26th Annual Meeting and Symposium of the
Desert Tortoise Council, March 16-18, 2001
Abstracts

Calcium and Phosphorus Availability in Native and Exotic Food Plants

Lisa C. Hazard, Danielle R. Shemanski, Kenneth A. Nagy
Department of Organismic Biology, Ecology and Evolution, P.O. Box 951606, University of California, Los Angeles, CA 90095-1606

Exotic plants can comprise a major component of the diet for some desert tortoises (Gopherus agassizii) in the Mojave Desert. Introduced plants may not be as nutritious as native plants. Nutrient availability in a native grass (Achnatherum hymenoides), an introduced grass (Schismus barbatus), a native forb (Malacothrix glabrata) and an introduced forb (Erodium cicutarium) were measured in one to two year old juvenile desert tortoises. We fed tortoises measured amounts of chopped foods daily for ~130 days (dry grass) or ~90 days (fresh or thawed forb). Orts and feces were collected daily and dried to constant mass, and calcium and phosphorus content of food and feces were measured. Calcium digestibilities did not differ significantly among diets. Because the grasses were lower in calcium content, calcium availability (mg obtained/g dry food) was significantly lower for grasses than forbs, and did not differ between grasses or between forbs. Phosphorus digestibility was relatively high for the forbs, but was negative for both grasses. Tortoises lost phosphorus while feeding on the grasses, but gained phosphorus while eating forbs. Calcium and phosphorus availability are better explained by the type of food (forb vs. grass) than by its geographic origin (native vs. exotic); however, if exotic grasses are replacing native forbs rather than native grasses, the nutritional quality of the tortoises' diet could be decreasing.