Olav T. Oftedal
Dept. of Conservation Biology, Conservation and Research Center, Smithsonian National
Washington DC 20008
In the absence of late spring or summer rains that permit drinking, desert tortoises must obtain sufficient water and nitrogen in food plants to excrete ingested potassium. They do this by selecting food plants having positive PEP (Potassium Excretion Potential) values. The amount of protein nitrogen available to support growth, reproduction and disease resistance will depend on the PEP value of the overall diet. I suggest that access to and ingestion of high PEP plants may be essential to short- and long-term nitrogen balance, overall health in the face of infectious disease, and female reproductive output. If so, a shortage of high PEP plants may underlie the precipitous population crashes evident in recent decades in much of the Mojave Desert.
Is there botanical evidence in support of this plant shortage hypothesis?
Desert tortoises apparently derive from southern arid regions in which summer rainfall is important. I speculate that they were able to colonize the Mojave and Colorado deserts (winter rainfall deserts) only because they could regularly find a sufficient abundance of high PEP plants. If so, any factor that reduces the temporal or spatial abundance of high PEP plants in these areas could have calamitous effects. In particular the abundance, biomass, reproductive success and size of the seed bank of high PEP plants may have been reduced by:
Unfortunately, it is difficult to evaluate the plant shortage hypothesis due to lack of data on historic trends in the abundance of annual plant species (other than the known massive increases in abundance and biomass of invasive species). I suggest two research strategies that may be fruitful: 1. Studies of stable isotope levels in tortoise carcass and shell materials that might indicate diet shifts by tortoises over time (including changes in the importance of high PEP legumes to tortoise diets), and 2. Manipulation of the abundance of high PEP plants (both via revegetation and removal experiments) to determine the impact on tortoise diets. If the current seed bank of high PEP plants is in fact depleted, habitat protection may be a necessary but not sufficient measure for the restoration of tortoise diets to a high nutritional plane.
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Esque, T. C. 1994. Diet and diet selection of the desert tortoise (Gopherus agassizii) in the northeast Mojave desert. M.S. thesis, Colorado State Univ., Ft. Collins.
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Oftedal, O.T. 2002. Nutritional ecology of the desert tortoise in the Mohave and Sonoran deserts. Pp. 194-241 in Van Devender, T.R. (ed) The Sonoran Desert Tortoise. Natural History, Biology, and Conservation. Univ. of Arizona Press, Tucson.
Oftedal, O.T., Hillard, S., and Morafka, D. J. 2002. Selective spring foraging by juvenile desert tortoises (Gopherus agassizii) in the Mojave Desert: Evidence of an adaptive nutritional strategy. Chelonian Conservation and Biology 4 (2):