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Invasive rats (Rattus rattus, R. norvegicus, R. exulans) are recognized as a major threat to native island ecosystems and biodiversity. On many islands, two or three invasive rat species co-occur, often sharing the same habitat; however few studies have focused on the effects of coexisting invasive rat species on native biodiversity. We investigated rat population ecology and diet in a NewCaledonian rainforest where black (Rattus rattus) and Pacific rats (R. exulans) coexist. Black rats dominated Pacific rats in relative abundance with a proportion varying between 80.9 and 88.9%. A total of 374 black rats and 87 Pacific rats were sampled for diet assessment through stomach and caecum analysis. Rat diet was mainly composed of plants, invertebrates and to a lesser extent Squamata, with black rats being more frugivorous and Pacific rats being more omnivorous. Ten of 15 endemic skink and gecko species were consumed, nine species by black rats and six species by Pacific rats. Thus, the presence of both rat species may strengthen the overall predation rate on each native prey species, and/or broaden the total number of native prey species impacted in the New-Caledonian rainforest. These results highlight the importance of preventing new rat species introduction on islands to avoid the strengthening and/or the broadening of negative effects on native biodiversity, and the importance of following the proportion of each rat species during rat control operations. Research to assess the threats generated by various assortments of rodent species on native biodiversity could improve priority setting in conservation actions.