Introduced species
An introduced species (also called an invasive species, invasive exotic or invasive alien species) is an animal or plant accidentally or deliberately taken to an area where it is not native, by human activity. This article discusses the concept and gives some key examples; there is a longer list of introduced species in a separate article.
The term introduced species is applied to both plants and animals. It does not refer to natural colonisation.
Introduced species often become serious pests, disturbing the natural ecosystems, displacing indigenous and endemic (native) species.
Because of this many are culled to reduce their numbers or even eradicate them.
Note that as well as introductions across continents, introductions also occur within continents, including within continental nations such as the United States and Australia, and these are no less potentially damaging than intercontinental introductions. Some introductions are ancient: for example the Rabbit and Fallow Deer, and both
species of resident rat, are all introduced species in the United Kingdom but have been living in the wild there for hundreds of years.
A very troublesome marine species in southern Europe is the seaweed Caulerpa taxifolia. Caulerpa was first observed in the Mediterranean Sea in 1984, off the coast of Monaco. By 1997, it had covered some 5000 hectares. It has a strong potential to overgrow natural biotopes, and represents a major risk for sublittoral ecosystems. The origin of the alga in the Mediterranean was thought to be either as a migration through the Suez Canal from the Red Sea, or as an accidental introduction from an aquarium.
Other outstanding examples of introduced animals include the gypsy moth in eastern North America, zebra mussel and alewife in the Great Lakes; and the Common Brushtail and Common Ringtail possums in New Zealand.
Problematic exotic disease introductions in the past century or so include the chestnut blight which has virtually extinguished the American chestnut, and Dutch elm disease, which has severely damaged the American elm.
There are several classic accounts of introduced species that have been causing problems for many decades. The sea lamprey began to make its way up into the Great Lakes Region when the St. Lawrence Seaway was completed in 1959, devastating the lake trout fishing industry. It continues to be a largely-controlled problem today, but costs millions in lamprecides, traps, physical barriers, and other control methods.
Rabbits were introduced into Australia with colonists in the 1800s and their devastation is ongoing in spite of the famous rabbit fences that were built along thousands of miles of territory with the futile intention of keeping them out.
The often unsuccessful use of biological control provides another historical perspecitive of the introduced species problem. When rats overwhelmed seaports and became crop pests during the 1800s in some islands in the Pacific, mongoose were introduced to control them. The mongoose preferred to eat native species that were easier to catch than the invasive rats, and became invaders themselves. As a result, modern day biological control is used only if extensive studies find that the biocontrol species will not have a negative effect on native populations.
Economically valuable crops and livestock may be introduced species, although they can have larger effects on ecosystems than even the most successful of unintentionally introduces species.
Some species, such as the Brown Rat, House Sparrow, Ring-necked Pheasant and European Starling have been introduced very widely. In addition there are some agricultural and pet species that frequently become feral; these include rabbits, goats, pigs and cats.
A special case of introduction is the attempt at the reintroduction of a species that has become locally endangered or extinct, done in the interests of conservation. Such reintroductions are not necessarily ecologically unproblematic, especially if the reintroduced stock differ genetically from a remnant population. Examples of successful reintroductions include wolves to the Yellowstone National Park in the US, and the Red Kite to parts of England.Terminology
Typical introduction
The great majority of introductions have involved the transfer of European species to New World and Australasian countries by settlers; in some countries, Acclimatization Societies were set up, particularly in the late nineteenth century, to promote this process. However there have also been introductions between New World countries, and of New World species to Old World countries.
Asian Tiger Mosquito (Aedes albopictus), a species introduced to the USA, biting a human.Introduced plants
Many non-native plants have been introduced into new territories, initially as either ornamentals or for erosion control, stock feed, or forestry. Whether or not an exotic will become invasive is seldom understood in the beginning, and many non-native ornamentals languish in the trade for years before suddenly naturalizing and becoming invasive.Introduced animals
One of the most egregious examples of introducing an exotic animal was perpetrated by one Eugene Scheiffer, a lover of the works of Shakespeare, who wanted to introduce all of the birds mentioned in Shakespeare's plays into the United States. He deliberately released eighty Starlings into Central Park in New York City in 1890, and another forty in 1891. Ironically, the Starling had been introduced previously into Ohio and had failed to survive.Invasive Exotic Diseases
History is rife with the spread of exotic diseases, such as the introduction of smallpox into the Americas, where it obliterated entire Native American civilizations before they were ever even seen by Europeans.Historical Perspective
Although it is assumed that invasive species have been a problem since man has been around to carry them, modern invasive species science began with the work of Charles Elton called The Ecology of Invasions, which was published in 1958. The next ground-breaking work dealing with the principles of invasions was Island Biogeography and Conservation Practice by Simberloff and Abele in 1976.The most commonly introduced species
Reintroduction