Wild Desert Tortoises, Gopherus agassizii, are eating different foods now than they were decades ago, because exotic plant species have invaded and flourished in the Mojave Desert over the last century. Reservations about the nutritional quality of exotic vegetation compared to native vegetation led us to conduct feeding experiments with growing, juvenile Desert Tortoises. We determined the digestibility of dry matter, energy, fiber, and nitrogen in four foods: Achnatherum hymenoides (a native grass), Schismus barbatus (an exotic grass), Malacothrix glabrata (a native forb), and Erodium cicutarium (an exotic forb). The largest nutritional differences among diets were between food types (fresh forbs and dry grasses) rather than between native and exotic species. The two grass diets were higher in fiber content and they contained less digestible energy than the two forb diets. The grasses contained little protein, and tortoises actually lost mass and body nitrogen while eating them. The exotic forb yielded more energy and nitrogen per unit dry mass than did the native forb, but this may be related to differences in phenological stages and associated fiber contents of these foods when they were collected. Juvenile tortoises gained weight rapidly when eating forbs and showed no evidence of having a lower digestive capability than did adults, despite their small size and immaturity. Estimates of nitrogen requirements compared to annual nitrogen intake on these diets suggested that growth of juveniles may be limited in part by dietary nitrogen.
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