The Founding Population - the Effect of Differences in Its Size, Nature and Season of Establishment on the Subsequent Dynamics of Populations of House Mice, Mus-Musculus L

Abstract

In south-eastern Australia, house mouse numbers in agricultural areas often reach plague proportions in the year following the breaking of a drought but numbers are usually low in subsequent years. The effects of differences in the size and nature of the groups invading ephemeral habitats on sub- sequent numbers, and on the time taken for the population to peak, were investigated by introducing groups of different size, age structure and composition into out-door pens where food supplies were abundant. The effects of differences in the season of invasion and the effects of allowing mice from one or two sources to invade empty home sites were also investigated. Varying the size of the founding groups had little effect on peak numbers or on the length of the establishment-peak interval. Peak numbers were usually higher in populations established with mice from a narrow band of age classes than in those established with mice from a wide band of age classes, and the establishment-peak interval was usually shorter in populations established with mice from one source than in populations established with mice from two sources. The establishment-peak interval was also shorter in populations established in spring than in those established at other seasons. When established groups were allowed access to empty home sites, the groups colonising the sites varied in size, age structure and composition. Peak heights in these colonies, too, appeared to be affected by the age structure of the founding group, and the establishment-peak interval was affected by the composition of the group. These results suggest that both between-year differences in the nature of the groups moving from the refuge areas to the growing crops, and between-year differences in the season at which the crops are colonised, could contribute to the observed differences in mouse numbers in the agricultural areas of south-eastern Australia. Possible reasons for between-year differences in the nature of the groups colonising the ephemeral habitats are discussed.