When frogs violate trophic hierarchy: conservation of the endangered San Francisco gartersnake

Introduced species often disrupt established food webs, but some native predators can come to rely on introduced prey. Understanding the net effects of the non-natives on imperiled predators is crucial for planning conservation measures. The invasive American bullfrog (.Lithobates catesbeianus) can be prey, predators, and competitors for the critically endangered San Francisco gartersnake (Thamnophis sirtalis tetrataenia). I examined the seasonal prey use and selection of a T. s. tetrataenia population that cooccurs with L. catesbeianus in San Mateo County, California. I quantified the snakes’ diet from analyzing their fecal DNA and also quantified anuran abundance as available prey. Prey selection varied by snake body size, although 95% credible intervals overlapped zero. If all prey types are equally available, T. s. tetrataenia selected California Red-legged frogs more than Sierran chorus frogs and L. catesbeianus. The degree of seasonal diet overlap between T. s. tetrataenia and L. catesbeianus was high. The strongest interaction between these species is likely competition rather than symmetric predation. Eradication of L. catesbeianus would likely benefit T. s. tetrataenia populations by reducing predation pressure on their shared amphibian prey.