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Individual-based modeling to project viability of gopher tortoise populations under alternative management scenarios: A case study with two Alabama Wildlife Management Areas


Tripp, Helen E.
Gitzen, Robert A.
Silvano, Amy L.


Population projection models are applied tools for considering the potential effects of land and population management alternatives. Incorporating spatially explicit processes and individual dynamics into these models can be important when assessing viability for relatively small populations in patchy habitats. We developed a spatially explicit, individual-based population simulation model (IBM) for gopher tortoises (Gopherus polyphemus) incorporating demographic rates from published studies throughout the range of the species. We then demonstrated this approach’s utility for evaluating potential viability under projected forest management with and without tortoise population augmentation on two areas of state-managed property in southern Alabama. Under all scenarios, projected populations declined to local extinction within 100–200 years. The IBM projected sharper declines compared to projections from a non-spatial, stochastic, stage-structured model, potentially indicating the importance of considering spatial dynamics and individual interactions in this context. The IBM approach is especially useful when dealing with actual management units because it identifies projected hotspots of consistent occupancy and important habitat connections on the landscape.