Contrasting Patterns of Demography and Population Viability Among Gopher Tortoise Populations in Alabama |
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Authors: | Brian Folt Jeffrey M Goessling Anna Tucker Craig Guyer Sharon Hermann Ericha Shelton-Nix Conor McGowan |
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Institution: | 1. Alabama Cooperative Fish and Wildlife Research Unit, 3305 School of Forestry and Wildlife Sciences, Auburn University, Auburn, AL, 36849 USA;2. Eckerd College, 075 James Center for Molecular and Life Sciences, St. Petersburg, FL, 33711 USA;3. U.S. Geological Survey, Patuxent Wildlife Research Center, 12100 Beech Forest Road, Laurel, MD, 20708 USA;4. Department of Biological Sciences and Auburn University Museum of Natural History, 331 Funchess Hall, Auburn University, Auburn, AL, 36849 USA;5. Department of Biological Sciences, 331 Funchess Hall, Auburn University, Auburn, AL, 36849 USA;6. Alabama Department of Conservation and Natural Resources, 64 Union Street, Montgomery, AL, 36130 USA;7. U.S. Geological Survey, Alabama Cooperative Fish and Wildlife Research Unit, School of Forestry and Wildlife Sciences, Auburn University, Auburn, AL, 36849 USA |
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Abstract: | Population viability analyses are useful tools to predict abundance and extinction risk for imperiled species. In southeastern North America, the federally threatened gopher tortoise (Gopherus polyphemus) is a keystone species in the diverse and imperiled longleaf pine (Pinus palustris) ecosystem, and researchers have suggested that tortoise populations are declining and characterized by high extinction risk. We report results from a 30-year demographic study of gopher tortoises in southern Alabama (1991–2020), where 3 populations have been stable and 3 others have declined. To better understand the demographic vital rates associated with stable and declining tortoise populations, we used a multi-state hierarchical mark-recapture model to estimate sex- and stage-specific patterns of demographic vital rates at each population. We then built a predictive population model to project population dynamics and evaluate extinction risk in a population viability context. Population structure did not change significantly in stable populations, but juveniles became less abundant in declining populations over 30 years. Apparent survival varied by age, sex, and site; adults had higher survival than juveniles, but female survival was substantially lower in declining populations than in stable ones. Using simulations, we predicted that stable populations with high female survival would persist over the next 100 years but sites with lower female survival would decline, become male-biased, and be at high risk of extirpation. Stable populations were most sensitive to changes in apparent survival of adult females. Because local populations varied greatly in vital rates, our analysis improves upon previous demographic models for northern populations of gopher tortoises by accounting for population-level variation in demographic patterns and, counter to previous model predictions, suggests that small tortoise populations can persist when habitat is managed effectively. © 2021 The Wildlife Society. |
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Keywords: | demography Gopherus polyphemus hierarchical model mark-recapture analysis minimum viable population population structure population viability analysis survival |
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