Background:

Three military installations in southwestern Arizona (U.S. Army Yuma Proving Ground,BarryM. Goldwater Air Force Range, and Marine Corps Air Station Yuma) represent the largest tracts of relatively undisturbed Sonoran Desert (outside of training and testing activities) in the southwestern United States. As such, they play an important role in the statewide conservation of the desert tortoise. A current lack of information regarding the genetic characteristics and habitat affinities of tortoises inhabiting the installations impedes a data-driven approach to tortoise management. Acknowledgement of this information gap led to the development and implementation of this on-going research study funded by the Department of Defense Legacy Program.

Studies in northwestern Arizona have shown a possible link between tortoise occurrence and soil type, specifically Aridisol soil sub-groups. Whereas Entisol soils are characterized by soils of recent origin with no distinct soil horizons, Aridisols have a sub-surface horizon comprised of clays, calcium carbonate, silica, and/or gypsum. We hypothesize that Aridisols provide the characteristics necessary for the construction of deep permanent burrows, a limiting factor in desert tortoise distribution. Our expectation is that soil characteristics can be used to predict desert tortoise occupancy at the landscape-scale.

Our objectives for this study are to:

1. Develop a landscape-scale, soil-based habitat model to predict tortoise activity centers.
2. Map tortoise distribution on the three installations based on the resulting model.
3. Clarify genetic designation as Sonoran or Mojavean.

Approach:

• We conducted standardized surveys for tortoises and their sign (i.e., carcasses, scat, tracks, etc.) on 3-ha surveys plots randomly located with each of the soil sub-groups found on the installations.

• We then used standard VHF radio-telemetry and GPS tracking units to track desert tortoise movements within the study area. At the completion of this study, we will examine desert tortoise movement patterns relative to soil sub-groups as a means to validate the habitat models developed for use in predicting areas with a high probability of tortoise occupancy.

For more information contact:
David Grandmaison, Research Biologist
Arizona Game and Fish Department
5000 W. Carefree Highway, Phoenix, AZ 85086
Phone: (520) 609-2164   E-mail: dgrandmaison@azgfd.gov

Ron Mixan, Research Biologist
Arizona Game and Fish Department
5000 W. Carefree Highway, Phoenix, AZ 85086
Phone: (520) 609-2165   E-mail: rmixan@azgfd.gov