1. Centre for Land Use and Water Resources Research, Porter Building, University of Newcastle, Newcastle upon Tyne NE1 7RU, U.K.
2. Department of Agricultural and Environmental Science, King George VI. Building, University of Newcastle, Newcastle upon Tyne NE1 7RU, U.K.
3. School of Biological Sciences, Queen Mary and Westfield College, London E1 4NS, U.K.