Species introductions are common in freshwater environments, with the potential to transform community and ecosystem structure. Predatory fishes and American bullfrogs (Rana catesbeiana = Lithobates catesbeianus) are both widespread aquatic invaders implicated in native amphibian declines. In lowland ecosystems, co-occurrence between native and non-native fishes and larval amphibians appears more common than in high-elevation ecosystems, though community interactions there are poorly studied. In this study, we used stable isotope analysis of carbon (δ¹³C) and nitrogen (δ¹⁵N) to examine native fish and amphibian trophic structure in lowland, floodplain waterbodies with and without non-native centrarchid fishes and bullfrogs. In the presence of non-native taxa, three-spine stickleback (Gasterosteus aculeatus), two native salamander larvae, and one of two native frog species shifted their food resources and/or fed lower on the food web, with only one species exhibiting a decline in niche size. The observed trophic shifts reflect changes in habitat and/or food resources, which may reduce competition or predation, and promote co-occurrence. Collectively, our study suggests that the co-occurrence of native and non-native amphibians and fishes in lowland, floodplain habitats may be facilitated by broad food resource availability and complex habitat structure.

Our study system was the Chehalis River Basin in southwestern Washington, USA. The Chehalis River floodplain contains extensive off-channel habitats, including oxbow ponds, wetlands, and seasonally flooded fields and forests. We sampled 16 sites in spring (18 April – 4 May 2017) and resampled eight of those sites in summer (20 June – 6 July 2017), with one site sampled in the spring excluded from further study as we observed none of our focal taxa. At each site, we examined a suite of habitat characteristics, collected water samples for chemical analysis, and sampled the biotic community. Specifically, we collected the following organisms for food web analysis: dominant macrophytes, seston (from water samples; see above), filamentous algae (where present), zooplankton, macroinvertebrates, amphibians, and fishes (where present). Vertebrates were weighed and measured prior to isotopic analysis. All isotope samples were analyzed for percent carbon (C), percent nitrogen (N), δ¹³C, and δ¹⁵N values. Details are provided in the associated manuscript (currently in review at Freshwater Science).

Three datasets are provided:

1. Environmental characteristics of the 15 study sites

2. Vertebrate measurements (e.g., mass and length) for the fishes and larval amphibians collected for the project

3. Stable isotope results, including percent C, percent N, δ¹³C, and δ¹⁵N values

Each file has associated metadata.