ASYMMETRIC COMPETITION IN AUSTRALIAN HEATHLAND RODENTS - A RECIPROCAL REMOVAL EXPERIMENT DEMONSTRATING THE INFLUENCE OF SIZE-CLASS STRUCTURE

The swamp rat (Rattus lutreolus) and the eastern chestnut mouse (Pseudomys gracilicaudatus) occupy the same coastal wet heath habitat in eastern Australia. Previous experimental removals have demonstrated asymmetric interspecific competition with the larger R. lutreolus excluding the smaller P. gracilicaudatus from preferred habitat and reducing its abundance. We report the results of a reciprocal manipulation experiment in summer with the abundance of the smaller P. gracilicaudatus being depressed by removals over a 40 d period, the abundance of the larger R. lutreolus increased more rapidly (p < 0.025) on the three experimental sites than on the three control sites. In the final trapping session of the removal period there was a significant difference (t = 2.67, p = 0.028, 1-tail) in R. lutreolus abundance on experimental (7.33 +/- 1.20) and control (3.67 +/- 0.67) sites. There were again no differences between these sites when retrapped 107 d after the removals ceased. A further reciprocal experiment removing P. gracilicaudatus was conducted in winter and failed to provide any evidence for the competitive release demonstrated by R. lutreolus in the summer experiment. We make comparisons between the results of these two experiments, which allows us to confirm that body size difference provides the mechanism which facilitates this asymmetric competition and determines access to better habitat. We analyse the response by R. lutreolus for two size classes separately, large (animals heavier than the largest P. gracilicaudatus) and small (lighter individuals). Interspecific competition in summer can be attributed to the small R. lutreolus size class alone, but, in winter neither size class exhibits competitive release. Normal growth of juveniles between summer and winter reduces the number of small R. lutreolus and also the intensity of competition with P. gracilicadatus. Only small R. lutreolus are vulnerable to competition with P gracilicaudatus and large individuals remain unaffected. The different outcomes of such competition are determined by the size class structure of the larger species, thus producing seasonally variable interspecific competition.