Habitat Suitability for Gopher Tortoises

Background/Question/Methods

The Gopher Tortoise (Gopherus polyphemus) is a keystone species of the original savanna-like habitats of the Southeastern United States. Along with its habitat, Gopher Tortoise numbers are in drastic decline since the arrival of the first European settlers in the region. Accordingly, the species is protected to various degrees throughout its range. Protected lands and military installations provide important refuges for tortoise populations. To protect populations on sites, and to determine potential alternative habitats off-sites, we use a statistical model approach to identify the environmental characteristics that define a suitable habitat for the tortoises. Using detailed data sets available from Ft. Benning, Georgia, we built a habitat model for the site, applying a resource selection probability function (RSPF). The available data sets include surveys of the burrow locations where the tortoises spend most of their time, detailed survey data about ground, midstory and overstory vegetation as well as sites of human activity. Additionally, detailed data about soil characteristics is available, and elevations and slope of the landscape.

Results/Conclusions

The model results show that a suitable burrow site for a Gopher Tortoise is dependent on most of the environmental variables we included in the model, i.e. overstory and midstory vegetation, soil characteristics, slope and elevation as well as closeness to human developments. A map of habitat quality as predicted from the model shows that only small areas within Ft. Benning provide suitable Gopher Tortoise habitat. Most of it is currently occupied by tortoise populations. Our model can be used to determine potentially suitable habitat for translocations of tortoise populations. However, habitat suitability is not the only factor that may be important in successful translocations. In addition, it has to be assured that the animals have ample mating opportunities, and the population sizes and their connectedness allow for sustainable genetic variability. In order to determine these factors, in a next step of the project, we will provide an individual-based spatial population model using the results from the RSPF to model the landscape of the tortoises.