Everything about Introduced Species totally explained
An
introduced species (also known as
naturalized species or
exotic species) is an
organism that isn't
indigenous to a given location but instead has been accidentally or deliberately introduced to a new location by
human activity or by natural means. Introduced species sometimes are damaging to the ecosystem they're introduced into, others negatively affect agriculture and other human uses of natural resources or impact on the health of animals and humans. A
list of introduced species is given in a separate article. Introduced species and their effects on natural environments is a controversial subject and one that has gained much scrutiny by scientists, governments, farmers and others.
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, 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", which removes from consideration as
introduced all of those species raised or grown in gardens or farms that don't 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". However, introduction of a species outside its native range is often all that's 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's
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".
Although some argue that "invasive" is a loaded word and harm is difficult to define,
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 Hawaii 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. 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. 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's 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.
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.
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. Increasing rates of human travel are providing accelerating opportunities for species to be accidentally transported into areas in which they're 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 don't 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 example of introducing an exotic animal was carried out by 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.
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 can't 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
species can be threatened with
extinction in a big way through the process of
genetic pollution for example 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. 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 isn't 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 can't be preserved however, hybridization with or without introgression may, nevertheless, threaten a rare species' existence.
Further Information
Get more info on 'Introduced Species'.
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