Competition in an invaded rodent community reveals black rats as a threat to native bush rats in littoral rainforest of south-eastern Australia.
Interspecific competition is a recognized but under-studied mechanism by which invasive species affect native fauna. We experimentally reduced populations of the introduced black rat Rattus rattus in relatively undisturbed littoral rainforest in south-eastern Australia to test its competitive impact on populations of the native bush rat Rattus fuscipes. Removal of R. rattus resulted in significant and sustained increases in populations of the native rodent due to immigration, juvenile recruitment and increases in residency of females. Native juveniles were particularly vulnerable to R. rattus, being largely absent from untreated populations despite indications of breeding in females, but responding rapidly to their removal. The absence of changes in body condition and reproduction of adult R. fuscipes with long-term removal of R. rattus suggests that direct interference by R. rattus rather than competition for resources best explains the low densities and poor juvenile recruitment of R. fuscipes. The behavioural dominance of R. rattus was not due to an inherent competitive ability because they did not re-establish post-removal as expected if competition was asymmetrical. Instead, competitive dominance seemingly shifted to R. fuscipes with changes in residency status of populations. Improved survivorship among native females reveals that they were particularly important in establishing resident populations following removal of R. rattus. Synthesis and applications. These findings identify R. rattus as a significant competitive threat to native R. fuscipes. Removal of invasive rodents and active conservation of common native species is recommended to forest managers based on the potential for R. fuscipes to prevent re-invasion subsequent to pest control efforts and maintain invasion resistance.