Conserving threatened and endangered species (TES) on Department of Defense (DoD) installations, without sacrificing vital military objectives, is necessarily a complex balancing act – especially given the uncertainties and risks associated with various natural hazards. This project advances the concept of critical habitat breadth as the foundation for rigorous TES conservation planning and vulnerability assessment. Gopherus tortoises - particularly Desert Tortoise (G. agassizii) and Gopher Tortoise (G. polyphemus) -- provide an excellent model system for applying the concept of critical habitat breadth because their populations have been extensively studied, and many prior translocations have been conducted which enable investigation into the inherent ability to acclimate to novel environments.

The project team compiled and re-analyzed previously collected data from across the range of both species to assess how demographic vital rates (e.g., survival, fecundity, age-at-maturity) respond to spatiotemporal environmental and climatic gradients. In addition, nesting surveys were conducted in the field to investigate how hatching success and hatchling sex ratios respond to climate gradients. These statistical models were integrated into comprehensive, spatially explicit predictive models capable of forecasting annual range-wide population dynamics for both species. This simulation model was used to forecast and visualize population dynamics through the year 2099, and to predict when and where populations are likely to be self-sustaining. Machine-learning methods were used to assess which environmental and climatic drivers were most influential for determining population sustainability.

The simulation results for Desert Tortoise indicated that range-wide population abundance appears to be relatively stable. This is due to compensatory effects of higher temperatures on different demographic rates – for instance, negative effects of warming on hatching success were balanced by positive population-level effects of warming on sex ratios. However, the simulations suggest that extreme warming could lead to slight range-wide population declines within the next century, and there could be negative effects of the highly skewed sex ratios (up to 80% female) which the model does not address (such as mating failures).

The simulations indicated that most Gopher Tortoise populations appear to be declining, but that these declines may be driven more by low habitat quality rather than increasing temperatures per se. In particular, the observed decline in survival rate with years since fire strongly influenced population dynamics. Higher temperatures, which are expected to reduce the frequency of prescribed fires across much of the southeastern United States, only exacerbated this effect in the simulations. Projected local extinction events in the Gopher Tortoise model tended to be most severe in the western and northern parts of the range, underscoring concerns about the status of Gopher Tortoise populations in this region.

The results of the data collection linking higher temperatures to growth, reproduction, hatching success and sex ratio in both species will be directly beneficial to all future efforts to model climate effects on Gopherus populations. The range-wide population viability analyses provide a tool to help resource managers prioritize conservation efforts in regions with projected climate resiliency, to identify potential future translocation sites outside of DoD lands that will remain suitable as habitat for many decades into the future, and to direct resources toward ameliorating the most damaging effects of climate change, including the reduced potential for critical habitat management (e.g. prescribed fire). Finally, a decision support tool was developed to enable more effective information transfer to land managers. However, harnessing the value of this tool for management will require further consultations with key stakeholders.