Niche complementarity between an alien predator and native omnivorous fish in the Wilge River, South Africa
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A major challenge in biological invasions is to predict community susceptibility to invasion. This study investigated trophic interrelationships between an alien predator, largemouth bass (Micropterus salmoides), and native omnivores, sharptooth catfish (Clarias gariepinus) and largescale yellowfish (Labeobarbus marequensis), in an invaded aquatic system to elucidate factors that might have aided their successful establishment. It tested the hypothesis that M. salmoides is able to co-exist with the two native omnivores by either utilising a previously vacant food niche or through niche complementarity. Gut content and stable isotope analyses were used to determine trophic interactions. There was no evidence that M. salmoides was utilising a previously vacant food niche but instead it occupied a restricted and specialised niche within a broad niche space utilised by the native omnivores. Differences in niche space and size have resulted in minimal niche overlaps that imply niche complementarity. The introduction of M. salmoides raises concerns about increases in predation pressure in the system. This, however, is difficult to ascertain because of complexities in measuring long-term trends in predator demands, abundance and community compensation mechanisms. There is a need for long-term monitoring of community structure, especially small-sized prey species that are vulnerable to increased predation pressure.