West Mojave Plan FEIR/S - Desert Managers Group

several seasons (Esque 1994 in Avery 1998). Possible reasons for desert tortoises to alter theirpreferences may include changes in nutrient concentrations in plant species, the availability ofplants, and the nutrient requirements of individual animals (Avery 1998). In Avery’s (1998)study in the Ivanpah Valley, desert tortoises consumed primarily green annual plants in spring;cacti and herbaceous perennials were eaten once the winter annuals began to disappear. Medicaet al. (1982 in Avery 1998) found that desert tortoises ate increased amounts of green perennialgrass when winter annuals were sparse or unavailable; Avery (1998) found that desert tortoisesrarely ate perennial grasses.Recent work by Dr. Olav Oftedahl, of the Smithsonian Institution, has shown thattortoises may selectively forage on plants that have a high Potassium Excretion Potential (PEPIndex) (Oftedahl 1996). Tortoises do not have salt glands or other physiological means ofgetting rid of high levels of potassium, although they may rid their systems of potentially lethallevels of potassium by voiding their bladders. It is speculated that water loss during droughtconditions could be lethal to tortoises voiding their bladders, unless additional rainfall becomesavailable shortly thereafter. Oftedahl’s studies have shown that tortoises regulate potassiumlevels by selecting plants that are high in water content and protein (nitrogen), which he refers toas “High PEP plants.” These plants are generally restricted to native, annual forbs such as desertdandelion (Malacothrix glabrata) and many legumes such as species in the Astragalus and Lotusgenera. As such, most of the High PEP plants are restricted in their availability to the springfollowing a winter of sufficient rainfall. In some years, no such plants are available, and mayresult in imbalances in potassium and other elements, which in turn could result in waterimbalance and other physiological stresses to tortoises.Oftedahl (pers. comm., Nov. 2002) expressed his concern that pervasive land uses, suchas cattle grazing, may have severely reduced or eliminated the seed bank and germinationpotential for High PEP annual plants. He suggested that removing cattle from grazing allotmentsmight not be sufficient to support new growth of these essential plants if they have already beeneliminated or replaced by non-native forb and grass species. There may be the need toreintroduce some of these species back into heavily impacted tortoise conservation areas.Desert tortoises can produce from one to three clutches of eggs per year. On rareoccasions, clutches can contain up to 15 eggs; most clutches contain 3 to 7 eggs. Multi-decadestudies of the Blanding’s turtle (Emydoidea blandingii), which, like the desert tortoise, is longlived and matures late, indicate that approximately 70 percent of the young animals must surviveeach year until they reach adult size; after this time, annual survivorship exceeds 90 percent(Congdon et al. 1993). Research has indicated that 50 to 60 percent of young desert tortoisestypically survive from year to year, even in the first and most vulnerable year of life. We do nothave sufficient information on the demography of the desert tortoise to determine whether thisrate is sufficient to maintain viable populations; however, it does indicate that maintainingfavorable habitat conditions for small desert tortoises is crucial for the continued viability of thespecies.Desert tortoises typically hatch from late August through early October. At the time ofhatching, the desert tortoise has a substantial yolk sac; the yolk can sustain them through the falland winter months until forage is available in the late winter or early spring. However, neonatesChapter 3 3-75