1. Community and Conservation Ecology Group, University of Groningen, Box 14, 9750 AA Haren, The Netherlands.
2. Department of Ecology and Evolutionary Biology, University of Connecticut, 75 North Eagleville Road, Storrs, CT 06269, USA.
3. Department of Botany, Terrestrial Ecology Research Unit, Nelson Mandela Metropolitan University, Box 77000, Port Elizabeth 6031, South Africa.