Biodiversity is rapidly declining worldwide1, and there is consensus that this can decrease ecosystem functioning and services 2-7. It remains unclear, though, whether few8 or many 9 of the species in an ecosystem are needed to sustain the provisioning of ecosystem services. It has been hypothesized that most species would promote ecosystem services if many times, places, functions and environmental changes were considered; however, no previous study has considered all of these factors together. Here we show that 84% of the 147 grassland plant species studied in 17 biodiversity experiments promoted ecosystem functioning at least once. Different species promoted ecosystem functioning during different years, at different places, for different functions and under different environmental change scenarios. Furthermore, the species needed to provide one function during multiple years were not the same as those needed to provide multiple functions within one year. Our results indicate that even more species will be needed to maintain ecosystem functioning and services than previously suggested by studies that have either (1) considered only the number of species needed to promote one function under one set of environmental conditions, or (2) separately considered the importance of biodiversity for providing ecosystem functioning across multiple years10-14, places15,16, functions14,17,18 or environmental change scenarios 12,19-22. Therefore, although species may appear functionally redundant when one function is considered under one set of environmental conditions, many species are needed to maintain multiple functions at multiple times and places in a changing world.
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Acknowledgements We thank J. Byrnes, L. Gamfeldt and M. Emmerson for comments on an earlier version of this manuscript. We thank the Swiss SystemsX.ch initiative (IPP-2008/23) for supporting this project. The BIODEPTH project was funded by the European Commission within the Framework IV Environment and Climate Programme (ENV-CT95-0008) and by the Swiss Federal Office for Education and Science (Project EU-1311).TheJenaExperimentwas fundedbythe DeutscheForschungsgemeinschaft (DFG, FOR 456), Friedrich Schiller University of Jena, Max Planck Society, University of Zurich, Swiss National Science Foundation (3100AO-107531) and ETH Zurich. The Wageningen experiment was funded by the Dutch Organisation for Scientific Research (NWO) within the framework of the Biodiversity Programme. Work on the Agrodiversity experiment was funded by the EU Commission through COST Action 852 and Science Foundation Ireland (09/RFP/EOB2546). The BioCON experiment was funded by the US Department of Energy (DOE/DE-FG02-96ER62291) and the US National Science Foundation (Biocomplexity 0322057, LTER DEB 9411972, DEB 0080382, DEB 0620652 and LTREB DEB 0716587). The MEND Irrigation, BioGEN and Rarity– Extinction experiments were funded by the US National Science Foundation (DEB 0639417). The Cedar Creek Biodiversity experiment was funded by the US National Science Foundation. M.L. was supported by The Natural Sciences and Engineering Research Council of Canada (Discovery Grant) and the Canada Research Chair program.