This content is only available as a PDF.

Author notes


School of Biosciences, Geoffrey Pope Building, University of Exeter, Stocker Road, Exeter EX4 4QD, UK. E-mail:


Department of Mathematics and Statistics, University of Lancaster, Lancaster LA1 4YF, UK. E-mail:

PHS Consulting Limited, Pryors Hayes Farm, Willington Road, Oscroft, Tarvin, Chester CH3 8NL, UK.

Article PDF first page preview

Article PDF first page preview