Global Test of Trophic Niche Conservatism of Non-Native Freshwater Fishes Between Their Native and Introduced Ranges

Non-native species represent a unique opportunity to understand the mechanisms that shape ecological niches, with important implications for characterizing the potential of species to adapt to rapid anthropogenic changes. Despite ample evidence that many introduced species are spreading outside of their native range, it remains unclear whether this process is facilitated by conservatism of broad native niches or rapid evolutionary changes in recipient ecosystems. Using a global database of stable isotope data characterizing the trophic attributes of 34 freshwater fishes, we tested whether the trophic niches of introduced species change between their native and non-native ranges and investigated different hypotheses that might explain consistent responses across species. We found evidence of trophic niche shifts for nearly half of the evaluated species, related to changes in both trophic position (inferred from δ¹⁵N) and main sources of energy (inferred from δ¹³C). Our findings further demonstrate high consistency in trophic niche differences both across geography and species trophic preferences, suggesting that niche shifts are governed by general mechanisms rather than location-specific differences. We argue that shifts in trophic niche between native and non-native populations is one underappreciated key factor in the establishment and integration phase of biological invasions and deserves more attention.