Changes in land use and exotic species invasions are thought to drive declines in biodiversity and ecosystem functioning globally. However, few experiments have been conducted to determine the mechanisms by which these drivers influence both biodiversity and ecosystem functioning. We established a field experiment to identify the mechanisms by which exotic species and high intensity grazing by cattle (henceforth overgrazing) influence grassland plant diversity and three ecosystem functions. We hypothesized that these drivers decreased biodiversity and ecosystem functioning by decreasing the complementarity effect (i.e., niche partitioning or facilitation). Perennial native and exotic species were carefully paired by phylogeny and functional group. For each native species mixture, there was an exotic mixture counterpart that was composed of the species paired with those in the native mixture. Seedlings were grown in a greenhouse and transplanted into field plots during May 2007 in western Iowa, USA. This allowed us to control the initial diversity, density, and biomass across treatments. All species were also planted in monoculture. Plots were allowed to establish for one year before half of the plots were intensely grazed by cattle in June of year two. Thus, we varied richness, origin, grazing, and species composition treatments in a factorial design.

Results/Conclusions

We found that exotic species and overgrazing decreased biodiversity and ecosystem functioning by decreasing the complementarity effect. Exotic species decreased the equity of biomass among species (evenness) in mixtures, and overgrazing decreased the number of species (richness) in native mixtures. Aboveground productivity, light interception, and resistance to weed invasion all increased with richness, and this effect was consistent across grazing and origin treatments. Additionally, we found that exotic species and overgrazing directly decreased ecosystem functions in an additive manner. Interestingly, we found that exotic species and overgrazing decreased the complementarity effect. These results suggest that ecosystem conservation may require consideration of changes in both the abundances of, and the interactions between, species.