Genetic Structure and Connectivity of the Endangered Butler's Gartersnake (Thamnophis butleri) Across the Fragmented Landscape of Southwestern Ontario

Abstract

Genetic patterns in many species are affected by environmental features that may enable or obstruct gene flow. Geographical isolation of conspecific individuals by uninhabitable expanses can lead to genetic separation. For species impacted by human activities, understanding how they interact with their environment can promote re-establishment of population connectivity and conservation. Thamnophis butleri (Butler’s gartersnake) is an Endangered species in Ontario, its range comprising three disjunct regional populations: Essex County, Lambton County, and Luther Marsh. Thamnophis butleri is a specialist of wetland-adjacent grassland-type habitat, thus its distribution is constrained by Southwestern Ontario’s largely agricultural landscape. Genetic differentiation among populations inhabiting these isolated regions was shown in a single previous study. I explored genetic structure more comprehensively, augmenting the geographic sampling and deploying landscape genetics approaches. In my first data chapter, I tested for population differentiation in both T. butleri and the generalist Thamnophis sirtalis sirtalis (eastern gartersnake) where the congeners occur sympatrically in Ontario. Assignment analyses revealed clear genetic structure within T. butleri, with clusters corresponding to geographical regions: one each in Lambton and Luther Marsh, and two in Essex, one along the shoreline and another in LaSalle, ON and the Ojibway Prairie Complex. Dispersal by river, either aquatically or along riparian corridors, may facilitate connection in the Lambton and Essex shoreline clusters, while the Ojibway/LaSalle cluster may have no need to disperse (occurring in good-quality habitat) or be impeded from dispersing by intervening urban areas. In contrast, I found that T. s. sirtalis comprises a single genetic population, implying that its generalist ecology allows for dispersal through this heavily altered landscape. In my second data chapter, I explored the relationship between patterns of T. butleri genetic differentiation and landscape composition across its Canadian range, using Mantel-based methods to test the correlation along either uniform or habitat-delineated distance matrices. The relationship between landscape and genetics varied among regional populations, from resistance-based in Lambton to distance-based in Essex. Treating open water as a potential impediment or facilitator of dispersal did not alter my conclusions, raising the possibility of aquatic dispersal that may occur at a different spatial scale than terrestrial dispersal.