| ecology : Are species-rich ecosystems the best? ELEANOR LAWRENCE Studies of the savanna grasslands of southern India reported in Nature1 (14 October) show that the most stable, productive and stress-resistant ecosystems are not always those with the greatest number of species, contrary to widely held assumptions. Mahesh Sankaran and Sam J. McNaughton of the University of Syracuse, New York, have an important message for conservation biologists: "we warn against concluding that species-rich ecosystems will necessarily ‘cope’ better than species-poor ones in the face of perturbations." Their study is an important contribution to the continuing debate over the ecological role of ‘biodiversity’ -- the rich variety of life on our planet. Biodiversity is now under severe threat. Forget the extinction of the dinosaurs at the end of the Cretaceous, 65 million years ago: in the next 50 to 100 years we may see more species lost than in any past episode of mass extinction. But while everyone agrees that the accelerating loss of biodiversity is a tragedy, ecologists have found themselves in something of a dilemma when they try to explain why biodiversity is ecologically important. Why is diversity apparently essential to the stability and productivity of some ecosystems, whereas other, low-diversity, ecosystems also seem to work perfectly well? As species are lost, will the ecosystem as a whole become less stable and more prone to damage by environmental changes? Sankaran and McNaughton studied how three different types of natural grassland recover from disturbance. Located within a remote tiger reserve in the Western Ghats Mountains of India, these savanna grasslands have been moulded by environmental factors, which include grazing animals and episodic fires that start in the tinder-dry grass. The grasslands differ in their biodiversity. Two of the grasslands were each massively dominated by a single grass species and had relatively low numbers of different plant species, whereas the third contained both of the grasses and was also of higher biodiversity. They found that the grassland of lowest diversity was not only the most productive, but that its species composition was most stable in the face of disturbance such as experimental burning and cutting. The grassland of highest diversity performed least well. Their conclusion is that the dominant grass in the low-diversity grassland was particularly well-adapted to recover from the type of disturbance they caused. Thus, whether a plant community stands up well to stress and disturbance depends not so much on the total number of species it contains but on the actual identities and properties of the species of which it is composed. This is turn is determined by the ecological history of the site and the type of disturbance it may have suffered in the past. |
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