-defined genetic structuring among our samples. Significant isolation by distance (IBD) was detected as well as positive spatial autocorrelation at distances less than 1 km. Our landscape genetic analysis identified the distance from roads surface to be our most supported model. In addition, either distance from roads or roads were included in the top three models suggesting the strong influence roads play in structuring patterns of gene flow of eastern massasaugas despite the low number of gravel roads and traffic volume on BBI. Our results suggest that roads act as barriers, reducing connectivity of eastern massasauga populations even in more rural areas with high quality habitat. Despite this, we still found considerable gene flow across the well-connected landscape of BBI