A test of a competition model with reference to three species of small mammals in south-eastern Australia

This study tests the hypothesis that competition occurs in a community of three species of small mammals (Antechinus stuartii and A. swainsonii, Marsupialia; Rattus fuscipes, Rodentia) in south-eastern Australia. The hypothesis was tested by using a simple model of competition that is based on two premises: I. competition results in a negative numerical or a negative spatial association between species, and II. competition is more intense where the densities of the competing species are relatively high. The model combines both premises and predicts that measures of association between competing species will be more negative where the species exist at high population densities than derived from the model and applied to data gathered in two study areas with contrasting mean animal densities. There was no evidence of numerical association among the species (three tests), but evidence of negative spatial association (three tests our of four). These latter findings were consistent with the predictions of the model. All species preferred the same broad kinds of habitat (macrohabitat), but they segregated into smaller, finer grained patches (microhabitat) when their numbers were relatively high. Such segregation was more evident between the two species of marsupials than between either species of marsupial and the eutherian rat. These findings suggested that competition occurs by interference, and reinforce the idea that an understanding of the behaviour of individuals is important to understanding competition between species.