Twenty-Fourth Annual Meeting and Symposium of the Desert Tortoise Council, March 5-8, 1999
Abstracts

The herbaceous plant species that desert tortoises eat are embedded in a complex web of interactions with other plants and with animals. Animals affect the reproductive success, survival, and population densities of plants in a variety of ways. Pollinators facilitate seed production; seed dispersers enhance the probability seeds will successfully germinate and grow; herbivores reduce plant survival and reproductive success; granivores kill seeds; and some animals physically disturb the soil, creating favorable or unfavorable microsites in the process.

In the Mojave and Sonoran deserts, the interaction between plants and granivorous rodents is especially prominent. These deserts support an abundant and diverse fauna of rodents in the family Heteromyidae, which includes kangaroo rats and pocket mice. Heteromyid rodents are specialized granivores that often harvest 90% or more of preferred seeds before they have a chance to enter the soil seed bank. Heteromyid rodents affect desert plants in three basic ways. First, by eating seeds they reduce the density of seeds in the soil and hence reduce the population density of preferred plant species, which tend to have large seeds. This in turn releases small-seeded plant species from competition, precipitating changes in species composition of the entire plant community. Second, heteromyid rodents consume some green vegetation in addition to seeds, a behavior that could put them into direct competition with desert tortoises as well as reducing reproduction of preferred plants. The tendency of some kangaroo rat species to clip grass tillers may be responsible for observed increases in small-seeded grasses when kangaroo rats were experimentally excluded in the Chihuahuan Desert. Finally, heteromyid rodents may affect plants in positive ways via their seed-caching activities.

All heteromyids avidly harvest seeds well beyond their immediate food requirements and store the excess in caches located either in the burrow (these are called larderhoards), or in shallow pits dug into the soil surface (these are called scatterhoards). Although larderhoarded seeds probably have been buried too deeply to germinate successfully, scatterhoarded seeds have been, in effect, planted in a favorable microsite for germination. There is growing evidence that the primary route to successful establishment of some desert plants is via an unrecovered scatterhoard, that exotic invaders are not able to benefit from being scatterhoarded because they are intolerant of crowding, and that the net impact of heteromyid rodents on desert plants can vary temporally, being positive (mutualistic) when cache recovery rates are low and negative when cache recovery is virtually complete. Too little is understood at present about the interactions between granivorous rodents and desert plants to know whether the net impact of rodents on the food of desert tortoises is positive or negative. This impact is, however, likely to be important enough to warrant consideration as we devise management strategies for desert tortoise habitat.

References

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McMurray, M. M., S. H. Jenkins, and W. S. Longland. 1997. Effects of seed density on germination and establishment of a native and an introduced grass species dispersed by granivorous rodents. American Midland Naturalist 138:322-330

Price, M. V. and S. H. Jenkins. 1986. Rodents as seed consumers and dispersers. Pp.191-235 in D. R. Murray, editor. Seed dispersal. Academic Press, Sydney, Australia.

Vander Wall, S. H. 1994. Seed fate pathways of antelope bitterbrush: dispersal by seed caching yellow pine chipmunks. Ecology 75:1911-1926