ALIEN SPECIES

The introduction of non-native species to an ecosystem is one of the major causes of decreased biodiversity. Termed alien species, they are also known as exotic, introduced, non-indigenous, or invasive species. As the names imply, these species do not belong to ecosystems in which they are either intentionally or unintentionally placed. They tend to disrupt the ecosystem's balance by multiplying rapidly. These specise are often plants, fishes, mollusks, crustaceans, algae, bacteria or viruses.




Many alien species are tranferred into marine ecosystems through the ballast water transported during commercial shipping operations. Ship ballast water may transport up to 3,000 species around the world every day. Alien species are often introduced into freshwater ecosystems such as estuaries, rivers, lakes and streams by humans discarding animals or plants formerly held in captivity. In some cases, species used as bait can invade freshwater ecosystems. Well-known invasive species include the Northern Snakehead fish, the Zebra mussel, the Sea Lamprey and the Asiatic Clam, Corbicula fluminea.



Effects on Humans



The introduction of an alien species is often responsible for an increase in predation and competition, habitat reduction, a variety of diseases, extinction of native plants or animals and genetic change in populations. Certain strains of cholera have been transported in ballast water ending up in oyster beds and infecting finfish destined for the dinner table.



Alien Species Aboard



Alien species are often transported to non-native habitats in the ballast of ships. The organisms are taken in when ships attempt to balance their load by letting water into their holding tanks. When they reach their destination, the ballast water is released and with it any organisms picked up earlier. Mollusks and other organisms whose habitat includes marine substrate also attach to the surfaces ocean-going vessels at the point of departure and then fall into the water at the destination.



Unintentional introductions



Aquarium plants and animals, such as the invasive algae Caulerpa, as well as ornamental plants like the Purple loosestrife are released innocently into waterways by humans. They quickly overgrow eventually choking native plants and interfering with the water flow of lakes, rivers, estuaries, and streams. Unwanted exotic fish, such as the Red Lionfish, Pterois volitans, have invaded the waters of the Southeastern United States. The introduction of this nonnative species may cause problems becaues of its poisonous spines that divers or swimmers may be unaware of and it may also pose a risk to native species through predation or competition.



Intentional Introductions



Alien species like the Cane toad have been introduced intentionally to reduce the number of a native species in the area. Unfortunately, this plan can backfire when the animal multiples quickly and takes over the habitat and beyond. In Hawaii, for example, the Mongoose has eliminated many species of birds but it was originally introduced to keep the rat population down. The kudzu plant that blankets much of the southern United States was the result of a program sponsored by the government to control erosion.