Variation of the stable isotope niches of native amphibians in ponds invaded by the red swamp crayfish

Relationships between native and invasive species can modify trophic interactions in food webs and the diet of native species, leading to substantial changes in their trophic niches. We quantified the stable isotope niche of native amphibians (two species of tadpoles and two species of newts) and the invasive red swamp crayfish (Procambarus clarkii) in 18 ponds of an area invaded for more than 30 years. We tested whether crayfish presence and abundance explained variation in the size and position of the amphibians’ stable isotope niches compared with proxies of pond resource availability and competition levels. Agile frog tadpoles (Rana dalmatina) had consistently low trophic positions, while tree frog tadpoles’ niches (Hyla arborea) showed signs of an opportunistic diet. Newts (palmate newt (Lissotriton helveticus) and marbled newt (Triturus marmoratus)) had high trophic positions consistent with a predatory diet. Crayfish showed a high level of trophic variability, but their trophic niche never overlapped with the trophic niche of amphibians. Amphibian niche size and position were associated with amphibian density and pond canopy cover rather than with crayfish presence or abundance. This study suggests limited changes in amphibian diets in the presence of red swamp crayfish in ponds compared with results from experimental studies, suggesting that complex environmental conditions and the long time since invasion might limit trophic interactions between these native and invasive species.