Desert Tortoises Get Real Estate Map

May 27, 2009 -- A new map shows where desert tortoises could relocate if displaced by climate change or human development.

While the project didn't turn up many surprises, it is the first real numbers-based tortoise map that is designed to adapt to changing environmental conditions. For now, the map mostly outlines where good tortoise habitats are today. Future modifications, researchers hope, will help them protect the threatened species.

"We can say, 'OK, what do we think the 100-year climate prediction is going to look like?'" said Kenneth Nussear, a wildlife biologist with the United States Geological Service in Las Vegas. "We can make that change and redo the map and see what the future might look like for tortoises. This is the first step needed to get there. That's where we want to go."

Tortoises are resilient animals that have been around for millions of years. Today, the desert tortoise lives throughout the Mojave and Sonoran deserts in California, Nevada, Arizona and Mexico. With development, climate change, and other human influences, researchers suspect that tortoise habitat is poised to shift. No one knows exactly how.

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Scientists have attempted to map tortoise habitat before, but previous efforts have relied on informed opinions and expert knowledge. Nussear and colleagues wanted something more quantitative.

To begin, the scientists identified 16 variables that determine whether tortoises can live in a given area. Variables included the volume of summer and winter rain, the abundance of shade, the region's elevation, and the types of plants available for the herbivores to eat.

The researchers also collected studies on where tortoises live now. By comparing the animals' known range with variables that can influence their range, the scientists nailed down which variables mattered most.

Finally, they overlaid all of the information into one map that divides the southwestern United States into 1-kilometer squares and ranks each square on its likelihood of containing desert tortoises. Not surprisingly, the final product shows that tortoises today live in the habitats most suitable for them.