Desert Tortoise habitat on the Florence Military Reservation (FMR) in south-central Arizona is considered atypical in that boulder strewn hillsides are scarce and much of the landscape is dominated by flat alluvial plains. As a result, tortoises use caliche burrows associated with deeply incised washes and appear to concentrate their activity around these washes and the few rocky hillsides occurring on the installation. Tortoises occur at lower density in all available habitats on the installation that provide adequate shelter sites, including upland areas on alluvial fans where they use soil burrows and other temporary shelters (i.e., under trees, shrubs, and inside packrat middens).
Much of the historically documented tortoise activity on the FMR is located adjacent to designated firing boxes where the Arizona Army National Guard focuses its training exercises. These exercises include the use of heavy vehicles, live artillery fire, and troop maneuvers. The topography within the firing boxes reflects that of the surrounding landscape (i.e., alluvial flats bisected by desert washes) and therefore provides habitat components important for desert tortoise habitat selection (i.e., incised washes and available shelter sites). However, impacts resulting from military training exercises and other forms of disturbance (e.g., recreation activity, OHV, etc.) may reduce habitat suitability for desert tortoises. We examined spatial patterns of desert tortoise radio-locations and micro-habitat selection of habitat components within their home ranges in response to concern regarding the possible impact of military training exercises and other forms of disturbance within desert tortoise habitat on FMR. We hypothesized that desert tortoises would concentrate their activity outside of firing box boundaries and that this spatial patterning would be related to reduced vegetation density and canopy cover resulting disturbance to vegetation within the firing boxes.
The Arizona Game and Fish Department (AGFD) used the following approach to evaluate desert tortoise micro-habitat selection on the FMR:
- We conducted Desert Tortoise surveys within, and adjacent to, the eight firing boxes on the FMR. One to four surveyors, spaced approximately 5 meters from each other, moved along each wash transect examining all vegetation and shelter sites for tortoises and their sign (i.e., scat, tracks, and egg shell fragments). We used flashlights and hand mirrors to illuminate dark burrows and boulder piles in order to detect inactive tortoises. We used a similar approach in upland areas were we surveyed for tortoises, tortoise sign, and burrows. We also conducted opportunistic “surveys” while radio-tracking tortoises in the study area.
- We instrumented 15 tortoises with VHF radio transmitters and monitored them 1 to 5 times per week from the date they were marked until their activity patterns indicated hibernation had begun (March through mid-November). Once hibernation had been initiated, we continued monitoring tortoises every other week until their first movement was detected in the spring.
- We evaluated desert tortoise micro-habitat selection by comparing used resources to available resources at the level of the individuals’ home range. We randomly selected a subset of 15 known tortoise locations within each home range to represent used resources for each individual. We then selected an equal number of random locations within each home range to represent available resources for that individual.
Results and Discussion:
We marked 33 tortoises over the course of the study (14 males, 15 females, and four juveniles). Fifteen tortoises were instrumented with VHF radio transmitters. We documented 1,676 tortoise locations between 27 July 2005 and 28 September 2007. Seven tortoises had home ranges that included portions of the firing boxes and were detected within firing boxes at least once. Among these 7 individuals, 32% of relocations using radio-telemetry fell within the firing-box boundaries.
Habitat selection analysis suggests that while Desert Tortoises on the Florence Military Reservation select home ranges based on the availability of shelter sites, micro-habitat selection is influenced by the proportion of canopy cover, absence of cattle activity, and location of washes and roads within their home range.
For more information contact:
Michael Ingraldi, Ph.D, Research Supervisor
Arizona Game and Fish Department
5000 W. Carefree Highway, Phoenix, AZ 85086