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

Does Nutritional Ecology Underlie the Greater Susceptibility of Mojave Tortoises than Sonoran Tortoises to Population Crashes?

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

It is apparent that tortoises as a group are particularly susceptible to dietary nutrient imbalance due to their inability to excrete potassium, a ubiquitous plant constituent, either via salt gland secretions or via concentration of urine. In most circumstances, tortoises require periodic access to drinking water, access to high moisture foods, and/or access to protein-rich foods in order to eliminate the potassium loads associated with feeding.

In most if not all of the Mojave Desert, summer rainfall events are rare and localized and thus cannot be depended on as a source of water. In the absence of drinking water, the ability of tortoises to utilize dietary protein for anything other than potassium excretion via urate production will depend on the relative proportions of potassium, water and protein (nitrogen) in the food. This relationship may be approximated by the Potassium Excretion Potential or PEP index. In moderately dry years, when high PEP plants are not available, tortoises may feed but remain in negative nitrogen balance, gradually losing body lean mass and condition, as demonstrated by Nagy and Medica (1986), Peterson (1996) and Henen (1994, 1997).

In good rainfall years, Mojave tortoises may be able to locate patchy or dispersed plants of high PEP, as demonstrated by Jennings (1993) and Oftedal et al. (in press; unpublished data from 2001). Diets of high PEP plants will allow positive nitrogen balance, and provide protein for growth and reproduction. However, Avery's data (1998) indicate that cattle grazing may selectively remove high PEP plants, such as desert dandelion (Oftedal et al., in press). Unfortunately, we have no good data to indicate the extent to which the abundance of high PEP plants has been affected by such factors as livestock grazing, soil compaction, invasive exotic species, air pollution, fire, climate change, or other human-impacted variables. The higher water needs of many high PEP plants may make them particularly susceptible to anything that reduces soil moisture retention. I believe it is likely that many high PEP plants that were essential components of diets that once allowed recovery of tortoise body condition in wet years are now greatly reduced, not only in terms of above-ground biomass, but also in terms of the available seed bank. This may be especially true of species with large seeds (such as legumes) that are not easily dispersed back into areas of former abundance.

If this is true, Mojave tortoises may be fighting a losing battle with environmental nitrogen, being unable to accumulate enough in body protein to consistently offset losses in dry years. A series of dry years, or a change in local flora due to invasive plants, or increased nitrogen losses associated with infection may be enough to initiate a health crisis that has been building over a long time period. Poor nitrogen status may also increase tortoise susceptibility to disease, as it does in other species. Both juvenile growth and egg production require substantial amounts of protein nitrogen. Thus long-term changes in nutritional ecology may ultimately cause population crises in terms of morbidity and mortality even though the proximal causes may vary from place to place.

This hypothesized scenario predicts that access to drinking water would reduce the dependence on high PEP plants, because tortoises that can drink can dispose of potassium without needing to utilize dietary or stored protein. Several studies also suggest that drinking of water frees tortoises to consume large amounts of dry or senescent material that can be used to generate body fat. Preliminary analysis of summer foraging observations at Ragged Top, AZ, in the Sonoran desert are consistent with the idea that tortoises that drink may be less selective in feeding (Oftedal, Averill-Murray and Christopher, unpublished data). Thus it appears that tortoises in the Sonoran desert where summer rainfall is regular would not have the same dependence on high PEP plants as do Mojave tortoises. Given the trend for summer rainfall to decline in the Sonoran desert from southeast to northwest, the only area in which a loss of high PEP plants might be critical is the Colorado subdivision of the Sonoran desert.

This hypothesized scenario is consistent with historical patterns of population crashes north and west of the Colorado River. If true, it suggests that tortoise health may be closely linked to changes in plant communities, and that simple management solutions, such as restrictions on grazing in low-rainfall years, may not address the underlying problem. Should attempts be made to experimentally alter the abundance of high PEP plants, or to look at the relation between disease prevalence and abundance of high PEP plants? These and other potential management and research actions warrant careful consideration.

_______ NB: For further elaboration on the fundamental considerations underlying this hypothesized scenario, and for references cited, see Oftedal review chapter (pp. 194-241) on tortoise nutritional ecology in Van Devender, T. (2002) The Sonoran Desert Tortoise: Natural History, Biology and Conservation. University of Arizona Press.