1. Department of Biology, University of York, PO Box 373, York YO10 5YW, UK
2. York Centre for Complex System Analysis, University of York, PO Box 373, York YO10 5YW, UK
3. Vegetationsökologie und Naturschutz, Universität Potsdam, 14469 Potsdam, Germany
4. Evolution and Ecology Program, International Institute for Applied Systems Analysis (IIASA), 2361 Laxenburg, Austria