28th Annual Meeting and Symposium of the
Desert Tortoise Council, February 21-23, 2003
Abstracts

Are Desert Tortoises Nutritionally Constrained By A Shortage Of High PEP Plants, And If So, What Do We Do?

Olav T. Oftedal
Dept. of Conservation Biology, Conservation and Research Center, Smithsonian National Zoological Park, 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?

• Plants that are high in PEP typically contain high levels of tissue water, high concentrations of leaf protein, and modest levels of potassium, all of which typify plants of high photosynthetic rate that are intolerant of low soil moisture (Oftedal 2002). Thus high PEP plants appear to be restricted in time and space to very moist conditions, and thus achieve abundance in numbers and biomass only in years of high winter rainfall and relatively cool springs, such as the 1991/92 and 1997/98 El Nino Southern Oscillations.
• In 1992 tortoises at the Desert Tortoise Research Natural Area (DTNA) were found to target high PEP plants (such as Lotus humistratus and Astragalus didymocarpus) that were both uncommon and highly patchy in distribution (Jennings 1993). Yet this was after the major tortoise population crash at DTNA, suggesting that a larger tortoise population would have been hard pressed to obtain a high PEP diet.
• Tortoises in Ivanpah Valley that foraged within exclosures that kept out range cattle selected high PEP plants (such as Malacothrix glabrata) that were largely unavailable in adjacent areas where cattle grazed, causing a significant shift in tortoise diet (Avery 1998). The calculated effect was that dietary PEP was reduced to one half outside the exclosures, compared to within.
• At Fort Irwin, although a high PEP evening primrose (Camissonia claviformis) was moderately abundant in both 1998 and 2001, juvenile tortoises could only rely on them in early spring when conditions remained moist (Oftedal et al. in press). As the season progressed these plants withered (and were damaged by tortoise and caterpillar foraging), leading tortoises to forage on lower PEP plants, with a reduction in overall dietary PEP.
• In years of lower rainfall, fewer plant species germinated and grew on the Beaver Dam Slope (AZ) and at City Creek (UT), leading tortoises to forage predominantly on lower PEP plants such as the invasive exotic annuals Bromus rubens, Schismus barbatus and Erodium cicutarium (Esque 1994). These plant species adapt to low soil moisture by reducing tissue water and/or nitrogen, and increasing tissue potassium, such that their PEP values are particularly low under such conditions.

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: 

• plant removal by grazing livestock (see point 3 above).
• changes in soil moisture associated with disturbance of the soil surface (by livestock, ORV, construction activities, tank maneuvers, etc.).
• competition for soil moisture and nutrients by ubiquitous invading species such as Bromus rubens, Schismus barbatus and Erodium cicutarium.
• changes in the amount and distribution of rainfall due to climate change associated with global warming.

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.

References

Avery, H. W. 1998. Nutritional ecology of the desert tortoise (Gopherus agassizii) in relation to cattle grazing in the Mojave desert. Ph.D. diss., Univ. Cal. Los Angeles.

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.

Jennings, W. B., 1993. Foraging ecology of the desert tortoise (Gopherus agassizii) in the western Mojave desert. M.S. thesis, Univ. of Texas, Arlington.

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):