The ability to predict ecosystem responses to change is dependent on understanding their normal functions. Biodiversity influences ecosystem functions (such as net primary production and nutrient cycling), but the challenge is to determine what levels of diversity are important in explaining ecosystem processes. Traditionally species effects have been evaluated at the trophic level. However, interactions within and among trophic levels challenges this assumption of equality among species. If all species are unique and complement one another, increasing the number of species (i.e. species richness) will increase ecosystem functions. If species that possess similar traits can be organized into functional groups that have similar effects on the ecosystem functions, functional diversity is more important. Understanding which level of biodiversity can adequately explain ecosystem functioning will increase our ability to predict ecosystem responses to change.

Multi-trophic systems have been found to be either top-down controlled by predators or bottom-up controlled by resources. The top-down control by predators varies with increased diversity, but is more effectively explained by functional diversity. Total herbivore diversity may dampen top-down control, but the effect of herbivore functional diversity is largely unknown. It is possible that one determinant of herbivore functional mode-foraging mode, such as sap-feeding vs. leaf-chewing-could influence the indirect effects of predators ecosystem functions like on plant productivity. I will examine the hypothesis that herbivore feeding mode diversity and total herbivore species diversity affect the indirect effects of the predators on plant primary productivity. I will experimental manipulate insect herbivore species to examine this hypothesis.