1. Department of Ecology and Evolutionary Biology, Princeton University, 106 Guyot Hall, Princeton, NJ 08544, USA
2. School of Natural Resources and Environment, Dana Building, 430 East University, University of Michigan, Ann Arbor, MI 48109-1115, USA
3. Department of Zoology, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway