Long-term (1977-1990) experimental exclusion of three species of kangaroo rats (Dipodomys) from study plots in the Chihuahuan Desert resulted in significant increases in abundance of a tall annual grass, Aristida adscensionis), and a perennial bunchgrass, Eragrostis lehmanniana. This mechanism producing this change probably involves a combination of decreased soil disturbance and reduced predation on large-sized seeds when kangaroo rats are absent. Species diversity of summer annual dicots was greater on plots where kangaroo rats were present, as predicted by the keystone predator models. The study site lies in a natural transition zone between Chihuahuan Desert scrub and grassland habitat, so the composition of local vegetation may be sensitive to small shifts in abiotic and biotic conditions. These results show that at this site the presence or absence of kangaroo rats had dramatic effects on the structure and composition of vegetation, whereas the presence or absence of livestock grazing has much less influence. Heske et al. note that while it is important to emphasize that the kangaroo rat effects were observed only where livestock has also been excluded, these results show that large grazing mammals are not the only animals that can profoundly influence the vegetation in the arid habitats of the southwestern U.S.
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