Competition between the Dasyurid marsupials, Antechinus stuartii and Antechinus swainsonii

Abstract

This thesis describes an experimental investigation of competition between the dasyurid marsupials, Antechinus stuartii and A. swainsonii. Most of the research was conducted in the Brindabella Range, Australian Capital Territory, between May 1978 and November 1981. Initial experiments, conducted between May 1978 and December 1979, were designed to establish whether competition occurred. Five natural, semi-isolated enclosures (four experimental, one control) were established in tall open-forest; each contained populations of both species. Removal of the larger species, A. swainsonii, resulted in many shifts in the population parameters and resource use of the smaller A. stuartii. These included increased numbers, enhanced survival of newly weaned young, increases in movements, home range areas, home range overlaps, the use of complex terrestrial habitat, and the proportion of large, terrestrial invertebrate prey in the diet and decreased arboreal activity. The reintroduction of A. swainsonii produced reciprocal shifts in most of these parameters. In contrast, the removal and reintroduction of A. stuartii had little evident effect on A. swainsonii. It was concluded that competition occurred between the two species but that the effects of A. swainsonii on A. stuartii were more marked than the converse situation. Subsequent manipulation experiments conducted between January 1980 and November 1981 were designed to investigate the effects on A. stuartii of reducing and enhancing the numbers of A. swainsonii. When the numbers of A. swainsonii were reduced, the numbers of A· stuartii increased, and movements and arboreal activity decreased.

Individuals also used more complex terrestrial habitat, and ate proportionately more terrestrial invertebrate prey. Enhancement of the numbers of A. swainsonii produced less marked, but reciprocal, responses in most of these parameters. It was concluded that the intensity of competition on A. stuartii depended, in part, on the relative abundance of A. swainsonii. Throughout the manipulation experiments, samples of invertebrates were obtained in the enclosures by pitfall trapping, litter sampling and sticky trapping. These samples, totalling over 120,000 specimens, showed that whereas sessile invertebrates in the forest litter were abundant all year round, freely-moving invertebrates on the litter surface and tree trunks disappeared in winter. Since A. swainsonii forages principally in the litter, but A. stuartii takes freely-moving invertebrates, it appeared that the latter species could face a seasonal shortage of food. In accordance with the shifts in diet exhibited by A. stuartii when the numbers of A. swainsonii were manipulated, these findings strongly suggested that food was the object of competition. To test the food competition hypothesis, a further experiment was conducted in 1981. Supplementary food was provided to two populations of A. stuartii, but not to a third, control population. The additional food resulted in increases in the survival, arboreal activity and body weights of A. stuartii, but a decrease in its movements. Reciprocal changes were observed when supplementary feeding was discontinued in one of the study areas. The findings demonstrated that A. stuartii was food limited, and also that competition occurred for this scarce resource. Interference was shown to be the mechanism of competition. Antechinus stuartii probably detects A. swainsonii by smell and possibly by sound, and it responds, on a moment to moment basis, by climbing into the trees. Direct harrassment of A. stuartii by A. swainsonii has probably resulted in selection for specific competitor recognition and avoidance in the smaller species. Transferrin allele freqencies were similar in all populations of A. stuartii, whether they were allopatric and sympatric with A. swainsonii, suggesting that gene flow occurred between them. It was proposed that gene flow between these populations could impede directional selection for divergence in sympatric A. stuartii, and so prolong competition in evolutionary time. However, some reduction in competition may be achieved by divergence in body size and possibly tail length: small body size and long tail length allow A. stuartii access to arboreal prey, whereas large body size allows A. swainsonii access to invertebrates buried in the forest litter. In general, competition strongly influenced the population parameters and resources used by Antechinus, especially A. stuartii. This research shows that competition, in addition to habitat and climate, is probably important in determining the patterns of distribution and abundance of these marsupials.