Is there a 'hypergeneralist' guild that dominates successful invasions? A case study of Oreochromis niloticu

Rapid changes in lacustrine ecosystems are often associated with invasions of exotic organisms. The success of an introduction is heavily dependent on the abilities of the invading species, even though there are no universal criteria for predicting colonizing success. This study is an attempt to examine the trophic biology of a successful invading species and compare it with the trophic biology of an endemic congeneric species that was replaced following that invasion.;The introduced fish species was the trophic generalist Oreochromis niloticus, and the endemic was the trophic specialist Oreochromis esculentus. The first set of experiments tested the prevalent notion that atrophic specialist is a more efficient feeder than atrophic generalist when it feeds on its preferred food, but is not very efficient in utilizing a wide spectrum of food resources. Contrary to the expected results, O. niloticus utilized the preferred food of O. esculentus with equal if not higher efficiency while at the same time it was able to explore a wider food spectrum. The second set of experiments focused on explaining these unexpected findings by examining morphological and behavioral differences between the two species. Predation pressures on juveniles of both Oreochromis spp. by endemic and exotic predators were evaluated. The results indicate that O. niloticus has an advantage over its congeneric due to its aggressive nature as well as morphological and behavioral differences.;O. niloticus belongs to an overlooked group of organisms (here termed "hypergeneralists") that utilize a generalist's niche with a specialist's efficiency. Hypergeneralists are represented across a broad taxonomic spectrum. The existence of such a guild can drastically alter the way one looks at fisheries and aquatic resource management. Introductions of hypergeneralist organisms to an ecosystem should be treated as a strong perturbation. It alters the dynamics of the system, and can lead to species replacements and extinctions of endemic taxa.