Species at risk of extinction often face multiple threats—habitat loss and fragmentation, invasive species, pollution, over-exploitation, and disease. In the last 20 years, a new, far reaching threat has been identified. With average global temperatures expected to rise between 1.4 and 5.8ºC in the coming century, temperature changes, together with increased carbon dioxide concentrations and altered precipitation patterns, will affect sea level, hydrological cycles, fire regimes, and ecological systems. Changes in the Earth’s climate have already led to shifts in species distributions and changes in phenology. Efficiently managing for the persistence of at-risk species requires an understanding of both the relative and cumulative effects of different stressors on wildlife.

The objective of this project was to develop and demonstrate a flexible, spatially explicit population model for investigating the effects of multiple stressors on at-risk populations. Demonstration involved using the model to assess the impacts of multiple stressors on three at-risk species on three Department of Defense (DoD) installations.

Technical Approach

This project had three phases. The first involved model development and specifically developing new code to modify the Environmental Protection Agency’s Program to Assist in Tracking Critical Habitat (PATCH). The new code converted a spatially explicit, individual-based, population model into a flexible population-modeling software platform capable of modeling a wide range of animal species, environments, and stressors. The model, now named HexSim, was adapted to (1) simulate complex interactions among stressors, (2) simulate interactions between multiple wildlife populations, and (3) produce a set of easily interpreted outputs and reports. In the second phase of the project, the improved model was parameterized and run for the red-cockaded woodpecker at Fort Benning, Georgia; the black-capped vireo at Fort Hood; Texas; and the desert tortoise at Fort Irwin, California, to evaluate the relative and cumulative impacts of military activities, environmental stochasticity, anticipated climate change, and other species- and site-specific threats. The last phase was technology transfer in which the model was presented to the DoD Conservation Committee and made publicly available via the web.


This project developed a flexible population-modeling platform capable of modeling multiple interacting species and stressors. The model is also capable of simulating dynamic landscapes, inherited traits (e.g., genetics), and a wide array of species. From the demonstration phase, climate- and vegetation-change projections indicate that temperatures are projected to increase across all three installations. Precipitation and vegetation changes are more variable across climate-change scenarios and installations. Habitat models were developed for all three species for the three installations. Simulated red-cockaded woodpecker populations were more susceptible to modeled effects of land-use change than to the modeled effects of climate change. Simulations also demonstrated the importance of recent longleaf pine restoration efforts and the potential for population decline without continued restoration efforts. Modeling results for the black-capped vireo stressed the importance of cowbird control and the potential positive effects of climate-induced increases in fire. Simulations for the desert tortoise project a continued rapid decline of the Fort Irwin population in response to upper respiratory tract disease.


The publicly available HexSim modeling tool can be used by DoD and others to develop population models for a wide range of animal species in a diversity of environments (http://www.hexsim.net/). The model can be used for exploring alternative development scenarios, implications of management actions, climate impacts, effects of invasive species and diseases, as well as for risk and population viability assessments. This project also provided guidance for managing three populations of at-risk species.