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

Low Rainfall Affects the Nutritive Quality as well as the Total Quantity of Food Available to the Desert Tortoise

Olav T. Oftedal
Department of Conservation Biology, Conservation and Research Center, National Zoological Park, Smithsonian Institution, Washington DC 20008

Each year, desert plants germinate and grow in response to the onset and intensity of rainfall. The numbers of species that germinate, the numbers of individual plants that survive, and the average mass of individual plants vary greatly from year to year, leading to tremendous annual variation in biomass production and in species diversity. This well-known variation in quantity of potential tortoise food is also coupled to variation in plant nutritive quality.

Desert tortoises are influenced by the relative amounts of water, nitrogen (protein) and potassium in their foods, and tend to choose foods with favorable ratios among these constituents, as indicated by the Potassium Excretion Potential (PEP) index. The water, nitrogen and potassium concentrations of plants may change substantially over the course of early growth, flowering, seed set, and senescence, but the direction and timing of these phenological changes vary from species to species. Thus tortoises are confronted not only with an array of species of differing PEP indices, but the overall pattern is continually changing as different species go through phenological changes at different times and rates. In at least some situations, tortoises alter food choice to take advantage of high PEP plants when they are available.

Annual variation in rates of precipitation and evapotranspiration further complicate the picture. If rains cease early and are succeeded by high temperatures, desert plants may be forced to adapt to dropping soil moisture and hence low soil water potential. This includes a reduction in tissue water and changes in relative proportions of nitrogen and potassium, so that the PEP index may be greatly reduced. Analytical data from both the western Mojave (Desert Tortoise Natural Area, Kern Co., CA) and the eastern Mojave (Piute Valley, Mormon Mesa, Clark Co., NV) demonstrate that most plant species with moderate and high PEP values in a wet year (1995, 1998) have low or negative PEP values in a dry year (1994, 1997). Thus the nutritional challenge facing desert tortoises in a dry year is exacerbated by the physiological responses of food plants to drought conditions.