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 gr asslands, we compiled data from published studies on the relationship betwe en precipitation (which is a predictor of primary productivity in grassland s), plant and herbivore standing crop biomass, and the results of large-her bivore exclosure experiments on plant abundance and composition. Three info rmative patterns emerged: (1) both producer and herbivore biomass increase across a natural precipitation gradient; (2) the relative effect of large h erbivores on plant biomass, inferred from exclosure studies, decreases with increasing precipitation; and (3) the effect of herbivores on changes in p lant species composition increases with precipitation. Simple resource-cont rolled and consumer-controlled food chain models can explain different subs ets of these patterns. However, models of heterogeneous food webs that inco rporate differences among species within trophic levels, and compositional turnover within such trophic levels, are consistent with all of the reviewe d patterns. We suggest that this compositional turnover of plant species, c ombined with the interactive controlling effects of consumers and resources , may help to explain why studies performed in different areas, and with di fferent methodologies, often draw different conclusions about the patterns and structuring processes in grassland ecosystems.