Niche expansion in a resilient endemic species following introduction of a novel top predator

Summary Introduced species can have profound direct ecological impacts on native species, yet their potential indirect effects remain relatively unexplored. For instance, introduced predators may directly affect some native species via predation, which may in turn have indirect consequences for other species that are released from competition. We explore this possibility in East Africa's Lake Victoria basin, where the introduction of the predatory Nile perch, Lates niloticus, in the 1950s and 60s contributed to the overall or local extinction of hundreds of native fish by the 1980s. We ask whether this dramatic change in assemblage composition has led to competitive release and niche expansion in Rastrineobola argentea, a resilient native cyprinid that has thrived in this highly perturbed ecosystem. To address this question, we compare the trophic ecology of R. argentea before (1966) and after (2011) the introduction of the Nile perch in Lake Victoria; and across eight satellite lakes that differ in their history of Nile perch invasion. Using this combination of spatial and temporal comparisons, we test for increases in dietary niche breadth (niche expansion) and changes in the level of individual specialisation of R. argentea in invaded versus uninvaded contexts. In our historical comparison, we find good evidence for dietary niche expansion and an increase in interindividual diet variation in R. argentea over time. Across lakes, however, strong bottom‐up effects (i.e. variation in prey availability) appear to obscure any potential top‐down effects of the Nile perch introduction on the trophic ecology of R. argentea. Overall, we find substantial temporal and spatial variation in the diet and niche breadth of R. argentea, but the underlying drivers remain uncertain, given the complexity of both anthropogenic and natural ecological changes in Lake Victoria over the past century. Understanding both the direct and indirect impacts of introduced species is challenging, but important for successful long‐term management of human‐altered ecosystems.