Relative effects of species composition and richness on ecosystem properties in ponds

Biological diversity is complex and can be described and quantified in various ways. Research exploring the consequences of biodiversity for ecosystem functioning has generally focused on the effects of four components of biodiversity: species richness and composition, and functional group richness and composition. Most of the research to date has focused on biodiversity effects within single trophic levels. The aim of this study was to compare the relative effects of these four components of biological diversity across pond food webs using aquatic mesocosm experiments in which all four community properties were manipulated independently. Functional groups were defined in terms of trophic groups, and consisted of aquatic macrophytes, periphyton grazers, and invertebrate predators. Species composition and species richness were manipulated within functional groups. Ecosystem response variables included the final biomass of the manipulated taxa (macrophytes, grazers, and predators), ecosystem rates (productivity and decomposition), and trophic structure (phytoplankton and periphyton biomass). Results reveal strong effects of species composition on a significant proportion of ecosystem response variables. In contrast, species richness, functional group richness, and functional group composition altered the biomass of the manipulated taxa, but had no additional effects on other ecosystem variables. Results suggest that the roles of species in ecosystems, when considered in a food web context, are often the result of both direct and indirect effects that are difficult to predict based on the association of a species to a particular functional group. If a goal of biodiversity research is to predict the response of ecosystems to biodiversity loss, effects of composition and species losses across food webs must be fully integrated into biodiversity-ecosystem functioning research.