1. Mathematical Ecology Research Group, Department of Zoology, South Parks Road; Oxford OX1 3PS U.K.
2. Department of Life Sciences; Imperial College London, Silwood Park Campus, Buckhurst Road; Ascot SL5 7PY U.K.
3. The Pirbright Institute, Ash Road; Pirbirght GU24 0NF U.K.
4. St Peter's College, New Inn Hall Street; Oxford OX1 2DL U.K.