The Effects of Productivity, Herbivory, and Plant Species Turnover in Grassland Food Webs

In order to evaluate the patterns and processes of food web structure in grasslands, we compiled data from published studies on the relationship between precipitation (which is a predictor of primary productivity in grasslands), plant and herbivore standing crop biomass, and the results of large-herbivore exclosure experiments on plant abundance and composition. Three informative patterns emerged: (1) both producer and herbivore biomass increase across a natural precipitation gradient; (2) the relative effect of large herbivores on plant biomass, inferred from exclosure studies, decreases with increasing precipitation; and (3) the effect of herbivores on changes in plant species composition increases with precipitation. Simple resource-controlled and consumer-controlled food chain models can explain different subsets of these patterns. However, models of heterogeneous food webs that incorporate differences among species within trophic levels, and compositional turnover within such trophic levels, are consistent with all of the reviewed patterns. We suggest that this compositional turnover of plant species, combined with the interactive controlling effects of consumers and resources, may help to explain why studies performed in different areas, and with different methodologies, often draw different conclusions about the patterns and structuring processes in grassland ecosystems.