Survival, demography, and growth of gopher tortoises (Gopherus polyphemus) from three study sites with different management histories

Abstract

Recovery or sustainable management of wildlife populations often entails management of habitat on which they depend. In this regard, turtles pose unique conservation challenges because of their life histories. The combination of late maturity, low survival when young, and dependence on high adult survival suggests they may be slow to respond demographically to conventional habitat management. Thus, long‐term studies are necessary to understand population dynamics and recovery potential in these species. We used 5–11 years of mark‐recapture data from 3 populations to evaluate survivorship, demography, and somatic growth of gopher tortoises (Gopherus polyphemus). Green Grove and Wade Tract (southwest GA) are ecological reserves with a history of land management compatible with tortoises. In contrast, Conecuh National Forest (south‐central AL) is a closed‐canopy pine plantation with prior intensive site preparation but where management intervention improved habitat for tortoises during the study. Apparent survival was high for mature tortoises (87–98%) compared to immature tortoises (70–82%). Adults comprised 57–79% of individuals captured, with Green Grove and Wade Tract populations dominated by larger individuals but Conecuh having a more uniform size distribution. The largest adults captured at Conecuh (297 mm maximum carapace length [CL]) were smaller than the largest adults from Green Grove (337 mm CL) or Wade Tract (341 mm CL), although characteristic growth constants from von Bertalanffy models were similar among sites. We suggest these results indicate a recovering population at Conecuh, where habitat conditions for gopher tortoises have improved despite a legacy of intense predation by humans and reduced habitat quality at the inception of this national forest. Further, we recommend using a combination of short‐term and long‐term monitoring metrics to assess population recovery in such long‐lived species. © 2014 The Wildlife Society.

Establishing Reference Demography for Conservation: A Case Study ofMacrochelys temminckiiin Spring Creek, Georgia

“…First, the observed and estimated population structure was characterized by an even sex ratio and a greater proportion of adults than juveniles. This structure is consistent with a general prediction for long-lived turtles, where high survival of adults followed by low postmaturity growth rates results in the accumulation of similar-sized adults from different age cohorts (Alford 1980); thus, greater abundance of adults than juveniles is a hypothesized characteristic of stable populations of long-lived turtles (Tuberville et al 2014). Second, regression analysis did not detect significant changes in observed adult body size during the course of the study, a result which was documented in the declining Oklahoma population (East et al 2013).…”

“…Similarly, Tuberville et al (2014) recently suggested that greater abundance of adults than juveniles should be expected in stable populations of another long-lived turtle species, Gopher Tortoises (Gopherus polyphemus), but our analyses were somewhat inconsistent with this notion for M. temminckii. The stable population structure produced from population projection analysis was dominated by juveniles at Spring Creek and the two declining western populations.…”

“…Because different age and sex groups of M. temminckii are thought to differ in life history strategies that may influence survival and detection probability, and our trapping effort varied through the study, we then used a multi-model approach to evaluate whether apparent survival and detection probability varied by all combinations of groups (females, males, juveniles) and by time-varying or constant apparent survival (phi) and recapture probability (P). Few of the juveniles in the dataset were recaptured after having transitioned to the adult stage, so individuals were grouped based on the assignment at the first capture; this structure allowed us to evaluate whether juveniles, females, and males differed in apparent survival or recapture probabilities in the population (Tuberville et al 2014). We ranked and evaluated relative support for each competing model with AIC and DAIC (Akaike 1974); we were unable to calculate AIC c because the number of parameters (n ¼ 72) approached the sample size of the dataset (n ¼ 73) for some models, which prohibits the calculation of AIC c .…”

Gopher Tortoise Demographic Responses to a Novel Disturbance Regime

“…In contrast, 10 populations in northern peninsular Florida tended to have high proportions of tortoises in intermediate size classes (Alford ). We suspect our study population is at carrying capacity, resulting in higher dispersal and emigration of subadults, which may be disadvantaged in social interactions with larger adults (McRae et al b , Tuberville et al ). As implied by the very high apparent survival estimates for tortoises in the ruderal field, adults tortoises in Hill Garden exhibited a high degree of site fidelity.…”

Precipitation quantity and timing affect native plant production and growth of a key herbivore, the desert tortoise, in the Mojave Desert

“…Survival and recruitment of juvenile tortoises also affects population persistence [36]. We thus focused on juvenile growth as a proxy for understanding possible demographic responses to altered climate because, it responds more quickly to changing resources and habitat quality than many other demographic metrics [37]. Desert tortoises also exhibit behavioral plasticity, which allow them to alter metabolic expenditures based on water availability [2].…”