1. School of Biological Sciences, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK.
2. Belvedere, Southampton Road, Whaddon, Salisbury, Wilts SP5 3DZ, UK.
3. Natural Environment Research Council Centre for Ecology and Hydrology, Monks Wood, Cambridgeshire PE28 2LS, UK.
4. Cardiff School of Biosciences, Cardiff University, Main Building, Museum Avenue, Cardiff CF10 3TL, UK.