Reproduction, growth and population dynamics of house mice in irrigated and non-irrigated cereal farms in New South Wales

Abstract

Populations of house mice (Mus musculus L.) irrupt at irregular intervals throughout the wheatlands of south-east Australia and in parts of the arid and semi-arid inland. A previous study in South Australia postulated that such plagues occurred after unusual rainfall or flooding caused, during the normally arid summers, concurrent abundance of food and moist soil in which mice could burrow. This thesis reports a capture-mark-release study of mice on an irrigated cereal farm, where summer flood irrigation and autumn harvest appeared likely to provide those twin requirements each year. In the last two years of the three-year study, an ancilliary, comparative examination of a population on an unirrigated cereal farm, in the same climatic region but on different soils, was conducted also. Concurrent with a widespread plague, both populations irrupted during the study: on the Irrigated Farm during the 1979-80 summer, and on the Dry farm during the following autumn. Changes in abundance of mice before, during, and after the irruptions are examined in terms of the quality of mice (growth rates; body composition and condition; and the incidence of chronic poly-articular arthritis caused by Actinobacillus Monliformis) an d their performance (survival; dispersal; length of breeding season; prevalence of pregnancies; and mean litter size). Mouse numbers on the Irrigated Farm exhibited annual cycles in which the peaks progressively increased from 1978 to 1980. The peaks also occurred progressively earlier in the year: late winter in 1978; early winter in 1979; and summer in 1980. The last peak, which is referred to as an 'outbreak' , is distinguished from the other two: first, mice invaded and damaged the rice crop before the flood irrigation waters were drained, and second, the population subsequently crashed to extinction. The immediate cause of the outbreak was the presence of many over-wintered, nulliparous females at the commencement of breeding in the spring of late 1979. Their presence is, in turn, attributed to a sequence of changes in the quality and performance of mice, and in the population structure. Those changes began no later than autumn of 1978. Mice continued to grow, and some to breed, during the winter. Rapid churning of the population continued through winter and spring, and population growth was slow. Mice were in their peak condition by spring, when an epizootic of actinobacillosis also peaked. At the beginning of the next breeding season {October 1978), the average litter size was 9.8 young per litter, compared to 6.4 a year earlier. The earlier population peak, in June 1979, is attributed to failure of mice to disperse. In the previous year, dispersal at crop draining returned the sex ratio to 1:1. In March 1979, however, an excess of females remained on the banks. Juveniles failed to mature, and adult males and females regressed so that the population was completely stagnant within six weeks. Individuals became obese in early winter then lost weight and increased in length very slowly compared to growth rates in the previous winter. Winter survival was subsequently high. After the outbreak(during the next breeding season) the condition of mice declined rapidly. Again, breeding ceased at crop-draining, and the population became extinct. Each of the three annual increase phases was simulated by a computer model based on litter size, number of adult females at the beginning of the breeding season, and dispersal patterns. The presence of high-quality food during the irrigation seasons and in the autumn of 1978, when there was very high rainfall, is concluded to determine the onset and duration of reproduction as well as growth rates of individuals. A food-quality, spacing-behaviour model is proposed, to account for the observed changes in abundance and population structure. It is also proposed that a suite of intercompensating forces regulated this population around two population densities, each associated with a particular social structure. The outbreak is seen as an eventual failure of that suite to regulate the population. It is also concluded that, under the conditions on the Irrigated Farm, the consecutive abundance of food and nesting sites during summer is not a sufficient condition to cause an outbreak; and that a prolongation of the breeding season by autumn rains providing high quality food for an extended period is also required. On the Dry Farm, mice permanently inhabited a piggery, which was a 1 so the only site on either farm where they were found after the irruption. The irruption on the Dry Farm, by occurring some 18 to 24 months after good rains, differed from those studied previously in other unirrigated areas. Further, this irruption did not build up in the wheatfield, which the mice vacated before the summer harvest, presumably because the sandy soils were too arid. Nor did mice reach plague proportions at a temporary wheat stores where food and shelter were in excess. It is concluded that the South Australian hypothesis requires modification to allow for differences in the landscape heterogeneity of winter and summer refuge habitats. Show more