Effects of Cattle Grazing
on the Desert Tortoise, Gopherus agassizii:
Nutritional and Behavioral Interactions


1 U.S. Geological Survey, Biology Department, University of California,
Riverside, CA 92421, USA / e-mail: hal_avery@usgs.gov

2 USDI Bureau of Land Management, Ridgecrest Resource Area,
300 South Richmond Road, Ridgecrest, CA 93555-4436, USA

        ABSTRACT:  As in most species of tortoises, the desert tortoise, Gopherus agassizii, relies inextricably on vegetation and soil for its survival.  In addition to meeting energy and nutrient needs, the desert tortoise uses vegetation as cover for thermal protection, predator avoidance, and intraspecific behavioral interactions.  Soil condition affects the ability to construct burrows and affects tortoise nutrition.  Tortoises acquire minerals and nutrients by consuming soil (geophagy), and they consume plants whose abundance and nutrient content are directly related to soil quality.  Studies on vegetation and soil changes are therefore of paramount importance to the conservation and management of wild tortoise populations.
        For over a century, range cattle and sheep have grazed the arid regions of the southwestern United States.  Because of the relatively slow recovery times of vegetation in desert ecosystems, the United States Bureau of Land Management is interested in determining long-term impacts of grazing on desert habitat and on populations of threatened and endangered wildlife.  We have been conducting research to determine the effects of cattle grazing on desert tortoise habitat and desert tortoise populations since 1991.  Vegetation and soil parameters were compared inside and outside a cattle exclosure, which has been in existence since 1980.  We also followed individual cattle and tortoises to document feeding behavior and to observe direct and indirect behavioral interactions of free-ranging cattle and wild desert tortoises.
        When comparing habitat in grazed and ungrazed areas, we found no significant difference in annual plant cover, biomass, or density between areas outside (grazed area) versus inside the cattle exclosure.  Individual volumes of two predominant shrub species, Larrea tridentata and Ambrosia dumosa, were greater outside the exclosure than inside the exclosure.  These shrubs were rarely or never consumed by range cattle or tortoises and are considered unpalatable.  In contrast, the densities and individual volumes of palatable perennial grass, Hilaria rigida, were greater in the grazed area versus the ungrazed area.  No significant difference was found in total cover of perennial plants between the grazed and ungrazed areas.
        Bulk density and penetration resistance of soils were greater in the grazed area versus the ungrazed area, indicating that soils are more compacted in the grazed area.  There was no measurable difference in hydraulic conductivity between the grazed and ungrazed areas, indicating that soil compaction was not sufficient to reduce the rate of water transit into the soil.
        Direct and indirect interactions were observed between cattle and tortoises.  Direct interactions included cattle making physical contact with tortoises (e.g., nudging and rubbing).  Indirect interactions included trampling of actively used burrows, attempts of tortoises to enter destroyed burrows, and the destruction of vegetation shading actively used burrows.  Cow dung was an insignificant component of tortoise diets (<0.5%).
        We evaluate how changes in vegetation and soils caused by cattle grazing affect the conservation biology of the desert tortoise within the framework of behavioral, thermal, and nutritional ecology.

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