Population Ecology of Feral House Mice

Abstract

Demographic changes which occurred during population fluctuations in the field were studied by live—trapping six populations during a two—year period. An attempt was made to determine the factors which prevent unlimited population increase. Four major population declines were observed. Epidemic disease caused mortality in one. A second population declined in the fall when insufficient food caused cessation of reproduction. The addition of supplemental food on two experimental grids, however, was sufficient to support both high growth rates and high recruitment of young during the winter, but supplemental food was not sufficient to maintain breeding indefinitely. These populations declined during the usual breeding season in the presence of supplemental and natural food, abundant cover, and few predators. It therefore seemed probable that these two populations were not limited by external factors, but by some intrinsic mechanism. Increased emigration and mortality, primarily of juveniles and subadults, occurred during the latter part of the population increase, but the decline of these populations was primarily caused by a large reduction in the number of weaned young per lactating female, and finally by cessation of breeding conditions in females. Several general characteristics of these populations were established. Body growth rates were highest during the late spring and early summer and lowest during the winter. Home ranges were first established by juveniles or subadults, and were rarely shifted unless the animal left the population. Similarly, the mean length of daily movement by any weight group did not vary greatly, even with a change in breeding activity or population density. Both mortality and dispersal, however, were affected by the breeding condition of the population. In breeding populations, mortality and dispersal of juveniles and subadults increased markedly as density increased, whereas during the nonbreeding season, those parameters were not affected by the weight of an animal, density, or supplemental food.