1. Dept of Global Ecology; Carnegie Inst. for Science; 260 Panama Street Stanford CA 94305 USA
2. Scientific Services, South African National Parks; Private Bag X402 Skukuza 1350 South Africa
3. Centre for African Ecology, School of Animal, Plant and Environmental Sciences, Univ. of the Witwatersrand; Private Bag 3 Wits 2050 South Africa
4. Dept of Biogeochemical Processes; Max Planck Inst. for Biogeochemistry; Hans-Knoll-Str. 10 Jena DE-07745 Germany