Non-native, exotic or introduced species fall into the category of aliens, whereas an invasive species is an alien that gives rise to ecological, economic, health or other concerns as a result of its establishment and spread, or has the potential to do so.Their effects include predation, competition and displacement, or hybridization with natives, as well as the transmission of parasites or pathogens.In cases where aliens are ecological engineers, the ramifications of their establishment are such that food-web architecture is disrupted, causing shifts in ecosystem structure and function.Predators (often piscivores) can cause marked changes in lakes (such as Victoria), but filter-feeing bivalves are also nuisance species.Fishes (often deliberately stocked), molluscs, crayfishes and other crustaceans, as well as aquatic macrophytes are frequently invasive, but aliens include a broad array of taxa.Both lakes and rivers in almost all continents are affected, especially those subject to human modification or with compromised water quality.The outcome of such invasions is replacement of natives, and on-going biotic homogenization of a formerly diverse global biota.