1. Institute of Terrestrial Ecology, Merlewood Research Station, Grange-over-Sands, Cumbria, LA11 6JU, U.K., Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen, Scotland, AB15 8QH, U.K., and School of Biology, University of Nottingham, Sutton Bonington, Loughborough, Leicestershire, LE12 5RD, U.K.