The Butler‟s gartersnake (Thamnophis butleri) – a diminutive, Great Lakes region wetland specialist - occurs in Wisconsin only as a disjunct population in the greater Milwaukee area in southeast Wisconsin (Rossman et al. 1996). This species is otherwise found only east of Lake Michigan in a distribution across the eastern half of Michigan and adjacent parts of southwestern Ontario, northeastern Indiana, and northwestern Ohio. BGS was listed as threatened in Wisconsin in 1997 because of long-term habitat loss in its geographically restricted range (Joppa and Temple 2005, Vogt 1981) and other threats. BGS presents a management and conservation challenge because it hybridizes with its close relative, the plains gartersnake (T. radix). The Eastern plains gartersnake is not a listed species in Wisconsin but is a species of Special Concern, and inhabits mesic prairies, dry-mesic prairies, and oak savanna with access to nearby water in the form of marshes, ponds, brooks, or rivers (Ernst and Ernst 2003, Vogt 1981). PGS occurs south of the Wisconsin BGS range – as currently understood - in extreme southeastern Wisconsin, northern Illinois, and northwestern Indiana, and west of the BGS range in south-central Wisconsin and states further west. A purported BGS-PGS hybrid zone occurs in southeastern Wisconsin where the two species are sympatric. The respective ranges of BGS, PGS, and BGS-PGS hybrids in this area – and the degree to which these ranges overlap - are not well understood. However, the hybrid zone is thought to encompass Walworth, Kenosha, and Racine counties, and at least the southern portions of Waukesha and Milwaukee counties.
