Four years in the making, the Millennium Ecosystem Assessment (see Nature 417, 112-113; 2002) is released this week (starting 30 March). This gigantic endeavour explores the link between human well-being, the status of ecosystems and their sustainable use. What has this assessment taught us about developing our planet, and will it, or should it, be continued? To answer the first part of this question, the assessment is an invaluable record of where we stand now, and why. But for it to be useful, the answer to the second part of the question must be 'yes'. We need to take a consistent approach to measuring the status and trends of the world's ecosystems. To take one example, the Convention on Biological Diversity has set the target of reducing the rate of global loss of biodiversity by 2010. But the data to evaluate whether this goal is being met are not readily available, as biological diversity is more than just an enumeration of species present or absent — itincludes parameters such as the populations of species and the ecosystems in which they reside.
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