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 n ot well understood. Theory predicts that herbivores might have opposing or compensatory effects by selectively consuming different competing plant spe cies. Alternatively, different herbivore species may consume the same plant species, thereby having additive effects on particular plant species. We r eview several recent field studies that manipulate the separate and combine d effects of different mammalian and insect herbivore species within grassl ands. We found evidence for both compensatory and additive effects of multi ple herbivore species on plant species composition, diversity and spatial h eterogeneity. Compensatory effects typically occurred when dominant plants competed for soil nutrients in the absence of herbivory, while additive eff ects occurred when dominant plants competed for water or light. When compen satory effects occurred, large herbivores typically consumed different plan t species than small herbivores. These patterns suggest that the impact of herbivore diversity on plant communities will depend on the resource for wh ich plants compete and the body size range of available herbivores. The res ource for which plants compete may depend on soil fertility and the supply ratios of different limiting resources. These conclusions represent hypothe ses that require future testing in field studies.