1. Department of Agroecology, Georg-August University, Waldweg 26, D-37073 Gõttingen, Germany
2. Department of Animal Ecology, Justus-Liebig University, Heinrich–Buff–Ring 26–32, D–35392 Giessen, Germany
3. Department of Animal Ecology, Georg–August University, Berliner Strasse 28, D–37073 Göttingen, Germany