Trophic interactions between native newts and introduced mosquitofish suggest invaded ponds may act as demographic sinks

Alien predator introduction is a global threat to amphibians. Yet, there is a lack of in situ studies of trophic interactions between alien predators and native amphibians, particularly concerning small predatory fish such as mosquitofish. Mosquitofish originate from the United States but have been introduced globally, including intentionally for mosquito control. They cause declines in many amphibian populations but the mechanisms involved have been seldom investigated. Trophic interaction studies (mainly ex situ) reveal negative effects on larval amphibian stages but do not consider interactions with adults. Some site-occupancy studies show no negative association with adult amphibians, suggesting potentially complex demographic impacts and calling for a better characterization of trophic interaction with adult amphibians. Here, we studied in situ trophic interactions between introduced Eastern mosquitofish (Gambusia holbrooki) and pond-breeding palmate newts (Lissotriton helveticus; larvae and adults) using gut content and stable isotope analyses. Mosquitofish had little trophic niche overlap with adult newts. Adult newts foraged mainly on burrowing benthic macroinvertebrates that were little exploited by mosquitofish, the latter focusing mainly on microcrustaceans. Both techniques suggested predation on newt eggs or larvae and cannibalism by mosquitofish. Since native newts were still abundant despite > 50 years of mosquitofish presence and reproductively active but without evidence of larval survival, we argue that ponds invaded by small predatory fish such as mosquitofish may pose a risk by acting as demographic sinks for newts due to their predatory impact on larvae and eggs, but potentially low impact on adults in terms of trophic niche overlap.