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.
That's a helpful starting point, said Roy Averill-Murray, Desert Tortoise Recovery Coordinator with the United States Fish and Wildlife Service in Reno.
"Everything in the past has been based on professional opinion and observation of tortoises and just extrapolating and drawing lines on a map," Averill-Murray said. "One of the things this will hopefully allow us to do is look at these environmental variables in light of potential climate change and see how tortoise habitat might look in the future."
The map could, for example, help inform decisions about where to build things like highways, railroads, wind farms and solar power stations. Those types of barriers can block tortoises from moving to new habitats as the environment changes.
Already, researchers have used the new map as a guideline for moving hundreds of desert tortoises to make way for the U.S. army's Fort Irwin National Training Center. Nussear's earlier work shows that tortoises do well with relocation, but the issue remains controversial.
In another study, scientists compared genetic information with the habitat map to see whether corridors existed to connect unrelated populations of tortoises with each other. Allowing for gene flow is an important strategy for conservation.
With more studies like these, Averill-Murray said, the new map should give tortoises a leg up.
"It's a huge starting point," he said. "It's going to be used a lot."
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