This study addresses the effects of different-sized mammalian herbivores on plant diversity at six different North American grassland sites. These sites represent two sets of sites that differ in their inherent site productivity. One hypothesis is that differences in productivity will lead to very different effects of herbivores on plant diversity. At low productivity sites, most plant species are of sufficiently high quality to be consumed by all herbivores, but are defended such that larger herbivores eat inferior plants, while smaller herbivores feed on seeds of dominant plants. At high productivity sites, competitively dominant species may be consumed only by larger herbivores that can accommodate their low N content. These trade-offs suggest that larger herbivores will have greater effects on plant biomass at high productivity sites, while smaller herbivores will have greater effects on biomass at low productivity sites. In addition, larger herbivores may increase diversity at high productivity sites and reduce it at low productivity sites, while smaller, more selectively foraging herbivores may have the opposite effects. The goal for this proposal is to begin monitoring for the first 3 years of a long-term (next 10 - 20 years) program as it is likely that changes in plant species composition and diversity will require that long to respond fully to treatments. The collaborative network of LTER and other sites committed to support this project should greatly facilitate such monitoring. The results should provide a general understanding of the role of herbivores in controlling grassland plant diversity, and should aid efforts to restore, preserve, and manage grassland ecosystems.
