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.
Terminology
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] However, introduction of a species outside its native range is
often all that is required to be qualified as an "introduced species" such that one can distinguish between introduced species
that may only occur in cultivation, under domestication or captivity whereas other become established outside their native range
and reproduce without human assistance. Such species might be termed "naturalized", "established", "wild non-native species", or
"invasive". The transition from introduction, to establishment and invasion has been described by Richardson
et al 2000 in the context of plants. Introduced species are essentially "non-native" species. Invasive species are those
introduced species that spread-widely or quickly, and cause harm, be that to the environment, biodiversity, human health, other valued resources or the economy. There have been calls from scientists to consider
a species "invasive" only in terms of their spread and reproduction rather than the harm they may cause Colautti
and MacIsaac 2004.
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.
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 (broad definition) |
INDIGENOUS
or ENDEMIC |
CULTIVATED
and
LIVESTOCK |
Established in the wild
INTRODUCED
(narrow definition) |
INVASIVE
(pest) |
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 Hawaiʻi 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 Schieffelin, a lover
of the works of Shakespeare, who, it is rumoured, 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 after a family vacation to Italy.[7]
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.[8]
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.[9] 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, but 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
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, the Cane Toad and Red fox in Australia, and the Common Brushtail
Possum 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.
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 first humans 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.
Genetic pollution
-
Purebred naturally evolved region specific wild species can be threatened with
extinction in a big way[10] through the process of Genetic Pollution i.e.
uncontrolled hybridization, introgression and
Genetic swamping which leads to homogenization or replacement of local genotypes as a result of
either a numerical and/or fitness advantage of introduced plant or animal[11]. Nonnative species can bring about a form of extinction of
native plants and animals by hybridization and introgression either through purposeful introduction by humans or through habitat
modification, bringing previously isolated species into contact. These phenomena can be especially detrimental for rare species
coming into contact with more abundant ones where the abundant ones can interbreed with them swamping the entire rarer gene pool
creating hybrids thus driving the entire original purebred native stock to complete extinction. Attention has to be focused on
the extent of this under appreciated problem that is not always apparent from morphological (outward appearance) observations alone. Some degree of gene flow may be a normal, evolutionarily constructive process, and all constellations of genes and genotypes cannot be preserved however, hybridization with or without
introgression may, nevertheless, threaten a rare species' existence[12][13].
See also
References
- ^ a b Carlton, James T. 2002. Introduced Species in U.S. Coastal Waters.
Pew Oceans Commission.
- ^ U.S. Environmental Protection Agency. Mid-Atlantic Integrated Assessment.
September 2003. Introduced species. Web site at US EPA
- ^ CEQ (1999). Web site page with Executive Order 13112 text.
- ^ Naylor, R.L., S.L. Williams, and D.R. Strong. 2001. Aquaculture—a gateway
for exotic species. Science 294: 1655-56.
- ^ 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.
- ^ Foster, J. and A. Sandberg. Friends or foe? Invasive species and public
green space in Toronto. 2004. The Geographical Review 94: 178-198.
- ^ Deichsel, G. & Gist, D. H. On the Origin of the Common Wall Lizards Podarcis muralis (Reptilia: Lacertidae) in Cincinnati, Ohio
- ^ Moritz, Craig. 1999. Conservation units and translocations: Strategies for
conserving evolutionary processes. Hereditas 130: 217-228.
- ^ Cohen, A.N. And J.T. Carlton. 1998. Accelerating invasion rate in a highly
invaded estuary. Science 279: 555-558.
- ^ Hybridization and Introgression; Extinctions; from "The evolutionary impact of invasive
species; by H. A. Mooney and E. E. Cleland" Proc Natl Acad Sci U S A. 2001 May 8; 98(10): 5446–5451. doi: 10.1073/pnas.091093398.
Proc Natl Acad Sci U S A, v.98(10); May 8, 2001, The National Academy of Sciences
- ^ Glossary: definitions from the following publication: Aubry, C., R. Shoal and V. Erickson. 2005.
Grass cultivars: their origins, development, and use on national forests and grasslands in the Pacific Northwest. USDA Forest
Service. 44 pages, plus appendices.; Native Seed Network (NSN), Institute for Applied Ecology, 563 SW Jefferson Ave, Corvallis,
OR 97333, USA
- ^ EXTINCTION BY
HYBRIDIZATION AND INTROGRESSION; by Judith M. Rhymer , Department of Wildlife Ecology, University of Maine, Orono, Maine 04469,
USA; and Daniel Simberloff, Department of Biological Science, Florida State University, Tallahassee, Florida 32306, USA; Annual
Review of Ecology and Systematics, November 1996, Vol. 27, Pages 83-109 (doi: 10.1146/annurev.ecolsys.27.1.83), [1]
- ^ Genetic Pollution from Farm Forestry using eucalypt species and hybrids; A report for the
RIRDC/L&WA/FWPRDC; Joint Venture Agroforestry Program; by Brad M. Potts, Robert C. Barbour, Andrew B. Hingston; September
2001; RIRDC Publication No 01/114; RIRDC Project No CPF - 3A; ISBN 0 642 58336 6; ISSN 1440-6845; Australian Government, Rural
Industrial Research and Development Corporation
External links
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
