The USGS has released a report today using a predictive modeling tool that incorporates "extensive amount of field data for desert tortoises, as well as environmental data related to landscape attributes, soil properties, annual rainfall patterns, and the influences of perennial and annual plants" to determine potentially suitable habitat for desert tortoises, to use in land use planning efforts.
[right, predicted habitat potential index values for desert tortoise]
Ref: Nussear, K.E., Esque, T.C., Inman, R.D., Gass, Leila, Thomas, K.A., Wallace, C.S.A., Blainey, J.B., Miller, D.M., and Webb, R.H., 2009, Modeling habitat of the desert tortoise (Gopherus agassizii) in the Mojave and parts of the Sonoran Deserts of California, Nevada, Utah, and Arizona: U.S. Geological Survey Open-File Report 2009-1102, 18 p.