Intrinsic Prey Preference and Selection of the Giant Gartersnake: A Threatened Predator in a Nonnative Prey-Dominated Community

Abstract

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 is of conservation concern. The giant gartersnake, Thamnophis gigas, an endemic predator of the Central Valley of California and a species of conservation concern at the state and federal levels, has declined as a result of conversion of its once vast wetland habitat to agriculture. Another anthropogenic factor contributing to this snake's changing ecology is the introduction of nonnative prey into the species' habitats. These introductions have resulted in a prey community that is almost completely composed of exotic species and have potential for considerable effects. In order to assess prey preference and selection we performed three sets of behavioral trials on naïve neonates. We examined 1) neonate prey preference in response to olfactory cues of prepared prey extracts, 2) neonate consumption of different live prey items presented simultaneously; and 3) terrestrial feeding behavior and/or latency to successful attack. Results from the olfactory study suggest that native Sierran treefrogs, Pseudacris sierra, are preferred by neonates. Results from consumption trials suggest that neonates are more likely to select frog species than fish species. This is the first study that we are aware of that examines prey selection of this threatened species and serves to inform its conservation and management.

Survival and establishment of captive‐reared and translocated giant gartersnakes after release

“…In July-August 2018, we collected gravid females from the 2 donor sites. We kept gravid females in 38-76-L glass aquaria at the University of California Davis vivarium on aspen substrate with a hide and a water bowl large enough for them to completely immerse themselves (Ersan et al 2020).…”

A tale of two valleys: endangered species policy and the fate of the giant gartersnake

“…century, many generalist fish species and American bullfrogs (Lithobates catesbeianus; hereinafter bullfrogs) were introduced from the eastern United States and have dramatically changed the prey composition available to giant gartersnakes. Despite the dominance of nonnative prey, giant gartersnakes retain a preference for native anurans and generally select anurans (native and introduced) over introduced fishes (Ersan et al 2020a(Ersan et al , 2020b. Naïve neonate giant gartersnakes in the laboratory respond most strongly to extracts of and most readily consume larval and adult Sierran treefrogs (Pseudacris sierra; Ersan et al 2020a), and wild giant gartersnakes also strongly select native treefrogs (Ersan et al 2020b).…”

“…Naïve neonate giant gartersnakes in the laboratory respond most strongly to extracts of and most readily consume larval and adult Sierran treefrogs (Pseudacris sierra; Ersan et al 2020a), and wild giant gartersnakes also strongly select native treefrogs (Ersan et al 2020b). Although bullfrog tadpoles and juveniles also are positively selected as prey (Ersan et al 2020a(Ersan et al , 2020b, adult bullfrogs are predators of small giant gartersnakes (Wylie et al 2003).…”

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“…Around the turn of the century, many generalist fish species and American bullfrogs (Lithobates catesbeianus; hereinafter bullfrogs) were introduced from the eastern United States and have dramatically changed the prey composition available to giant gartersnakes. Despite the dominance of nonnative prey, giant gartersnakes retain a preference for native anurans and generally select anurans (native and introduced) over introduced fishes (Ersan et al 2020a(Ersan et al , 2020b. Naïve neonate giant gartersnakes in the laboratory respond most strongly to extracts of and most readily consume larval and adult Sierran treefrogs (Pseudacris sierra; Ersan et al 2020a), and wild giant gartersnakes also strongly select native treefrogs (Ersan et al 2020b).…”

“…For example, the estimated annual loss of snake recruitment caused by bullfrogs (Wylie et al 2003) might be overcompensated by the snakes frequently preying on pre-metamorphic and juvenile bullfrogs. Because giant gartersnakes prefer bullfrogs over invasive fish prey (Ersan et al 2020a(Ersan et al , 2020b, it is possible that where Sierran treefrogs are rare or extirpated, bullfrogs may be an important prey source to sustain the snake population (Rose et al 2018b(Rose et al , 2019.…”