An Experimental Study of Competition Between Two Species of Dasyurid Marsupials

Abstract

This study describes an experimental investigation of the hypothesis of interspecific competition between the dasyurid marsupials Antechinus stuartii and A. swainsonii, conducted between May 1978 and December 1979. Antechinuses were live—trapped, marked, and released in five natural enclosures (four experimental, one control), each established in dense forest along a valley floor in the Brindabella Range near Canberra, Australia. The enclosures were similar in size, shape, vegetation, and climate, and semi—isolated from one another by creeks or by dry, open habitat unsuitable for the marsupials. Antechinus stuartii was removed from two enclosures in July 1978 and later reintroduced to one, whereas A. swainsonii was removed from two other enclosures and later returned to one. Neither species was manipulated in the control enclosure. Comparisons of the population parameters and resource use were made for each species when it occurred in the presence and in the absence of the other. Removal of the larger and more terrestrial species, A. swainsonii, produced several effects on A. Stuartii that were consistent with the interpretation of competition. These included increased numbers; enhanced survival of newly weaned young; increases in extent of movements, home range areas, and use of structurally complex terrestrial habitat; and decreased arboreal activity. The proportion of large, terrestrial invertebrate prey in the diet also increased. The experimental reintroduction of A. swainsonii after ≈8 mo produced reciprocal shifts in most of these parameters. An anomalous finding was that the mean body mass of A. stuartii declined when A. swainsonii was excluded. This was probably due to increased levels of intraspecific interference, since the decline coincided with unusually high population densities in A. stuartii. Competition with A. swainsonii had little evident effect on the time of activity of A. Stuartii. In contrast, manipulations of the numbers of A. stuartii produced no changes in the population parameters of the pattern of resource use of A. swainsonii; hence it was concluded that the competitive effects of A. stuartii on its larger congener are small. Predation, the presence of a superfluous third species (a eutherian rat) during the experiments, and genetical differences between the enclosure populations of Antechinus were rejected as possible alternative explanations of the results; none was as convincing as that of interspecific competition. probably occurs for food, and is therefore most intense in winter, when surface litter invertebrates are scarce. This study demonstrates that competition has asymmetrical effects on the two study species, and challenges the notion that competitive interactions are weak or absent in marsupial communities.