1. Department of Environmental Studies, University of Nijmegen, P.O. Box 9010, NL-6500 GL Nijmegen, The Netherlands, Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Nieuwe Achtergracht 166, NL-1018 WV, Amsterdam, The Netherlands, and Netherlands Energy Research Foundation ECN, P.O. Box 1, NL-1755 ZG, Petten, The Netherlands