1. Department of Ecology and Natural Resource Management; Norwegian University of Life Sciences; P.O. Box 5003 1432 Ås Norway
2. Norwegian Institute for Nature Research; P.O. Box 5685 Sluppen 7485 Trondheim Norway
3. CEFE UMR 5175, CNRS, Université de Montpellier; Université Paul-Valéry Montpellier; EPHE; 1919 Route de Mende 34293 Montpellier Cedex 5 France