1. Department of Ecology & Evolution; University of Chicago; 900 E 57th St Chicago IL 60637 USA
2. Biology Department; Radford University; PO BOX 6931 Radford VA 24141 USA
3. Department of Applied Mathematics; University of Colorado; 526 UCB Boulder CO 80309 USA
4. Department of Science, Systems and Models; Roskilde University; Universitetsvej 1, DK-4000 Roskilde Denmark