Natural ecosystems and biodiversity must be made a bulwark against climate change, not a casualty of it, argue Will R. Turner, Michael Oppenheimer and David S. Wilcove. This week's Nature is the third of three special issues (previously 20 November 2008 and 12 February 2009) marking the Darwin bicentenary. The theme is biodiversity and how to preserve it. Though still endangered, the golden lion tamarin is a conservation success story in an area where successes are rare: with re-introductions and efforts to preserve its Brazilian forest habitat, the population of the primate has been stabilized. The ecosystem services approach exemplified by the 1997 Nature review by Robert Costanza and colleagues, which puts a value on everything including biodiversity, is seen by many as a key to encouraging conservation: Gretchen Daily, a leading proponent, is optimistic. In an Opinion piece, Pavan Sukhdev puts the case for governmental investment in environmental services, with biodiversity properly costed as part of the broader picture. Taxonomists use part of the mitochondrial cytochrome c oxidase gene as a 'DNA bar code': more controversially, Dan Mishmar and colleagues propose that the bar code is not just a useful handle on the relatedness of species but a driver of diversity. With the Copenhagen conference looming, Will Turner, Michael Oppenheimer and David Wilcove explain how natural ecosystems and biodiversity can mitigate climate change. Robert Smith and colleagues stress the importance of local agencies — as opposed to NGOs and incomers — in setting biodiversity goals. The best biodiversity data set we have is provided by palaeontologists: Douglas Erwin thinks they could do more with it though, by adopting the techniques used by physicists. For a listing of all Darwin-related content from this issue, see the Editorial. All of this content, plus web-only material, is available at http://www.nature.com/darwin .