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Plant diversity and niche complementarity had progressively stronger effects on ecosystem functioning during a 7-year experiment, with 16-species plots attaining 2.7 times greater biomass than monocultures. Diversity effects were neither transients nor explained solely by a few productive or unviable species. Rather, many higher-diversity plots outperformed the best monoculture. These results help resolve debate over biodiversity and ecosystem functioning, show effects at higher than expected diversity levels, and demonstrate, for these ecosystems, that even the best-chosen monocultures cannot achieve greater productivity or carbon stores than higher-diversity sites.

Ecosystem processes are thought to depend on both the number and identity of the species present in an ecosystem, but mathematical theory predicting this has been lacking. Here we present three simple models of interspecific competitive interactions in communities containing various numbers of randomly chosen species. All three models predict that, on average, productivity increases asymptotically with the original biodiversity of a community. The two models that address plant nutrient competition also predict that ecosystem nutrient retention increases with biodiversity and that the effects of biodiversity on productivity and nutrient retention increase with interspecific differences in resource requirements. All three models show that both species identity and biodiversity simultaneously inf luence ecosystem functioning, but their relative importance varies greatly among the models. This theory reinforces recent experimental results and shows that effects of biodiversity on ecosystem functioning are predicted by well-known ecological processes.Recent studies have shown that several community and ecosystem processes are correlated with species diversity (1-12), but why this should occur remains mathematically largely unexplained. Here we report that three different ecological models predict that ecosystem productivity, standing crop, and resource use depend on species diversity, much as has been experimentally observed (3, 4, 6). Our models provide simple mechanisms that explain how such dependencies can arise and help resolve the controversy over the importance for ecosystem functioning of species identity versus species diversity.The functioning of ecosystems has long been known to depend on the identities of the species the ecosystems contain (13-19), and hypothesized to depend on the number of species. However, recent work, plus early observations by Darwin (20) Because the functioning of an ecosystem may depend both on the identities and the numbers of its species, it is necessary to distinguish between these two dependencies in both experimental and theoretical studies. This requires, first, that the group of all potential species, called the ''species pool,'' be defined. Then, to attribute effects to species diversity, effects must occur in comparisons of the average responses of two or more levels of diversity. At each level of diversity, there must be numerous replicate ecosystems, each with a random and independent combination of species chosen from the species pool. By having many random species combinations drawn from a large species pool, the mean response among replicate ecosystems at a given level of diversity becomes independent of particular species combinations. The differences among mean responses for different levels of diversity then measure the effect of diversity. The variance among the various species combinations at a given diversity level measures the effects of alternative species compositions.We apply this approach to two models of interspecific plant competition for nutrient...

Biofuels derived from low-input high-diversity (LIHD) mixtures of native grassland perennials can provide more usable energy, greater greenhouse gas reductions, and less agrichemical pollution per hectare than can corn grain ethanol or soybean biodiesel. High-diversity grasslands had increasingly higher bioenergy yields that were 238% greater than monoculture yields after a decade. LIHD biofuels are carbon negative because net ecosystem carbon dioxide sequestration (4.4 megagram hectare(-1) year(-1) of carbon dioxide in soil and roots) exceeds fossil carbon dioxide release during biofuel production (0.32 megagram hectare(-1) year(-1)). Moreover, LIHD biofuels can be produced on agriculturally degraded lands and thus need to neither displace food production nor cause loss of biodiversity via habitat destruction.

Three markedly different models of multispecies competition-one mechanistic, one phenomenological, and one statistical-all predict that greater diversity increases the temporal stability of the entire community, decreases the temporal stability of individual populations, and increases community productivity. We define temporal stability as the ratio of mean abundance to its standard deviation. Interestingly, the temporal stability of entire communities is predicted to increase fairly linearly, without clear saturation, as diversity increases. Species composition is predicted to be as important as diversity in affecting community stability and productivity. The greater temporal stability of more diverse communities is caused by higher productivity at higher diversity (the "overyielding" effect), competitive interactions (the "covariance" effect), and statistical averaging (the "portfolio" effect). The relative contribution of each cause of temporal stability changes as diversity increases, but the net effect is that greater diversity stabilizes the community even though it destabilizes individual populations. This theory agrees with recent experiments and provides a degree of resolution to the diversity-stability debate: both sides of the longstanding debate were correct, but one addressed population stability and the other addressed community stability.