Studies of natural populations of Mus. VI. Sizes of populations inhabiting corn cribs in southwestern Ontario

Theoretical studies concerned with genetic variability in natural populations have generally considered genetically effective population sizes (N e) of 104 or greater. This study shows that N e in some highly polymorphic populations such as those of the house mouse, Mus musculus, may be considerably smaller. The mouse populations considered inhabit corn cribs in southwestern Ontario and are polymorphic at a number of loci. They are ideal for this work because the entire population can be collected over a short period of time, and because the various cribs provide replicas of the same general microhabitat. The mean population sizes were 14.4 and 54.4 in the winter and summer, respectively. No correlation was observed between population size and crib size. N e, based on the mean numbers of mice observed in the winter and summer collections and the assumption that all mice are reproductively active, was approximately 20. This could be as high as 70 in some populations and as low as 10 in others. To determine factors that might affect population size, data were collected on 16 environmental variables. Stepwise multiple regression analysis indicated that four environmental variables, compass orientation of a crib (P < 0.001), number of predators (P < 0.01), distance from an east-west line (P < 0.01), and distance from a north-south line (P < 0.05) were significantly associated with population size. Based on the male:female ratio in the summer populations, only one crib appeared to be fully utilized by the mice. In this exception, a crib with a high population density, a deficiency of males suggested competition for territory.