Diet and prey preference of giant gartersnakes (Thamnophis gigas) in the Sacramento Valley of California

The introduction of exotic species into an environment can introduce great change in the trophic dynamics of native species. This is of even greater concern if the native species are threatened or endangered. The giant gartersnake (Thamnophis gigas), an endemic predator of the Central Valley of California, is listed as threatened, its decline because of the conversion of its once vast wetland habitat to agriculture. Another anthropogenic factor contributing to this snake’s changing ecology and potentially its decline is the introduction of non-native prey species into their habitats. These introductions have resulted in a prey community that is almost completely composed of exotics. Exotic prey can be detrimental to predators, potentially causing injury, parasitic infection, and intraguild predation. A first step toward understanding the effect(s) of exotic prey on giant gartersnakes is to determine whether and the degree to which these snakes have an affinity toward and will actually consume these species. I examined dietary choices in the giant gartersnake using laboratory and field studies to determine whether these snakes are selecting their prey or if they are simply consuming what is abundant. The laboratory component of my investigation was comprised of two sets of behavioral trials in which naïve neonates of wild-caught adult females from three different areas served as the focal animals. I examined: 1) neonate prey preference in response to olfactory cues of prepared prey extracts and 2) what neonates actually consumed when provided with a simultaneous choice of different live prey items. The field studies involved an analysis of prey selection where I: 1) quantified the available prey species in several known giant gartersnake habitats and 2) examined stomach contents to reveal the composition of the snakes’ actual diet. Results from these studies revealed that giant gartersnakes both preferred and selected native Sierran treefrogs. These results will contribute to a further understanding of the giant gartersnake’s diet, its role as a predator and the degree of impact, if any, exotic species might be having on them. This, in turn, can direct management and conservation efforts.