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The relationships between biodiversity and stability were determined for both population and ecosystem traits in a long—term study of 207 grassland plots. Results demonstrate that biodiversity stabilizes community and ecosystem processes, but not population processes. Specifically, year—to—year variability in total aboveground plant community biomass was significantly lower in plots with greater plant species richness both for the entire 11—yr period and for the nine non—drought years. The change in total plant community biomass from before the drought to the peak of the drought was also highly dependent on species richness. For all three measures of total community biomass stability, multiple regressions that controlled for covariates showed similar significant relationships between plant diversity and stability. In contrast, year—to—year variability in species abundances was not stabilized by plant species richness for either all years or non—drought years. This difference between species vs. community biomass likely results from interspecific competition. When climatic variations harm some species, unharmed competitors increase. Such compensatory increases stabilize total community biomass, but cause species abundances to be more variable. These results support both the predictions of Robert May concerning the effects of diversity on population stability and the diversity—stability hypothesis as applied to community and ecosystem processes, thus helping to reconcile a long—standing dispute.