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The impact of biodiversity loss on the functioning of ecosystems and their ability to provide ecological services has become a central issue in ecology. Several experiments have provided evidence that reduced species diversity may impair ecosystem processes such as plant biomass production. The interpretation of these experiments, however, has been controversial because two types of mechanism may operate in combination. In the 'selection effect', dominance by species with particular traits affects ecosystem processes. In the 'complementarity effect', resource partitioning or positive interactions lead to increased total resource use. Here we present a new approach to separate the two effects on the basis of an additive partitioning analogous to the Price equation in evolutionary genetics. Applying this method to data from the pan-European BIODEPTH experiment reveals that the selection effect is zero on average and varies from negative to positive in different localities, depending on whether species with lower- or higher-than-average biomass dominate communities. In contrast, the complementarity effect is positive overall, supporting the hypothesis that plant diversity influences primary production in European grasslands through niche differentiation or facilitation.

Studies of experimental grassland communities1,2,3,4,5,6,7 have demonstrated that plant diversity can stabilize productivity through species asynchrony, in which decreases in the biomass of some species are compensated for by increases in others1,2. However, it remains unknown whether these findings are relevant to natural ecosystems, especially those for which species diversity is threatened by anthropogenic global change8,9,10,11. Here we analyse diversity–stability relationships from 41 grasslands on five continents and examine how these relationships are affected by chronic fertilization, one of the strongest drivers of species loss globally8. Unmanipulated communities with more species had greater species asynchrony, resulting in more stable biomass production, generalizing a result from biodiversity experiments to real-world grasslands. However, fertilization weakened the positive effect of diversity on stability. Contrary to expectations, this was not due to species loss after eutrophication but rather to an increase in the temporal variation of productivity in combination with a decrease in species asynchrony in diverse communities. Our results demonstrate separate and synergistic effects of diversity and eutrophication on stability, emphasizing the need to understand how drivers of global change interactively affect the reliable provisioning of ecosystem services in real-world systems. Fil: Hautier, Yann. University of Minnesota; Estados Unidos. Universitat Zurich; Suiza Fil: Seabloom, Eric. University of Minnesota; Estados Unidos Fil: Borer, Elizabeth T. University of Minnesota; Estados Unidos Fil: Adler, Peter. State University of Utah; Estados Unidos Fil: Harpole, E. Stanley. University of Iowa; Estados Unidos Fil: Hillebrand, Helmut. University Oldenburg; Alemania Fil: Lind, Eric M.. University of Minnesota; Estados Unidos Fil: McDougall, Andrew S.. University of Guelph; Canadá Fil: Stevens, Carly J.. Lancaster University; Reino Unido Fil: Bakker, Jonathan D. University of Washington; Estados Unidos Fil: Buckley, Yvonne M.. University of Queensland; Australia. Trinity College Dublin; Irlanda Fil: Chu, Chengjin. Lanzhou University; China Fil: Collins, Scott L.. University Of New Mexico; Estados Unidos Fil: Daleo, Pedro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencia Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; Argentina Fil: Damschen, Ellen I.. University of Wisconsin; Estados Unidos Fil: Davies, Kendi F.. State University of Colorado Boulder; Estados Unidos Fil: Fay, Philip A.. United States Department of Agriculture Agricultural Research Service; Estados Unidos Fil: Frin, Jennifer. Queensland University of Technology; Australia Fil: Gruner, Daniel S.. University of Maryland; Estados Unidos Fil: Jin, Virginia L.. United States Department of Agriculture Agricultural Research Service; Estados Unidos Fil: Klein, Julia A.. State University of Colorado - Fort Collins; Estados Unidos Fil: Knops, Johannes M. H.. University Of Nebraska; Estados Unidos Fil: La Pierre, Kimberly J.. University of California at Berkeley; Estados Unidos Fil: Li, Wei. Southwest Forestry University; China Fil: McCulley, Rebecca L.. University Of Kentucky; Estados Unidos Fil: Melbourne, Brett A.. State University of Colorado - Fort Collins; Estados Unidos Fil: Moore, Joslin L.. University of Melbourne; Australia. Monash University; Australia Fil: O'Halloran, Lydia R.. Oregon State University; Estados Unidos Fil: Prober, Suzanne M.. CSIRO Ecosystem Sciences; Australia Fil: Risch, Anita C.. Swiss Federal Institute for Forest; Suiza Fil: Sankaran, Mahesh. University of Leeds; Reino Unido. National Centre for Biological Sciences; India Fil: Schuetz, Martin. Swiss Federal Institute for Forest; Suiza Fil: Hector, Andy. University of Oxford; Reino Unido

Biodiversity loss can affect ecosystem functions and services. Individual ecosystem functions generally show a positive asymptotic relationship with increasing biodiversity, suggesting that some species are redundant. However, ecosystems are managed and conserved for multiple functions, which may require greater biodiversity. Here we present an analysis of published data from grassland biodiversity experiments, and show that ecosystem multifunctionality does require greater numbers of species. We analysed each ecosystem function alone to identify species with desirable effects. We then calculated the number of species with positive effects for all possible combinations of functions. Our results show appreciable differences in the sets of species influencing different ecosystem functions, with average proportional overlap of about 0.2 to 0.5. Consequently, as more ecosystem processes were included in our analysis, more species were found to affect overall functioning. Specifically, for all of the analysed experiments, there was a positive saturating relationship between the number of ecosystem processes considered and the number of species influencing overall functioning. We conclude that because different species often influence different functions, studies focusing on individual processes in isolation will underestimate levels of biodiversity required to maintain multifunctional ecosystems.

A consequence of Darwin's 'principle of divergence' is that loss of species can harm the functioning of ecosystems. A study of algal communities in artificial streams suggests that he was right. See Letter p.86 Studies in recent years have suggested that the conservation of biodiversity improves the ability of an ecosystem to retain nutrients and remain productive. These papers have proved controversial, in part because of a lack of direct evidence for a mechanism to explain the phenomenon. Now, in experiments involving manipulation of the number of algal species in model stream systems, Bradley Cardinale provides one such mechanism. Uptake of nitrogen nutrients increased linearly with species richness in response to changes in flow habitats and disturbance regimes. But when niche structure was experimentally removed, the relationship disappeared. This suggests that habitats with more species take greater advantage of the niche opportunities in an environment than do less-species-rich habitats, allowing the more diverse systems to capture a greater fraction of biologically active resources such as nitrogen.

The term 'biodiversity' is a simple contraction of 'biological diversity', and at first sight the concept is simple too: biodiversity is the sum total of all biotic variation from the level of genes to ecosystems. The challenge comes in measuring such a broad concept in ways that are useful. We show that, although biodiversity can never be fully captured by a single number, study of particular facets has led to rapid, exciting and sometimes alarming discoveries. Phylogenetic and temporal analyses are shedding light on the ecological and evolutionary processes that have shaped current biodiversity. There is no doubt that humans are now destroying this diversity at an alarming rate. A vital question now being tackled is how badly this loss affects ecosystem functioning. Although current research efforts are impressive, they are tiny in comparison to the amount of unknown diversity and the urgency and importance of the task.

Nature, 412, 72–76 (2001). In Figures 2 and 3, some regression lines were very faint or missing. The corrected panels are reproduced below. All panels in both figures should also have shown the null hypothesis as a horizontal dashed line through zero.