1. F. S. Chapin III and D. U. Hooper are in the Department of Integrative Biology, University of California, Berkeley, CA 94720–3140, USA. B. H. Walker is in the Division of Wildlife and Ecology, Commonwealth Scientific and Industrial Research Organisation (CSIRO), Post Office Box 84, Lyneham, ACT 2602, Canberra, Australia. R. J. Hobbs is in the Division of Wildlife and Ecology, CSIRO, LMB 4 Post Office Midland, Western Australia 6056, Australia. J. H. Lawton is at the National Environmental Research...