Freshwater floodplain habitats buffer native food webs from negative effects of non-native centrarchids and bullfrogs

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.