1. Department of Zoology, University of Aberdeen, Tillydrone Avenue, Aberdeen AB24 2TN, UK
2. Biomathematics and Statistics Scotland, Environmental Modelling Unit, Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB15 8QH, UK
3. Forest Research,Woodland Ecology Branch, Northern Research Station, Roslin, Midlothian EH25 9SY, UK