Introduced species

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Sweet clover (Melilotus sp.), introduced and naturalized to the U.S. from Eurasia as a forage and cover crop.

An introduced species (also known as naturalized species or exotic species) is an organism that is not indigenous to a given place or area and instead has been accidentally or deliberately transported to this new location by human activity. Introduced species can often be damaging to the ecosystem it is introduced to. 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 (having the same or similar meanings) with introduced are acclimatized, adventive, alien, bioinvasive, exotic, escaped, feral, foreign, invasive, non-native, naturalized, immigrant, non-indigenous, and xenobiotic. Nonetheless, distinctions can and should be made between some of these terms.

In the broadest and most widely used 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, some sources add to that basic definition: "...and are now reproducing in the wild",[1] which removes from consideration as introduced all of those species raised or grown in gardens or farms that do not survive without tending by people. With respect to plants, these latter are in this case defined as either ornamental or cultivated plants.

The following definition from the United States 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".[2]

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".[3]

Although some argue that "invasive" is a loaded word and harm is difficult to define,[1] 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 Spec ies"
INTRODUCED (broad definition)
Established in the wild
(narrow definition)
All others
not listed*

*Not listed in any "official" source as a pest species

The nature of introductions

By definition, a species is considered “introduced” when its transport into an area outside of its native range is human mediated. Introductions by humans can be described as either intentional or accidental. Intentional introductions have been motivated by individuals or groups who believe that the newly introduced species will be in some way beneficial to humans in its new location. Unintentional or accidental introductions are most often a byproduct of human movements, and are thus unbound to human motivations. Subsequent range expansion of introduced species may or may not involve human activity.

Intentional introductions

Species that humans intentionally transport to new regions can subsequently become successfully established in two ways. In the first case, organisms are purposely released for establishment in the wild. It is sometimes difficult to predict whether a species will become established upon release, and if not initially successful, humans have made repeated introductions to improve the probability that the species will survive and eventually reproduce in the wild. In these cases it is clear that the introduction is directly facilitated by human desires.

In the second case, species intentionally transported into a new region may escape from captive or cultivated populations and subsequently establish independent breeding populations. Escaped organisms are included in this category because their initial transport to a new region is human motivated.

Perhaps the most common motivation for introducing a species into a new place is that of economic gain. Examples of species introduced for the purposes of benefiting agriculture, aquaculture or other economic activities are widespread.[4] Eurasian carp was first introduced to the United States as a potential food source. The apple snail was released in Southeast Asia with the intent that it be used as a protein source, and subsequently to places like HawaiTemplate:Okinai to establish a food industry. In Alaska, foxes were introduced to many islands to create new populations for the fur trade. The timber industry promoted the introduction of Monterey Pine (Pinus radiata) from California to Australia and New Zealand as a commercial timber crop. These examples represent only a small subsample of species that have been moved by humans for economic interests.

Introductions have also been important in supporting recreation activities or otherwise increasing human enjoyment. Numerous fish and game animals have been introduced for the purposes of sport fishing and hunting. The introduced amphibian (Ambystoma tigrinum) that threatens the endemic California salamander (Ambystoma californiense) was introduced to California as a source of bait for fishermen.[5] Pet animals have also been frequently transported into new areas by humans, and their escapes have resulted in several successful introductions, such as those of feral cats and parrots.

Many plants have been introduced with the intent of aesthetically improving public recreation areas or private properties. The introduced Norway Maple for example occupies a prominent status in many of Canada's parks.[6] The transport of ornamental plants for landscaping use has and continues to be a source of many introductions. Some of these species have escaped horticultural control and become invasive. Notable examples include water hyacinth, salt cedar, and purple loosestrife.

In other cases, species have been translocated for reasons of “cultural nostalgia,” which refers to instances in which humans who have migrated to new regions have intentionally brought with them familiar organisms. Famous examples include the introduction of starlings to North America by Englishman 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. Yet another prominent example is the introduction of the European rabbit to Australia by one Thomas Austin, a British landowner who had the rabbits released on his estate in Victoria because he missed hunting them. A more recent example is the introduction of the wall lizard to North America by a Cincinnati boy, George Rau, in the 1950s.

Intentional introductions have also been undertaken with the aim of ameliorating environmental problems. A number of fast spreading plants such as Garlic Mustard and kudzu have been introduced as a means of erosion control. Other species have been introduced as biological control agents to control invasive species and involves the purposeful introduction of a natural enemy of the target species with the intention of reducing its numbers or controlling its spread.

A special case of introduction is the reintroduction of a species that has become locally endangered or extinct, done in the interests of conservation. Examples of successful reintroductions include wolves to Yellowstone National Park in the U.S., and the Red kite to parts of England and Scotland. Introductions or translocations of species have also been proposed in the interest of genetic conservation, which advocates the introduction of new individuals into genetically depauperate populations of endangered or threatened species.[7]

The above examples highlight the intent of humans to introduce species as a means of incurring some benefit. While these benefits have in some cases been realized, introductions have also resulted in unforeseen costs, particularly when introduced species take on characteristics of invasive species.

Accidental introductions

Unintentional introductions occur when species are transported by human vectors. For example, three species of rat (the Black, Norway and Polynesian) have spread to most of the world as hitchhikers on ships. There are also numerous examples of marine organisms being transported in ballast water, one being the zebra mussel. Over 200 species have been introduced to the San Francisco Bay in this manner making it the most heavily invaded estuary in the world.[8] Increasing rates of human travel are providing accelerating opportunities for species to be accidentally transported into areas in which they are not considered native.

Introduced plants and algae

Many non-native plants have been introduced into new territories, initially as either ornamental plants or for erosion control, stock feed, or forestry. Whether 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.

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 introduced 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.

Dandelions are also introduced species to North America.

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 km². 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.

Japanese knotweed grows profusely in many nations. Human beings introduced it into many places in the 19th century. It is a source of resveratrol, a dietary supplement.

Introduced animals

Male Phasianus colchicus (Common Pheasant), a widespread introduction

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. The starling had been introduced previously into Ohio and had failed to survive.

Other examples of introduced animals include the gypsy moth in eastern North America, the zebra mussel and alewife in the Great Lakes, the Canada Goose and Gray Squirrel in Europe, the Muskrat in Europe and Asia, 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, dogs, goats, fish, pigs and cats. It is also hard to believe that something as common as a dandelion is an alien species that was imported here at some point in time.

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.

The field of island restoration has developed as a field of conservation biology and ecological restoration, a great deal of which deals with the eradication of introduced species.

New Zealand

In New Zealand the largest commercial crop is Pinus radiata, the Monterey Pine from California, which grows better in New Zealand than in California. However, the pine forests are also occupied by deer from North America and Europe and by possums from Australia. All are exotic species and all have thrived in the New Zealand environment. The pines are seen as beneficial while the deer and possums are regarded as serious pests.

Common gorse, originally a hedge plant in Scotland, was introduced to New Zealand for the same purpose. Like the radiata pine, it has shown a favour to its new climate and is regarded as a noxious plant which threatens to obliterate native plants in much of the country and is hence routinely eradicated, though it can also provide a nursery environment for native plants to reestablish themselves.

Rabbits, introduced as a food source by sailors in the 1800s, have become a severe nuisance to farmers, notably in South Island. The myxomatosis virus was illegally imported and illegally released but it had little lasting effect upon the rabbit population other than to make it more resistant to the virus.

Rats, brought either by the second population group to arrive in New Zealand (the Maori) or by Europeans have had a devastating effect upon native birdlife, particularly as many New Zealand birds are flightless. Feral cats and dogs which were originally brought as pets are also known to kill large numbers of birds. A recent (2006) study in South Island has shown that even domestic cats with a ready supply of food from their owners may kill hundreds of birds in a year, including natives.

Sparrows, which were brought to control insects upon the introduced grain crops, have displaced native birds as have rainbow lorikeets and cockatoos (both from Australia) which fly free around areas west of Auckland City such as the Waitakere Ranges.

In much of the New Zealand the Australian black swan has effectively eliminated the existence of the previously introduced mute swan.

Two notable varieties of spiders have also been introduced: the white tail spider and the black widow spider. Both may have arrived inside shipments of fruit. Prior to this the only spider (and the only poisonous animal) dangerous to humans was the native katapo which is very similar to the black widow and which is known to successfully interbreed with the more aggressive North American variety.

See also


  1. 1.0 1.1 Carlton, James T. 2002. Introduced Species in U.S. Coastal Waters. Pew Oceans Commission.
  2. U.S. Environmental Protection Agency. Mid-Atlantic Integrated Assessment. September 2003. Introduced species. Web site at US EPA
  3. CEQ (1999). Web site page with Executive Order 13112 text.
  4. Naylor, R.L., S.L. Williams, and D.R. Strong. 2001. Aquaculture—a gateway for exotic species. Science 294: 1655-56.
  5. Riley, SPD, H.B. Shaffer, S.R. Voss, B.M. Fitzpatrick. Hybridization between a rare, native tiger salamander and its introduced congener. 2003. Ecological Applications 13: 1263–1275.
  6. Foster, J. and A. Sandberg. Friends or foe? Invasive species and public green space in Toronto. 2004. The Geographical Review 94: 178-198.
  7. Moritz, Craig. 1999. Conservation units and translocations: Strategies for conserving evolutionary processes. Hereditas 130: 217-228.
  8. Cohen, A.N. And J.T. Carlton. 1998. Accelerating invasion rate in a highly invaded estuary. Science 279: 555-558.

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