First evidence of aggressive interactions by native rakali towards invasive black rats

The mechanisms by which native species might compete with invasive alien species are poorly understood, especially for competing cryptic small mammals. We document the first evidence of an aggressive interaction between a rakali (water rat, Hydromys chrysogaster) and a black rat (Rattus rattus) in the natural environment in Australia. Our camera trap recorded a rakali ambush a black rat in bushland on the Sydney Harbour foreshore. This finding suggests there is potential for rakali to play a positive role in biotic resistance against invasive black rats, additionally providing ecological justification for the conservation and promotion of rakali especially in urban environments where introduced rats are common.