Twenty-Third Annual Meeting and Symposium of the
Desert Tortoise Council, April 3-5, 1998
Abstracts

Organ Pipe Cactus National Monument lies in southwestern Arizona, on the U.S.-Mexico border, near the geographic center of the Sonoran Desert. The monument is comprised of approximately 133,600 hectares (330,000 acres), 95% of which is designated wilderness. Topography ranges from rugged mountains to steep bouldery slopes, to bajadas grading downslope to broad desert valleys. Vegetation varies from relatively dense and diverse Sonoran Desertscrub, Arizona Uplands Subdivision, to monotypic creosotebush flats and other Lower Colorado River Subdivision plant associations. In recent years, the monument has sought to clarify the status and distribution of desert tortoises and their habitat. This was pursued through inventory and monitoring activities, and use of geographic information system technology. Survey transects in 1988 and 1995, and monitoring plot data gathered in 1996 and 1997, have confirmed that tortoises are widespread in typical habitat, at densities in the range of 25-30 per km2. Tortoises were also found lower on bajadas and valley floors than expected, probably because of the availability of microhabitats in incised arroyos for shelter sites and burrows. GIS data management has allowed estimation of the distribution, quality, and quantity of tortoise habitat, using the parameters of slope, elevation, and vegetation. Management of the tortoise in Organ Pipe Cactus National Monument involves a number of issues, including: poaching; abandonment of pet tortoises; invasion by non-native plants (e.g. buffelgrass); roadkill; and habitat impacts from increasing off-road travel related to border activities (e.g. drug smuggling and interdiction, illegal immigration and interdiction).