1. Department of Environmental Sciences, Utrecht University, P.O. Box 80115, 3508 TC Utrecht, The Netherlands; Erosion and Soil and Water Conservation Group, Wageningen University, Nieuwe Kanaal 11, 6709 PA Wageningen, The Netherlands; and Department of Plant Sciences, Wageningen University, Postbus 9101, 6700 HB Wageningen, The Netherlands;2. Tropical Nature Conservation and Vertebrate Ecology Group, Wageningen University, Bornsesteeg 69, 6708 PD Wageningen, The Netherlands;3. Section Population Biology, Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Kruislaan 320, 1098 SM Amsterdam, The Netherlands;4. School of Mathematics, University of Natal, P/Bag X01, 3209 Scottsville, Pietermaritzburg, South Africa;5. Spatial Ecology Department, Netherlands Institute of Ecology, P.O. Box 140, 4400 AC Yerseke, The Netherlands;6. Department of Plant Biology, University of Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands;7. ITC International Institute of Aerospace Survey and Earth Sciences, Agriculture, Conservation and Environment Division, P.O. Box 6, 7500 AA, Enschede, The Netherlands