Using Demographic Sensitivity Testing to Guide Management of Gopher Tortoises at Fort Stewart, Georgia: A Comparison of Individual-Based Modeling and Population Viability Analysis Approaches

Abstract

Population Viability Analysis (PVA) models have been used as a decision-support tool for managing wildlife populations, both game and nongame species. However, PVA models require extensive population-level data; without such data, an individual-based model (IBM) may be a more appropriate tool. The at-risk gopher tortoise (Gopherus polyphemus) is one species of interest about which numerous individual studies have been published, but with little published documentation of population dynamics. Using NetLogo (http://ccl.northwestern.edu/netlogo/), the authors developed a spatially explicit IBM for the gopher tortoise population that inhabits Fort Stewart, GA, a US Army installation. The model was used to perform demographic sensitivity analyses and compare the results to sensitivity analyses conducted using a PVA model based on the same combinations of demographic parameters. The comparison showed a significant congruence in results from the two approaches. Several parameters—particularly juvenile and egg-to-age 1 mortality—appeared to disproportionately affect simulation results and are likely to be influenced by habitat manipulation. Based on their results, the authors conclude that IBMs can be useful to perform demographic sensitivity analysis and evaluate the capacity for habitat manipulation alone to provide the means for ensuring long-term persistence of gopher tortoise populations at Fort Stewart.