Some notes on plagues of small mammals in Australia

Summary

Plagues of small mammals in Australia are nearly confined to rodents, both native and introduced. Only in the arid regions do small marsupials reach plague numbers.

Records of plagues have been found as early as 1845, but in many of them there is insufficient information to identify the species involved.

Plagues can develop under three main conditions, namely (a) when species are introduced into new environments, (b) in the artificial environment of crop plants, and (c) in the natural environment.

There are records of the introduced Rattus rattus forming plagues. On colonizing Lord Howe Island in 1918 the species built up to plague numbers in two years. On other islands the species is likely to have built up large populations also. Of various reported plagues on the Australian mainland, Rattus rattus has been identified definitely on only one occasion, in western New South Wales in 1887. There are earlier reports of the introduced rats and mice occurring in large numbers in unsettled areas, but it seems more likely that the animals were native species, wrongly identified, than introduced species which had spread into the areas concerned.

Two groups of species build up to plague numbers in the artificial environment of crop plants. These are Rattus conatus and some other native species which infest sugar-cane in Queensland, and Mus musculus which often infests wheatfields but also forms plagues in other habitats. Both groups infect the crop from a neighbouring natural habitat. Detailed ecological studies have been carried out on both these types of infestation. Reports of dispersal of plagues of Mus musculus need investigation.

Mus musculus can form plagues over a wide range of habitats, from the arid regions of central Australia to the high rainfall areas of tropical Queensland. The status of the form inhabiting the arid regions needs study.

The arid regions of Australia provide a special environment in which live a number of species of small mammals, both eutherians (rodents) and marsupials. In this habitat breeding occurs when there is a sufficient rainfall, and is not regulated at its extremes by an annual climatic cycle. Some of the riverine lands within the region receive drainage from outside, and this modifies the relationship with rainfall in the Lake Eyre Basin particularly. Under favourable conditions several species of the small mammals can build up to plague numbers. In this they appear to differ markedly from the small mammals of other deserts of the world. The special features of the Australian arid environment which allow this are unknown.

Plagues of Rattus villosissimus are generated in the tablelands to the south of the Gulf of Carpentaria, and hordes of the rodents disperse in large numbers, mostly moving out of the home territory along the course of rivers draining it to the south, and sometimes also to the north. The little information available concerning the populations suggests similarities with the cyclic changes which take place in populations of voles and lemmings in the northern hemisphere. There is no good information about the period of recurrence of the plagues, present records showing a range from three to seventeen years.

The little information available about the plagues of small mammals in Australia suggests parallels with such plagues elsewhere. The common factors are a small size, an herbivorous (? or insectivorous) diet, a high reproductive potential and a polymorphism which permits quick adaptation to the different population structures found under different population densities.