NUTRITIONAL QUALITY OF NATIVE AND INTRODUCED FOOD PLANTS OF WILD DESERT TORTOISES

Digestibilities of dry matter, energy, water, and nitrogen were determ ined for four foods of desert tortoises (Gopherus agassizii) that were kept individually in outdoor pens where food intake and feces output could be measured quantitatively. Two native plants, the forb Malacoth rix glabrata and the grass Achnatherum (Oryzopsis) hymenoides, and two exotic plants, the forb Erodium cicutarium and the grass Schismus bar batus, were collected in the field during the seasons that wild tortoi ses consumed them (spring for the forbs, summer for the then-dead and dry grasses), and were then offered to the penned tortoises. The diges tibilities of the nutrients in the two forbs were similar, ranging fro m 63-70% for dry matter, 69-73% for energy, 72-79% for nitrogen, and 7 0-75% for water. Which forb was the more nutritious depends on what de termines feeding rate (appetite) in tortoises. If tortoises eat to obt ain a given volume of food (''full stomach'') daily, then the exotic f orb provides significantly more digestible energy and nitrogen, but if tortoises eat to obtain a given amount of digestible energy (''meet e nergy needs'') daily, then the native forb provides significantly more nitrogen and water. The dry matter and energy digestibilities of the two grasses were much lower than the forbs, but were similar to each o ther, ranging from 46-50%. However, both grasses provided little or no nitrogen, and the tortoises lost more water than they gained while pr ocessing grasses. The type of food plant (forb or grass) and its pheno logical stage, rather than its geographic origin (native or exotic), b est predicted its nutritional value.