SREL Reprint #3618

Survival of immature gopher tortoises recruited into a translocated population

Tracey D. Tuberville1, Rebecca K. McKee2, Heather E. Gaya2, and Terry M. Norton3

1University of Georgia, Savannah River Ecology Laboratory, Drawer E, Aiken, SC 29802, USA
2University of Georgia, Warnell School of Forestry and Natural Resources, Athens, GA 30602, USA
3St. Catherines Island Foundation, 182 Camellia Road, Midway, GA 31320, USA

Abstract: Population manipulations such as translocation and head-starting are increasingly used as recovery tools for chelonians. But evaluating success of individual projects can require decades of monitoring to detect population trends in these long-lived species. Furthermore, there are often few benchmarks from stable, unmanipulated populations against which to compare demographic rates, particularly for the immature stages. We used 8 years of mark-recapture data to estimate apparent survival of immature gopher tortoises (Gopherus polyphemus) recruited into an introduced population of gopher tortoises first established on St. Catherines Island, Georgia, USA, in 1987. During 2006–2013, we conducted targeted trapping of immature gopher tortoises and compared survival of the hatchling, juvenile and subadult stages among treatments: individuals released shortly after hatching from eggs obtained from gravid female founders (direct releases), individuals reared in captivity for 6–9 months following hatching (head-starts), and individuals first encountered as free-ranging, wild-recruited offspring (wild recruits). Among the candidate models we examined, the best fit model included additive effects of tortoise stage and treatment; however, overlapping 95% credible intervals among treatments (CrI) suggested that survival did not vary significantly among treatments. Annual apparent survival increased over the immature period, highlighting the importance of calculating separate estimates for the different immature stages. Across all treatments, the additive model estimated annual apparent survival probability to be 0.37 (CrI=0.25–0.48) for hatchlings, 0.71 (CrI=0.61–0.81) for juveniles, and 0.83 (CrI=0.74–0.94) for subadults. Our study, in combination with previous monitoring efforts at St. Catherines Island, provides strong evidence that the translocation and subsequent population augmentation efforts have been successful in establishing a robust population of gopher tortoises. Additionally, our results provide estimates of demographic rates for life stages that are poorly understood but critical to understanding population dynamics of this imperiled species.

Keywords: apparent survival, augmentation, demographic rates, gopher tortoise, Gopherus polyphemus, immature, life stage, population viability, recovery, translocation

SREL Reprint #3618

Tuberville, T. D., R. K. McKee, H. E. Gaya, and T. M. Norton. 2021. Survival of immature gopher tortoises recruited into a translocated population. Journal of Wildlife Management 85(4): 631-639.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).