1. Centre National de la Recherche Scientifique Ecologie des Hydrosystèmes Fluviaux, Université Lyon 1, 69622 Villeurbanne Cedex, France;
2. School of Biological Sciences, Queen Mary and Westfield College, University of London, London E1 4NS, United Kingdom;
3. Department of Environmental Science, Policy and Management, University of California, Berkeley, California 94720;