Survival of Immature Gopher Tortoises Recruited into a Translocated Population

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.

Waif Gopher Tortoise Survival and Site Fidelity Following Translocation

“…Because we used a multistate modeling framework to distinguish stage-specific transition probability from survival, our study is among the first to separately estimate annual apparent survival for subadult (0.91 ± 0.07) and non-hatchling juvenile gopher tortoises (0.25 ± 0.15). Using a similar analytical approach, a recent study estimated apparent annual survival to be 0.71 for juveniles and 0.83 for subadult gopher tortoises recruited into a population following translocation (Tuberville et al 2021). Although estimates from additional populations are required to fully address this historical gap in life-history information, our estimates will serve as an important benchmark, particularly for manipulated populations.…”

Toward a predictable cask theory of species extinction assessment in the Anthropocene

“…2020; Tuberville et al . 2021), highlights the importance of determining the colonization potential of species when developing conservation metrics in a changing world.…”

Special Section: Gopher Tortoise Demographic Variables Estimated from Long‐Term Mark‐Recapture Data

“…This study reveals the importance of site-specific demographic information for estimating the probability of population persistence. Tuberville et al (2021) followed an original study (Tuberville et al 2008) of a tortoise population translocated to St. Catherine's Island, a coastal barrier island in Georgia.…”