title: Population growth and habitat distribution in cyclic small rodents: to expand or to change?
abstract: Habitat selection determined by intraspecific interactions (social behaviour), being either free or despotic, should result in the largest densities in the most favourable habitat at least in late increase and decline phases of cyclic populations. Habitat distribution determined by interspecific effects such as herbivore-plant or predator-prey interactions may result in higher densities in inferior habitats at late peaks and/or declines due to overgrazing of preferred habitats, or invasion of such habitats by specialist predators. An examination of the distributions of the rodent species Clethrionomys glareolus and Microtus agrestis during a population cycle on forest clearcuts at Grimsö, south-central Sweden, demonstrated clear changes towards less preferred habitats during the decline phase, particularly in M. agrestis. Intraspecific competition could be excluded as a cause since the numbers of both species declined simultaneously. There was no sign of overgrazing, while weasels, Mustela nivalis, invaded the preferred M. agrestis habitat during peak-decline. Predation thus appears more important for the local occurrence at early decline, rather than territoriality or other social behaviour. It is concluded that distributions in various habitats do not always reveal any primary habitat selection but, rather, habitat-dependent survival.
authors.affiliation: Department of Conservation Biology, SLU, S-750 07 UPPSALA, Sweden Fax: +46-18-67 35 37; e-mail: Lennart.Hansson@nvb.slu.se, , , , , , SE.
authors.forename: Lennart
authors.initials: L
authors.lastname: Hansson
keywords: Clethrionomys; Habitat selection; Key words Population cycles; Microtus; Survival