Habitat Use by Desert Tortoises (Gopherus agassizii) on Alluvial Fans in the Sonoran Desert, South-Central Arizona

Desert Tortoises (Gopherus agassizii) in the Sonoran Desert typically occur on rocky slopes and bajadas and are absent from intermountain valley floors. Tortoises also occur along deeply incised washes emanating from rocky bajadas, using caliche caves as shelter sites. The Florence Military Reservation (FMR), in south-central Arizona, is typified by gently sloping alluvial fans bisected by steeply incised washes. One 10.9-ha hill consisting of volcanic outcrops and boulders occurs at the northern end of the reservation. Tortoise locations at FMR were concentrated around incised washes with dense caliche caves or near the volcanic hill. Home ranges of male and female tortoises were not significantly different, and the sexes used shelter types similarly. Tortoises used caliche caves as shelter more than other shelter types, especially those tortoises without access to the rocky hill. Compositional analysis of the three principal habitat types used by tortoises at FMR revealed that they selected incised washes over the other habitat types. However, we did not find tortoises in washes with few caliche caves. These results suggest that availability of shelter sites strongly influences tortoise distribution at FMR.