Several influential experiments have shown that high plant diversity enhances ecosystem productivity, animal diversity, and invasion resistance. Yet theory predicts that plant and herbivore diversity, which often co vary in nature, should have countervailing effects on ecosystem properties. In the July issue of Ecology Letters, Duffy, Canuel, and Richardson provide experimental support for this prediction. In a seagrass system, increasing grazer diversity reduced both algal biomass and total community diversity, and facilitated dominance of a grazing-resistant invertebrate.
In parallel with previous plant results, however, grazer diversity enhanced grazer biomass production, an important determinant of fish yield in aquatic ecosystems. Moreover, ecosystem responses at high grazer diversity often differed considerably from those predicted by summing impacts of individual species. Thus, complex interactions, often opposing plant diversity effects, arose as emergent consequences of changing consumer diversity, advising caution in extrapolating conclusions from plant diversity experiments to more complete food webs.
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