1. Laboratoire d'Ecologie, CNRSURA 258, Université Pierre etMarie Curie, Bâtiment A, 7è meé tage, 7 quai St Bernard, Case 237, F–75252 Paris Cedex 05, France
2. Department of Biological and Environmental Science, University of Jyväskylä, PO Box 35, FIN– 40351 Jyväskylä, Finland