Affiliation: | a Department of Terrestrial Ecology, National Environmental Research Institute, Vejlsøvej 25, P.O. Box 314, DK-8600 Silkeborg, Denmark b Department of Zoology, University of Otago, Dunedin, New Zealand c present address: Department of Zoology, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa |