Mouse plagues in South Australia cereal-growing areas III. Changes in mouse abundance during plague and non-plague years, and the role of refugia

Abstract

Mouse populations were monitored at 15 sites between 1980 and 1990, during which time one severe mouse plague, in 1980, and one minor outbreak, in 1984, were recorded. Smaller annual peaks in autumn to early winter were followed by winter population declines. Crops were colonised each year in late winter or early spring by mice from winter refuge habitats with dense, low vegetation, including roadsides and grassland along a railway line. In most years mouse numbers in crops declined during summer, but in 1983-84 they rose continuously during summer and autumn, and reached very high levels. Crops planted in 1984 were invaded by large numbers of mice which had survived through winter in the paddocks, but population levels again crashed in late spring and summer. Recorded population changes were generally consistent with plague probabilities predicted from environmental variables, except in 1985 when numbers failed to reach the predicted high levels at most sites. Population changes in crops during late spring appear to be critical in the development of mouse plagues. Large litter sizes and pregnancy rates, and variable survival rates and size of the breeding population, appear to be important factors at that time.