1. Invasive species are a key driver of global environm ental change, with frequently strongnegative consequences for native biodiversity and ecosystem processes. Understanding com-petitive interactions between invaders and functionally similar native species pro vides animportant benchmark for predicting the consequences of invasion. However, even thoughhaving a broad dietary niche is widely considered a key factor determ ining invasion success,little is known about the effects of competition with functionally similar native competitorson the dietary nich e breadths of invasive species. 2. We used a combination of ﬁeld experiments and ﬁeld surveys to examine the impacts ofcompetition with a functionally similar native crab specie s on the population densities, growthrates and diet of the globally widespread invasive red swamp crayﬁsh in an African riverecosystem. 3. The presence of native crabs triggered signiﬁcant dietary niche constriction within the inva-sive crayﬁsh population. Further, growth rates of both species were reduced signiﬁcantly, andby a similar extent, in the presence of one another. In spite of this, crayﬁsh maintained posi-tive growth rates in the presence of crabs, whereas crabs lost mass in the pr esence of crayﬁsh. Consequently, over the 3-year duration of the study, crab abundance declined at those sitesinvaded by the crayﬁsh, becoming locally extinct at one. 4. The invasive crayﬁsh had a dramatic effect on ecosystem structure and functioning, halv-ing benthic invertebrate densities and increasing decomposition rates fourf old compared tothe crabs. This indicates that replacement of native crab s by invasive crayﬁsh likely alters thestructure and functioning of African river ecosystems signiﬁcantly. 5. This study provides a novel example of the constriction of the dietary niche of a successfulinvasive population in the presence of competition from a functionally similar native species.This ﬁnding highlights the importance of considering both environmental and ecologi calcontexts in order to predict and manage the impacts of invasive species on ecosystems.