Impact of the introduced small Indian mongoose (Herpestes auropunctatus) on abundance and activity time of the introduced ship rat (Rattus rattus) and the small mammal community on Adriatic islands, Croatia

The small Indian mongoose (Herpestes auropunctatus) is one of the world’s 100 worst invasive species (IUCN 2000). It has negative impacts on several small mammals on islands where it was introduced. We assess the abundance of small mammal populations and the activity time of introduced ship rats (Rattus rattus) on three mongoose-infested and three mongoose-free islands in the Adriatic Sea, Croatia. We set up three transects on each island with a trapping system consisting of 30 small live traps to capture small mammals under 30 grams and 30 larger traps to capture ship rats and mongooses, on each transect. Our results support an already large but mostly speculative literature that suggests inability of the small Indian mongoose to reduce high abundances of introduced R. rattus. Further, we suggest that the low abundance of native small mammals is probably not solely caused by the mongoose but also by high R. rattus populations on all six islands. In addition, we provide evidence that R. rattus has changed its activity time to become more nocturnal on mongoose-infested islands, possibly to avoid predation by the mongoose. As R. rattus became more nocturnal, the diurnal mongoose may have become the main predator on amphibians, reptiles, and poultry.