Ecology of alien species with special reference to stream salmonids

Because of their economical value, salmonid fishes have been transferred across national boundaries everywhere in the world. Here, we review the invasion patterns and mechanisms of successful invasions in stream-dwelling salmonids. The high invasion success of salmonids may partly result from extremely high propagule pressure (number of individuals introduced and number of introduction events). Interspecific interactions may also play a role, and the close ecological similarity among salmonids may result in niche overlap and interspecific aggressions. Often newly invading species take advantage of their species-specific core habitats, forcing other fishes to species-specific refugia where their fundamental niche requirements are met. This may result in disruption of the original distribution patterns of species along the headwater-to-mainstream continuum. Moreover, salmonid introductions may have wide-reaching impacts on the trophic organization of stream communities and, further, on terrestrial organisms such as insects, spiders, amphibians, reptiles, birds and mammals.