1. School of Biological Sciences, The University of Reading, Reading, Berkshire RG6 6AS, UK
2. Centre for Environmental Policy, Imperial College London, London SL5 7PY, UK
3. National Institute of Agricultural Botany (NIAB), Cambridge, Cambridgeshire CB3 0LE, UK
4. Warwick HRI, Wellesbourne, Warwickshire CV35 9EF, UK
5. Natural Environment Research Council, Swindon, Berkshire SN2 1EU, UK
6. Rothamsted Research, Harpenden, Hertfordshire AL5 2JQ, UK
7. Present address: Institute of Biological, Environmental and Rural Sciences, Edward Llwyd Building, Aberystwyth University, Aberystwyth SY23 3DA, UK