| Sweet clover (
Melilotus sp.), introduced and naturalized to the U.S. from Eurasia as a forage and cover crop, supports insect biodiversity.
An introduced species (also known as an exotic species) is a organism that is not native to the place or area where it is considered introduced and instead has been accidentally or deliberately transported to the new location by human activity. This article discusses the concept and gives numerous examples of introduced species. A list of introduced species is given in a separate article.
The terminology associated with introduced species is presently in flux for a variety of reasons. Other terms that are used sometimes interchangeably with introduced are acclimatized, adventive, alien, bioinvasive, exotic, escaped, feral, foreign, injurious, invasive, non-native, naturalized, immigrant, non-indigenous, and xenobiotic. Nonetheless, distinctions can and should be made between some of these terms. In the broadest sense, an introduced species is synonymous with non-native and therefore applies as well to most garden and farm organisms — these adequately fit the basic definition given above. However, most sources add to that basic definition: "...and are now reproducing in the wild" (Carlton, 2002), which removes from consideration as introduced all of those species raised or grown in gardens or farms that do not survive without tending by humans. With respect to plants, these latter are perhaps best defined as either ornamental or cultivated plants. The following definition from the U.S. Environmental Protection Agency, although perhaps lacking ecological sophistication, is more typical: introduced species are .."[s]pecies that have become able to survive and reproduce outside the habitats where they evolved or spread naturally" (EPA, 2003). And at IUFRO (FAO, undated): "An established species not native to the ecosystem, region or country" — wherein "established" conveys the ability to reproduce in the wild.
There is valid disagreement as to whether the term invasive species is exactly synonymous with introduced species. A species that is invasive is one that has been introduced and become a pest in its new location, spreading (invading) by natural means. The term is used to imply both a sense of urgency and actual or potential harm. For example, U.S. Executive Order 13112 (1999) defines "invasive species" as "an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health" (CEQ, 1999). Although some argue that "invasive" is a loaded word and harm is difficult to define (Carlton, 2002), the fact of the matter is that organisms have and continue to be introduced to areas where they are not native, sometimes with, usually without, much regard to the harm that could result. Ecological purists would argue that all non-natives capable of becoming established in the wild are harmful where they are introduced. From a regulatory perspective, it is neither desirable nor practical to simply list as undesirable or outright ban all non-native species (although the State of Hawaii has adopted an approach that comes close to this). Regulations require a definitional distinction between non-natives that are deemed especially onerous and all others. Introduced pest species that are officially listed as invasive, best fit the definition of an invasive species.
Table of terms related to "Introduced Species"
|NATIVE ||NON-NATIVE |
"Introduced" in broadest sense
|Established in the wild |
|All others |
* Not listed in any "official" source as a pest species
The nature of introductions
Peaches, for example, originated in Persia, and have been carried to much of the populated world. Tomatoes are native to the Andes. Squash (pumpkins), maize, and tobacco are native to the Americas, but were carried back to the old world. Many introduced species require continued human intervention to survive in the new environment. Others may become feral, but do not seriously compete with natives. They simply increase the biodiversity of the area.
Several 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.
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 the 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.
Introduced plants and algae
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.
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 50 kmē. 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.
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.
Other outstanding examples of introduced animals include the gypsy moth in eastern North America; the zebra mussel and alewife in the Great Lakes; and the common brushtail and common ringtail possums in New Zealand.
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.
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.
The most commonly introduced 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 Yellowstone National Park in the U.S., and the red kite to parts of England.
Introduced species on islands
Perhaps the best place to study problems associated with introduced species is on islands. Depending upon the isolation (how far an island is located from continental biotas), native island biological communities may be poorly adapted to the threat posed by exotic introductions. Often this can mean that no natural predator of an introduced species is present, and the non-native spreads uncontrollably into open or occupied niche.
An additional problem is that birds native to small islands may have become flightless due to the absence of predators prior to introductions, and cannot readily escape danger. The tendency of rails in particular to evolve flightless forms on islands has led to the disproportionate number of extinctions in that family.
- Carlton, James T. 2002. Introduced Species in U.S. Coastal Waters. Pew Oceans Commission. Available on WWW at Pew Oceans Commission (http://www.pewoceans.org/reports/introduced_species.pdf) (.PDF file)
- CEQ (1999). Web site page with Executive Order 13112 (http://ceq.eh.doe.gov/nepa/regs/eos/eo13112.html) text.
- IUFRO (undated web site). Introduced species. Reference definition. Multilingual glossary, Forest genetic resources, IUFRO, FAO. Web site at IUFRO (http://www.iufro.org/iufro/silvavoc/glossary/29_0en.html)
- U.S. Envronmental Protection Agency. Mid-Atlantic Integrated Assessment. September 2003. Introduced species. Web site at US EPA (http://www.epa.gov/maia/html/intro-species.html)
- Introduced species in the British Isles (http://www.introduced-species.co.uk/)