EFFECTS OF KANGAROO RAT EXCLUSION ON VEGETATION STRUCTURE AND PLANT-SPECIES DIVERSITY IN THE CHIHUAHUAN DESERT

Long-term (1977-90) experimental exclusion of three species of kangaro o rats from study plots in the Chihuahuan Desert resulted in significa nt increases in abundance of a tall annual grass (Aristida adscensioni s) and a perennial bunch grass (Eragrostis lehmanniana). This change i n the vegetative cover affected use of these plots by several other ro dent species and by foraging birds. The mechanism producing this chang e probably involves a combination of decreased soil disturbance and re duced predation on large-sized seeds when kangaroo rats are absent. Sp ecies diversity of summer annual dicots was greater on plots where kan garoo rats were present, as predicted by keystone predator models. How ever, it is not clear whether this was caused directly by activities o f the kangaroo rats or indirectly as a consequence of the increase in grass cover. No experimental effect on species diversity of winter ann ual dicots was detected. Our study site was located in a natural trans ition between desert scrub and grassland, where abiotic conditions and the effects of organisms may be particularly influential in determini ng the structure and composition of vegetation Under these conditions kangaroo rats have a dramatic effect on plant cover and species compos ition.