Stuck amongst introduced species: Trophic ecology reveals complex relationships between the critically endangered Niau kingfisher and introduced predators, competitors and prey

The introduction of mammals on oceanic islands currently threatens or has caused the extinction of many endemic species. Cats and rats represent the major threat for 40 % of currently endangered island bird species. Direct (predation) and/or indirect (exploitative competition for food resource) trophic interactions are key mechanisms by which invaders cause the decrease or extinction of native populations. Here, we investigated both direct and indirect trophic interactions amongst four predator species (i.e. animals that hunt, kill and feed on other animals), including three introduced mammals (Felis silvestris catus, Rattus rattus and Rattus exulans) and one critically endangered native bird, the Niau kingfisher (Todiramphus gertrudae). All four species' diets and prey availability were assessed from sampling at the six main kingfisher habitats on Niau Island during the breeding season. Diet analyses were conducted on 578 cat scats, 295 rat digestive tracts (218 R. exu/ans and 77 R. rattus) and 186 kingfisher pellets. Despite simultaneous use of morphological and PCR-based methods, no bird remains in cat and rat diet samples could be assigned to the Niau kingfisher, weakening the hypothesis of current intense predation pressure. However, we determined that Niau kingfishers mainly feed on introduced and/or cryptogenic prey and highlighted the potential for exploitative competition between this bird and both introduced rat species (for Dictyoptera, Coleoptera and Scincidae). We recommend removing the cats and both rat species, at least within kingfisher breeding and foraging areas (e.g. mechanical or chemical control, cat sterilisation, biosecurity reinforcement), to simultaneously decrease predation risk, increase key prey availability and boost kingfisher population dynamics