1. Department of Mathematics, Tufts University, 177 College Ave., Medford, MA 02155, USA
2. Department of Mathematics, City University of Hong Kong, 83 Tat Chee Ave., Kowloon, Hong Kong
3. Conservation Research Institute, University of Cambridge, Downing Street, Cambridge CB2 3EA, UK
4. Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EA, UK
5. Departments of Mathematics and Computer Science, Wake Forest University, 1834 Wake Forest Rd., Winston-Salem, NC 27109, USA