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 utilizing 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 utilizing a previously vacant food niche but instead it occupied a restricted and specialized niche within a broad niche space utilized 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.
Trophic niche analysis can be used to assess ecological opportunities available to alien species in areas of introduction that might aid their establishment, define their functional role and inform on their potential impacts. This study assessed the trophic niche utilized by an invasive population of the Australian redclaw crayfish, Cherax quadricarinatus, in the Inkomati River Basin, South Africa. It evaluated the hypothesis that the euryphagous feeding strategy of redclaw crayfish may allow it to shift its niche width and niche position by altering its feeding strategy in response to fluctuations in resource availability. Gut content and stable isotope analyses were used to determine trophic niche and trophic interactions. Redclaw crayfish were shown to be omnivores and their diet consisted mainly of algae, plant material and invertebrates. Small‐sized individuals had a constricted niche width and fed primarily on invertebrates, whereas larger individuals expanded their niche width to include larger proportions of plant material. Crayfish caught from lotic environments had a higher proportion of invertebrates in their diet than crayfish from lentic environments, and the species exploited a wider niche in summer than in winter. These differences are likely related to differences in productivity amongst habitats and fluctuations in resource availability. There was significant niche overlap (>60%) between redclaw crayfish and Sidney's river crab (Potamonautes sidneyi), but not with other native invertebrates. Both species are omnivores and have similar functional roles, possibly making redclaw crayfish functionally redundant in this ecosystem. Even though both species mainly feed on resources (plant material and invertebrates) that are not normally limiting, the redclaw crayfish invasion might be an accelerator of ecosystem processes such as shredding and decomposition rates of plant material. There is, therefore, a need for further studies to examine potentially altered ecosystem functions caused by redclaw crayfish invasion in the Inkomati River system.
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