We continue to lose species and genetic diversity locally, nationally, and planet wide. In deciding priorities for conservation, there is an urgent need for criteria that help us to recognize losses with potentially serious consequences. It would be naive to assume that species-poor ecosystems are always malfunctional; some of the world's most extensive and ancient ecosystems - boreal forests, bogs, and heathlands - contain few species. For both species-rich and species-poor ecosystems, we need to establish whether current losses in biodiversity are likely to seriously impair functioning and reduce benefits to humans.

Model communities with controlled levels of species diversity have been created in the Ecotron at Silwood park in southern England and at the Cedar Creek Reserve in Minnesota to assess the effects of diversity on various ecosystem properties such as primary productivity, nitrogen mineralization, and litter decomposition. Early publications from both sites claimed to demonstrate benefits to ecosystem function arising from higher levels of biodiversity, and these have been highlighted by commentators excited by the prospect of a scientific underpinning for conservation measures.

This view that "biodiversity begets superior ecosystem function" is not shared by all ecologists. There are obvious conflicts with published evidence from work on natural rather than synthesized ecosystems.

Science includes three contributions to this important debate. One is a report of results from the Cedar Creek synthesized plant assemblages, whereas the two others describe biodiversity-ecosystem studies conducted on natural systems (mediterranean grassland in California and northern forest in Sweden). In all three, variation in ecosystem properties is found to be related to differences in The functional characteristics, especially resource capture and utilization, of the dominant plants, and there is no convincing evidence that ecosystem processes are crucially dependent to higher levels of biodiversity.

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