Discordant patterns of population structure for two co-distributed snake species across a fragmented Ontario landscape


Correspondence: Stephen C. Lougheed, Department of Biology, Queen’s University, Kingston, ON, Canada K7L 3N6.
E-mail: steve.lougheed@queensu.ca


Aim  The goal of our study was to investigate the effects of a fragmented landscape on the genetic population structure of two sympatric snake species that differ in habitat preference. The eastern garter snake (Thamnophis sirtalis sirtalis) is a common, habitat generalist, whereas the endangered eastern foxsnake (Mintonius [Elaphe] gloydi) is rarer, geographically restricted, and a marsh-specialist. We were most interested in comparing the genetic population structure of both species and identifying any natural and human-created features of the landscape that overlap with genetic disjunctions.

Location  Southwestern Ontario, Canada, surveying over half of the remaining range of the eastern foxsnake.

Methods  We utilized DNA microsatellite markers to examine genetic population structure of both species. The number of genetically distinct clusters for each species was determined using both Bayesian spatial assignment and spatial principal component analyses (sPCA). Genetic clusters were overlaid onto a habitat map to deduce possible physiognomic barriers to gene flow.

Results  Spatial assignment revealed three genetic clusters for garter snakes and five for foxsnakes. Each individual garter snake had a near equal probability of membership to two or more clusters with no cluster mapping onto a discrete geographic region, indicating that garter snakes comprise a single genetic population. The identified foxsnake clusters correspond to geographically circumscribed locations on the landscape, roughly coincident with isolated patches of suitable habitat. sPCAs revealed significant global allelic structure for foxsnakes, but not for garter snakes. No significant local structure was found for either species.

Main Conclusions  Our results imply that foxsnakes and garter snakes are differentially impacted by the same landscape or have dramatically different effective population sizes. Unsuitable intervening habitat such as agricultural tracts and roads between existing populations of foxsnakes appears to act as barriers to gene flow, while garter snake movement appears unrestricted by these features. Our findings have important implications for the management of eastern foxsnakes.