The effect of herbivore diversity on plants: A test at multiple grassland sites

Background/Question/Methods

Well known patterns exist between diversity and ecosystem function. These patterns, however, have most often been demonstrated with primary producers or consumers in freshwater or marine systems. How consumer diversity affects terrestrial ecosystem functioning remains ambiguous at best.  We used an enclosure experiment to test how grasshopper species richness and functional composition affects plant biomass and composition (grass, forb, shrub) in three grasslands along a latitudinal gradient (Texas, Kansas, Montana). We expected that grasshopper presence would have the largest effect on plant biomass at the highest latitude, lowest production site (Montana) because of the high consumer to plant ratio. We expected that functional composition would have the largest effects on plant composition at the lowest latitude site (Texas); plants at lower latitudes are often lower quality, leading to greater diet specialization to meet nutritional needs. We held grasshopper density constant at all sites (5 g/m², adult dry weight), while varying grasshopper species richness (0, 1, 2, or 4 species) and functional composition of two feeding guilds (grass feeders and mixed feeders). We also included a “natural community” treatment consisting of the 10 most abundant species at each site. After grasshoppers matured, we harvested plants and sorted them to functional group.

Results/Conclusions

Grasshopper presence significantly reduced aboveground biomass at all sites, but the relative reduction in biomass was highest in Montana, the highest-latitude and least productive site. Grasshopper functional composition strongly affected plant biomass: grass feeders reduced plant biomass significantly more than mixed feeders. The effects of functional composition varied among sites: functional composition had a greater effect at low latitude sites compared to high latitudes (Texas versus Montana). In contrast to the strong effects of grasshopper functional richness, grasshopper species richness did not affect plant aboveground biomass. These results suggest: 1) that grasshopper functional composition has greater effects than grasshopper species richness on plant biomass, and 2) that the effects of grasshopper functional richness vary across latitude. Because the effects of biodiversity may vary across latitude because of changes in available resources or other abiotic factors, we suggest that future studies of how diversity affects ecosystem functioning be done along latitudinal gradients.