Anthropogenic disturbances within the range of the desert tortoise (Gopherus agassizii) may reduce habitat suitability for this species (Krzysik 1997, Berry et al. 2006). For more information on desert tortoises and their conservation needs go to http://www.azgfd.gov/w_c/deserttortoisemanagement.shtml. Concerns regarding the possible impacts of military training, infrastructure development, fire management, and other anthropogenic impacts to desert tortoise habitat and tortoise persistence resulted in efforts to better understand tortoise distribution on the Florence Military Reserve (FMR). The goal of these efforts was to collect information about the distribution of desert tortoises on the FMR and develop recommendations that will help minimize impacts to tortoises while maintaining the National Guard’s military readiness mission. In addition, the information gained through an extensive survey of the installation can be used to develop an optimal monitoring plan for evaluating future population trends and provide the baseline dataset upon which future desert tortoise survey efforts on the FMR can be compared.
The FMR is bounded on the east by the Mineral Mountains, west by State Route 79, north by US 60, and south by the Gila River Valley. The installation's training Area covers approximately 1,552 hectares of gently sloping to flat alluvial plains, rugged hills with deeply incised washes, and mountainous terrain. Erosional processes caused by seasonal precipitation events create washes that often expose the caliche layer which stabilizes soil above it while allowing tortoises to evacuate shelters underneath (Barrett 1990). Previous research had identified desert washes as a major component of tortoise habitat on the FMR (Grandmaison et al. in review, Reidle et al. 2008) more information on this can be found at the following web address http://www.azgfd.gov/w_c/research_micro_habitat.shtml.
The Arizona Game and Fish Department (AGFD) used the following approach to model desert tortoise occupancy on the FMR:
- We established a grid of 3-ha survey plots along desert washes to increase detection rates.
- We conducted standardized surveys on a total of 228 randomly selected survey plots from July to September in 2008 and 2009. Each plot was surveyed 4 times.
- A plot was considered occupied when a live tortoise or tortoise sign were detected.
- We used Program PRESENCE to estimate detection probabilities (p) and the proportion of area occupied (PAO).
- We examined the influence of site- and survey-specific covariates (e.g., presence of shelter sites, roads and cattle activity, timing of survey, temperature, relative humidity).
Results & Discussion
Desert tortoises or their sign were detected on 40 (17%) of the 228 sites surveyed in 2008 and 2009. The average detection probability across all the sites surveyed was 0.307 (SE = 0.054), meaning that on average, 30.7 ± 5.4% of the tortoises present on a survey plot were detected. The probability of desert tortoise detection was influenced by the time of day during which the survey took place with high detection in the morning and lower detection as the day progressed. Tortoises were 1.67 times more likely to be present with each caliche cave present on the plot. Desert tortoise presence decreased by 0.45 and 0.35 times when roads and cattle sign were present on the plot, respectively. The overall proportion of area occupied was estimated at 0.216 (SE = 0.055). In other words, we estimate that 21.6 ± 5.5% of the desert washes we surveyed were occupied by desert tortoises. Our estimates of detection probability and proportion of area occupied can be used to develop an optimal monitoring plan to evaluate changes in desert tortoise occupancy over time. Low detection rates will continue to pose significant challenges for monitoring desert tortoises. Increasing detection probability will increase the level of precision for the occupancy estimate. Given the results of our study, surveys should be conducted in the early morning to maximize detection. Surveys should also coincide with seasonal peaks in tortoise activity to further increase detection probability.
For more information contact:
Michael Ingraldi, Ph.D, Research Supervisor
Arizona Game and Fish Department
5000 W. Carefree Highway, Phoenix, AZ 85086
Berry, K. H., T. Y. Bailey, and K. M. Anderson. 2006. Attributes of desert tortoise populations at the National Training Center, Central Mojave Desert, California, USA. Journal of Arid Environments 67:165-191.
Barrett, S. L. 1990. Home range and habitat of the desert tortoise (Xerobates agassizi) in the Picacho Mountains of Arizona. Herpetologica 46:202-206
Grandmaison, D. D., M. F. Ingraldi, and F. R. Peck. In review. Desert tortoise micro-habitat selection on the Florence Military Reservation, Arizona. Journal of Herpetology.
Krzysik, A. J. 1997. Desert Tortoise populations in the Mojave Desert and a half-century of military training activities. Proceedings of the Conservation, Restoration, and Management of Tortoises and Turtles – An International Conference. 1997:61-73.
Riedle, J. D., R. C. Averill-Murray, C. L. Lutz, and D. K. Bolen. 2008. Habitat use by Desert Tortosies (Gopherus agassizii) on alluvial fans in the Sonoran Desert, south-central Arizona. Copeia 2008:414-420.