Effects of Experimental Fire and Nonnative Grass Invasion on Small Mammals and Insects

摘要

A goal of efforts to restore altered ecosystems is to reestablish natural disturbance processes, such as fire, that govern ecosystem structure and function. In ecosystems where structure has changed in response to human activities, however, ecosystem drivers may function differently and their reestablishment could yield unexpected or undesired consequences. Lehmann lovegrass, Eragrostis lehmanniana, a perennial bunchgrass from Africa, was introduced to grasslands in the southwestern United States in the 1930s and has since increased in distribution and dominance. Reintroducing fire has been proposed as a mechanism by which to restore semi-desert grasslands by reducing the dominance of nonnative plants, despite the altered plant community.To assess the effect of nonnative grass on animals, between 2000 and 2004 we sampled 54 plots across a gradient of invasion by nonnative grass and quantified variation in presence, abundance, and richness of insects and small mammals. For small mammals, we used a framework we developed to estimate abundance when data are sparse. The number of insect orders, families, and morphospecies, as well as overall abundance decreased as nonnative grass increased. Many insect families that decreased in abundance as nonnative grass increased were comprised of herbivorous species, suggesting that increases in nonnative grass may have reduced abundance and quality of plant foods. Abundance of several species of murid rodents increased and several species of heteromyid rodents decreased as nonnative grass increased, indicating clear changes in habitat for these species of small mammals.To assess the interactive effects of prescribed fire on small mammals in these altered ecosystems, we performed a randomized experiment where we applied prescribed fire on 36 of 54 plots. The effects of fire on many small mammal populations and the composition of the small mammal community varied along the invasion gradient, suggesting that fire functions differently inareas dominated by nonnative plants relative to those dominated by native plants. Invasion by this nonnative grass has clearly shifted the composition of faunal communities in semi-desert grasslands and has altered ecosystem processes, therefore reestablishing fire is not likely to be an omnibus solution for restoration.