FACTORS INFLUENCING THE ESTABLISHMENT OF AQUATIC INSECT INVADERS

Because of their small size and limited mobility aquatic insects are seldom recognized as invasive species and only where verifiable historical data exist can introductions of exotic species be confirmed. Records of some generally considered cosmopolitan species could in fact be undocumented introductions. Of the entirely aquatic orders of insects only the Odonata are noted as regular invaders. Trichoptera, Ephemeroptera, Plecoptera and Megaloptera are all poor colonizers. In contrast, the Hemiptera, Diptera and Coleoptera have many species which show excellent colonizer and invader qualities. Diptera are recognized as the most deleterious of introduced aquatic insects because many of the species are pests and disease vectors of man and his livestock. Although no alien aquatic insect pests have been recorded in South Africa, the potential for the introduction of problem species exists. There are certain life history styles which make an aquatic insect predisposed to colonize a new region successfully. There are also qualities of the habitat which enable successful colonization of the region. Both of these aspects are discussed and demonstrated with case histories. Streams in the southern Cape contain a large number of biogeographically important palaeoendemic aquatic insects. These animals have evolved without the presence of fish predators and their behaviour and life history styles would make them highly vulnerable to extinction by the introduction of alien fish predators or even translocation of indigenous species. It is important to conserve this biologically important stock of invertebrates in as near a pristine state as possible and prevent all introductions or translocations of fauna alien to their streams or sections of streams where natural barriers keep out fish.