Journal Article

Implications from Monitoring Gopher Tortoises at Two Spatial Scales

TUBERVILLE, TRACEY D., HERMANN, SHARON M., GOESSLING, JEFFREY M., STOBER, JONATHAN M., GUYER, CRAIG, GYENGO, SYBIL G.

2021

Overview

Conservation biologists need to effectively monitor species given resource limitations and the inherent challenges of assessing long-term demographic processes. We assessed gopher tortoise (Gopherus polyphemus) abundance at a landscape scale and at the scale of 3 local populations within the Conecuh National Forest (CNF), Alabama, USA, between 1991 and 2017. We collected landscape-level data from line transect distance sampling arranged uniformly across the CNF during a single season (2011); we obtained data for local populations from long-term mark-recapture of individuals at 3 sites selected based on prior knowledge of high density at each. At a landscape scale, we estimated 5,242 (95% CI=3,538–7,768) tortoises occurred across the approximately 34,000-ha forest, yielding a density of 0.14–0.32 tortoises/ha. These low densities across the landscape suggest that, on average, management activities across the property have not allowed tortoise populations to retain the social structure needed for long-term persistence. The 3 local populations, however, contained 25–60 individuals and densities of 1.9–6.9 tortoises/ha. Over the study period, populations at 2 sites were stable and the third experienced significant population growth. Mean annual survival of individuals was 0.89 and invariant across size classes. Overall, line transect distance sampling is important for assessing landscape-scale abundance of tortoises but may fail to detect local clusters of high-density sites important for population persistence. Our mark-recapture efforts at the local scale revealed that small populations on these high-density sites can exhibit long-term stability or growth even though they do not meet current established criteria for viability. Improved models that incorporate immigration and emigration and better reflect the dynamics of peripheral populations would assist in determining how such populations best contribute to species recovery and regional conservation targets.

• Publisher: Wiley,Blackwell Publishing Ltd
• Subject: Abundance / Alabama / Biological effects / Conecuh National Forest / Conservation organizations / demography / Emigration / forests / Gopherus polyphemus / Immigration / Landscape / landscapes / line transect distance sampling / Local population / mark-recapture studies / mark‐recapture / minimum viable population / National forests / Peripheral populations / Population density / Population Ecology / Population growth / Population studies / Reptiles & amphibians / Sampling / Social conditions / social structure / Testudines / Tortoises / viability / Wildlife / Wildlife conservation / Wildlife management