Stable isotope analyses reveal predation on amphibians by a globally invasive fish (Gambusia holbrooki)

1. Biodiversity loss caused by invasive species is particularly problematic in freshwater ecosystems, which are among the world's most threatened habitats. Invasive fish such as the eastern mosquitofish, Gambusia holbrooki, have been implicated in the decline of amphibians, which suffer high extinction rates globally. Although G. holbrooki is one of the most studied freshwater fish, its diet shows wide geographic variation and its impact on amphibian populations remains unclear. 2. Stable isotopes ¹³C and ¹⁵N were used in 10 urban ponds in Sydney, Australia to compare the diet of G. holbrooki in January, April and May 2013 using a stable isotope mixing model. 3. Gambusia holbrooki was carnivorous and fed on invertebrates (24-39%), tadpoles (25-32%) and conspecifics (20-45%). In contrast to previous studies, primary producers were a negligible part of Gambusia holbrooki diet (<10%). Its diet in late autumn comprised a high proportion of conspecifics (up to 45%) owing to the depletion of other food sources before winter (metamorphosis of larvae). 4. This study provides evidence of high rates of predation on native tadpoles and invertebrates by a highly invasive fish. This knowledge should be incorporated into amphibian releases through head-starting tadpoles or using soft releases where tadpoles are placed in predator-free enclosures until larvae are large enough to avoid predation. Considering the dire conservation status of amphibians globally and the growing interest for invertebrates, it is suggested that stable isotopes are valuable to identify threats from predation in order to target conservation practice toward suitable priorities.