A leading cause of biodiversity loss in many aquatic ecosystems is the introduction of non-native or exotic species deliberately or accidentally introduced into a new habitat. Such species include plants, fishes, algae, mollusks, crustaceans, bacteria, and viruses. Non-native species that are able to reproduce and survive outside of the habitats where they evolved are also referred to as alien, introduced, invasive, non-native, or nonindigenous.
There are many ways that exotic species are introduced into freshwater areas. They include the release of pet fish or foreign species brought in for entertainment in public or commercial aquariums or for education. In addition, the demand for bait fish, the captive rearing of fish, and the unintentional transfer of species by ship ballast water all contribute to the introduction of exotic species in freshwater areas. However, typically exotic species are introduced into marine areas by means of transplanting or commercial shipping. It has been reported that ship ballast water is responsible for the transport of approximately 3000 species worldwide each day.
Exotic species can have many negative impacts on the environment, the economy, and human health. When species are introduced into an area, they may cause increased predation and competition, disease, habitat destruction, genetic stock alterations, and even extinction. Approximately 68% of fish species lost in North America over the last century were caused by an invasion of exotic species. The invasion of exotic species has also caused the economy to suffer through the obstruction of industrial and municipal water pipes and the displacement or elimination of important commercial and sport-fishing species. Public health may also be negatively impacted. For example, in a number of coastal areas in the United States, cholera strains carried in the ballast water of some commercial trade ships contaminated numerous oyster and fin-fish populations, making them unsafe for consumption.
Non-native species have earned their negative reputation as invader species because of their great success at survival. A non-native animal may survive better than a native one, not only because it has no natural enemies in the new environment, but because it grows more quickly or in less-favorable conditions than natives. This causes increased competition for resources by native species.
Exotics typically crowd out sunlight and nutrients from native species; consume food sources that native species would eat, leaving insufficient food; crowd out other native species and consume eggs, young, and adults of native species; jeopardize animals dependent on native vegetation; overgrow, leading to excessive populations, which in turn leads to decay and excess oxygen depletion and aquatic organism losses; crowd out navigation channels; clog water intakes and machinery; and occupy safe or supportive habitat, leaving a reduced amount of habitat for natives.
Losses for humans from exotic species include higher transportation and navigation costs; reduced food availability for subsistence fishing; reduced commercial and sport-fishing; and reduced water quality or supply.
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