Microtus population biology: dispersal in fluctuating populations

KREBS, C. J . , I. WINGATE, J. LEDUC, J. A. REDFIELD, M. TAITT, and R. HILBORN. 1976. Microtus population biology: dispersal in fluctuating populations of M. townsendii. Can. J . Zool .54: 79-95. If spacing behavior limits the breeding density of small mammals, the colonization of vacant areas by surplus animals ought to show how and when population regulation is achieved. Two 0.8-ha areas near Vancouver, British Columbia, were cleared of voles from May 1971 to December 1973 and the colonization of the cleared areas was monitored every 2nd week. All colonists were removed. The colonization rate of the experimental areas was most rapid when populations were increasing rapidly in the adjacent control areas, and much of the loss of individuals in increasing control populations was due to dispersal rather than death. In declining populations very little dispersal occurred. Voles of high body weight (>60 g) were characteristic of late increase and peak populations, but few of these heavy voles dispersed to the vacant experimental areas. Weight at sexual maturity was lower in colonizing voles, particularly among females. About 25% more males than females dispersed into the vacant areas. Colonizing voles were not a genetically random subsample from the control populations. Some leucine aminopeptidase (EC 3.4.11.1) genotypes were more prone to dispersal, particularly when populations were increasing. These results are in agreement with the results of earlier experiments on Microtus pennsylvanicus and M. ochrogaster, and they point to the need for more detailed studies of the dispersal process in voles.