Herbivore diversity and plant dynamics: compensatory and additive effects

Ecosystems typically contain a variety of herbivore species but the effects of diverse herbivore assemblages on plant communities and succession are not well understood. Theory predicts that herbivores might have opposing or compensatory effects by selectively consuming different competing plant species. Alternatively, different herbivore species may consume the same plant species, thereby having additive effects on particular plant species. We review several recent field studies that manipulate the separate and combined effects of different mammalian and insect herbivore species within grasslands. We found evidence for both compensatory and additive effects of multiple herbivore species on plant species composition, diversity and spatial heterogeneity. Compensatory effects typically occurred when dominant plants competed for soil nutrients in the absence of herbivory, while additive effects occurred when dominant plants competed for water or light. When compensatory effects occurred, large herbivores typically consumed different plant species than small herbivores. These patterns suggest that the impact of herbivore diversity on plant communities will depend on the resource for which plants compete and the body size range of available herbivores. The resource for which plants compete may depend on soil fertility and the supply ratios of different limiting resources. These conclusions represent hypotheses that require future testing in field studies.