Abstract
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. exulans 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.
Highlights
Islands, which host almost 40% of the critically endangered species on Earth on less than 6% of its total land area, are vulnerable to biological invasions and represent a global conservation priority (Tershy et al 2015)
Introduced cats and rats have been identified as a prime cause of more than half the extinctions of island bird species recorded over the last centuries and still represent the major threat to 40% of currently endangered island bird species (Doherty et al 2016; McCreless et al 2016)
This study aimed to identify possible trophic interactions between three introduced mammals and the Niau kingfisher during its reproductive season
Summary
Islands, which host almost 40% of the critically endangered species on Earth on less than 6% of its total land area, are vulnerable to biological invasions and represent a global conservation priority (Tershy et al 2015). Alien (i.e. introduced) species inevitably disturb the natural balance of island food webs by modifying trophic links. This may affect the demography and abundance of native (and introduced) species, with further cascading effects (Courchamp et al 2003; White et al 2006; David et al 2017). Direct predation (i.e. top-down effect) is the predominant mechanism by which invaders can dramatically decrease populations of native species or even cause their extinction (Doherty et al 2016; David et al 2017). Local species restrict their realised niche and/or shift their niche to sub-optimal habitats to reduce niche overlap with introduced species (Reitz and Trumble 2002; Rankin et al 2018)
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