GIANT GARTERSNAKES (THAMNOPHIS GIGAS) SELECTIVELY FORAGE ON NATIVE ANURANS

Julia S Martins Ersan; CSU, East Bay/ USGS; ersan@ucdavis.edu; Brian J Halstead, Erica L Wildy, Michael L Casazza, Glenn D Wylie

The introduction of exotic species can alter trophic dynamics of native species. Threatened giant gartersnakes (Thamnophis gigas) forage on a prey community that is comprised almost entirely of exotic species. We examined prey selection of giant gartersnakes by analyzing trap by-catch and regurgitated gut contents of snakes in four basins in the Sacramento Valley of California to determine whether these snakes actively select their prey or if they are simply consuming what is abundant. Although non-native anuran and fish species dominate the available prey community, giant gartersnakes most strongly selected native Sierran treefrogs (Pseudacris sierra) metamorphs. If all prey types were equally available, giant gartersnakes would have selected Sierran treefrogs with a probability of 0.596 [0.0254 - 0.855]. Giant gartersnakes also were more likely to select introduced American bullfrog (Lithobates catesbeianus) adults over all tested introduced fishes. Managing prey communities to increase native amphibian abundance within the giant gartersnake's range might benefit populations of this rare snake.

Ecology and Conservation of Amphibians and Reptiles     Student Paper