UC Davis

Translocation, although often criticized, is gaining popularity as a conservation management tool for imperiled species. Many species today face habitat loss, leaving populations isolated and with few options for recolonizing remaining habitat. Giant gartersnakes (Thamnophis gigas) are a semi-aquatic snake endemic to the Central Valley of California and are currently facing this very predicament. Giant gartersnakes historically inhabited wetlands and sloughs, but after losing >90% of their original wetland habitat and being nearly extirpated from the San Joaquin Valley, remaining populations are scattered among remnant marshes and rely heavily on rice agriculture as surrogate habitat. To recover the species, wildlife managers have prescribed the reintroduction of snakes into the San Joaquin Valley and, given the level of habitat loss throughout the region, translocation will likely be required to accomplish these goals. Translocation has never been attempted in giant gartersnakes, so the focus of this dissertation was to examine the survival, movement, and habitat selection of giant gartersnakes from one wetland donor site and one rice agriculture site to a third restored wetland site all within the Natomas Basin in Sacramento, CA, USA. Survival is often lower in translocated animals and because there have been no previous giant gartersnake translocation studies, our goal in the first chapter was to examine the survival of translocated wild-caught adult snakes to the survival of snakes in each of the two donor sites. We also monitored the survival of captive-reared juvenile snakes over a 2-yr rearing period in captivity and in the first four months following release. We found low survival in translocated adult snakes, consistent with several studies of other snake species; just 8% of translocated adult giant gartersnakes survived >801 days (95% CI = 1–64%), compared with 39% of resident snakes at the donor sites surviving >1,154 days (95% CI = 23–68%). This equated to annualized survival rates of translocated adults (0.32, 95% CI = 0.12–0.82) that were roughly half that of resident snakes ( 0.74, 95% CI = 0.63–0.89). Juvenile survival was more promising, with 76% of juvenile snakes surviving captivity and, once released, 60% surviving (95% CI = 38–94%) for the 4-month monitoring period before winter. We found no effect of average distance moved or surface activity on snake survival. Increased movement after translocation is often linked to survival, which prompted us to examine the movement and home ranges of snakes in the second chapter. In chapter two, we compared 100% and 95% MCP and 95% a-LoCoh home range estimates and examined movement between groups using the metrics of total distance traversed, start-to-end distance of movement paths, sinuosity of movement paths, and three measures of net-displacement. We found that results varied depending on the specific metric, but overall, translocated snakes had intermediate values for home range size, movement distances, and sinuosity compared to the two resident groups. Although translocation affected movement and space use relative to resident snakes, the metrics we used remained similar in individuals before and after translocation, and the landscape type of the study site (rice agriculture versus wetland) appeared to have the greatest effect on home range size and movement. We also did not see any signs of translocated snakes attempting to return to their origin sites, which has been a problem in translocation projects in some other species of snakes. Another important consideration in translocation projects is how well the animals acclimate to the release site, with animals that can acclimate more quickly usually being more successful. We examined this question in part in chapter two by looking at snake movement and built on this further in chapter three by quantifying habitat and vegetation selection of snakes from the donor sites and translocated snakes in the recipient site during the active (April–October) and overwintering (November–March) seasons. Across all three sites, we found that snakes avoided exposed habitat types and selected for habitats that provided cover during the active season. The preferred cover type and strength of selection differed among sites, especially between the wetland donor site and rice canal donor site. During the over-wintering season, habitat selection values were more neutral (neither selected nor avoided) which was expected because snakes are largely brumating underground. Habitat selection of translocated snakes in the recipient site shifted to selecting more typical wetland vegetation, such as tules, especially considering most translocated snakes originated from the canal donor site. The strength of selection still differed between snakes at the wetland donor site and recipient site, however, which may be an effect of natal preferences from the snake’s origin site, existing differences between the two wetland sites, effects of translocation itself, or a combination of factors. Snakes in the recipient site appeared to use rock and rip/rap more heavily compared to the other two sites, especially over winter. This could suggest that man-made refugia benefited translocated snakes in the recipient site, which was restored more recently than the wetland donor site and may therefore not have had as many naturally occurring burrows as the more mature donor wetland. Overall, despite lower survival, snake movement and habitat selection, which are often linked to survival, appeared to show that giant gartersnakes acclimated to the recipient site and did not attempt to emigrate from the recipient site after translocation. Future research building on our findings that improve survival of translocated adult snakes or additional efforts using captive-reared juveniles will help to ensure that larger-scale reintroductions into the San Joaquin Valley and other areas of the giant gartersnake’s range are successful.