Population genetic structure at the northern range limit of the threatened eastern hog-nosed snake (Heterodon platirhinos)

Habitat loss and fragmentation have important consequences for population persistence by reducing dispersal, connectivity, and population size. Characterizing patterns of population genetic structure and gene flow, and testing for the genetic signature of population declines and inbreeding can provide key insights into conservation for at-risk species. We use 12 microsatellite DNA markers to quantify the spatial structuring across two regional populations of the threatened eastern hog-nosed snake (Heterodon platirhinos) in Ontario, Canada. We further test for inbreeding and recent, rapid population declines, for the latter using both a ‘heterozygosity excess’ approach and Approximate Bayesian computation. Spatial Bayesian assignment tests revealed four distinct genetic clusters that were not apparent from species occurrence data, suggesting that the current designation of a single management unit for this species is invalid. We found some evidence for inbreeding in all studied locales. However, heterozygosity excess tests provided only weak evidence for recent bottlenecks and varied according to mutational model specified. In contrast, ABC provided strong support for a recent and rapid reduction in population size within one sample locale (Wasaga Beach), where individuals are largely confined to a small area circumscribed by expanding urban areas. For three other locales investigated, we found equal support for either recent rapid or gradual population declines. Immediate action is required for Wasaga Beach if this population is to persist.