Non-native rainbow trout (Oncorhynchus mykiss) occupy a different trophic niche to native Breede River redfin (Pseudobarbus burchelli) which they replace in South African headwater streams
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Recent research has revealed that non-native rainbow trout Oncorhynchus mykiss have largely replaced a native cyprinid, the Breede River redfin Pseudobarbus burchelli, as the dominant species of fish in many headwater streams in the Cape Floristic Region (CFR) of South Africa. Moreover, differences in the composition of benthic communities in CFR headwater streams with and without trout suggest that trout do not functionally compensate for the native redfin which they have replaced in these food webs. In this study, we used gut content and stable isotope analyses to characterise and compare the trophic niches and diet compositions of allopatric populations of trout and redfin in six CFR headwater streams (three containing trout, three containing redfin). Results indicate that native redfin exploit a broader trophic niche, and a more omnivorous diet, than do trout. Gut content analyses showed terrestrial invertebrates to be an important prey source for trout, which could potentially offset predation pressure on aquatic invertebrates and explain why benthic invertebrate density in streams with trout is higher than that in streams with no trout. Contrastingly, redfin diet appeared to be dominated by aquatic invertebrates, with terrestrial prey a less important food item in the guts of redfin. That redfin and trout exploit nonequivalent trophic niches may have consequences for benthic community composition in CFR headwater streams, and this study highlights the importance of quantifying how the functional role of predators changes following a predator replacement for understanding and managing the consequences of non-native predator invasions.