Introduced species
Overview
 
An introduced species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

— or neozoon, alien, exotic, non-indigenous, or non-native species, or simply an introduction, is a species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 living outside its indigenous
Indigenous (ecology)
In biogeography, a species is defined as native to a given region or ecosystem if its presence in that region is the result of only natural processes, with no human intervention. Every natural organism has its own natural range of distribution in which it is regarded as native...

 or native
Native plant
Native plant is a term to describe plants endemic or naturalized to a given area in geologic time.This includes plants that have developed, occur naturally, or existed for many years in an area...

 distributional range
Range (biology)
In biology, the range or distribution of a species is the geographical area within which that species can be found. Within that range, dispersion is variation in local density.The term is often qualified:...

, and has arrived in an ecosystem or plant community by human
Human
Humans are the only living species in the Homo genus...

 activity, either deliberate or accidental. For examples of various introduced plant species, see the separate article List of introduced species.

Some introduced species are damaging to the ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

 they are introduced into.
Encyclopedia
An introduced species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

— or neozoon, alien, exotic, non-indigenous, or non-native species, or simply an introduction, is a species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 living outside its indigenous
Indigenous (ecology)
In biogeography, a species is defined as native to a given region or ecosystem if its presence in that region is the result of only natural processes, with no human intervention. Every natural organism has its own natural range of distribution in which it is regarded as native...

 or native
Native plant
Native plant is a term to describe plants endemic or naturalized to a given area in geologic time.This includes plants that have developed, occur naturally, or existed for many years in an area...

 distributional range
Range (biology)
In biology, the range or distribution of a species is the geographical area within which that species can be found. Within that range, dispersion is variation in local density.The term is often qualified:...

, and has arrived in an ecosystem or plant community by human
Human
Humans are the only living species in the Homo genus...

 activity, either deliberate or accidental. For examples of various introduced plant species, see the separate article List of introduced species.

Some introduced species are damaging to the ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

 they are introduced into. Others have no negative effect. Some can be beneficial: to humans—as an alternative to pesticides in agriculture for example; and to ecosystems—as in New Zealand where some introduced species of flora from North America may increase biodiversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...

 and bioproductivity. Introduced species and their effects on natural
Nature
Nature, in the broadest sense, is equivalent to the natural world, physical world, or material world. "Nature" refers to the phenomena of the physical world, and also to life in general...

 environments can be a complex and controversial subject. It receives ongoing: research by scientists; and discussion by government officials, environmentalists, politicians, farmers, and the public.

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, naturalized, immigrant, 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 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
United States Environmental Protection Agency
The U.S. Environmental Protection Agency is an agency of the federal government of the United States charged with protecting human health and the environment, by writing and enforcing regulations based on laws passed by Congress...

, 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 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 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, 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.

An invasive species
Invasive species
"Invasive species", or invasive exotics, is a nomenclature term and categorization phrase used for flora and fauna, and for specific restoration-preservation processes in native habitats, with several definitions....

 is one that has been introduced and become a pest and noxious weed in its new location, spreading (invading) by natural means. The term is used to imply both a sense of urgency, and potential or actual 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, 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 usually not practical to list as undesirable or outright ban all non-native species (although the State of Hawaii
Hawaii
Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...

 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.

In Great Britain
Great Britain
Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...

 the Wildlife and Countryside Act 1981
Wildlife and Countryside Act 1981
The Wildlife and Countryside Act 1981 is an Act of Parliament in the United Kingdom and was implemented to comply with the Directive 2009/147/EC on the conservation of wild birds...

 prevents the introduction of any animal not naturally occurring in the wild, and also any of a list of both animals or plants which have been introduced previously and have proved to be invasive.
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

Nature of introductions

By definition
Invasion biology terminology
The need for a clearly defined and consistent invasion biology terminology has been acknowledged by many sources. Invasive species, or invasive exotics, is a nomenclature term and categorization phrase used for flora and fauna, and for specific restoration-preservation processes in native habitats...

, 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 herrera. 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
Agriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...

, aquaculture
Aquaculture
Aquaculture, also known as aquafarming, is the farming of aquatic organisms such as fish, crustaceans, molluscs and aquatic plants. Aquaculture involves cultivating freshwater and saltwater populations under controlled conditions, and can be contrasted with commercial fishing, which is the...

 or other economic activities are widespread. 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
Norway Maple
Acer platanoides is a species of maple native to eastern and central Europe and southwest Asia, from France east to Russia, north to southern Scandinavia and southeast to northern Iran....

 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
Invasive
Invasive may refer to:*A military invasion*An invasive species*An invasive medical procedure*The invasively progressive spread of disease from one organ in the body to another, especially in reference to cancer...

. Notable examples include water hyacinth
Water hyacinth
The seven species of water hyacinth comprise the genus Eichhornia. Water hyacinth are a free-floating perennial aquatic plant native to tropical and sub-tropical South America. With broad, thick, glossy, ovate leaves, water hyacinth may rise above the surface of the water as much as 1 meter in...

, salt cedar, and purple loosestrife
Purple loosestrife
Lythrum salicaria is a flowering plant belonging to the family Lythraceae, native to Europe, Asia, northwest Africa, and southeastern Australia. It should not be confused with other plants sharing the name loosestrife that are members of the family Primulaceae...

.

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
Eugene Schieffelin
Eugene Schieffelin belonged to the and the New York Zoological Society. He was responsible for introducing the starling to North America.-Starling release:...

, 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
European Rabbit
The European Rabbit or Common Rabbit is a species of rabbit native to south west Europe and north west Africa . It has been widely introduced elsewhere often with devastating effects on local biodiversity...

 to Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

 by one Thomas Austin
Thomas Austin
Thomas Austin was an English settler in Australia who is generally credited with the introduction of rabbits into Australia in 1859.- Personal life :...

, 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
Wall lizard
The common wall lizard is a species of lizard with a large distribution in Europe and well-established introduced populations in North America, where it is also called the European wall lizard. It can grow to about in total length.-Identification:The common wall lizard is a small, thin lizard...

 to North America by a Cincinnati boy, George Rau, around 1950 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
Garlic Mustard
Garlic mustard is a biennial flowering plant in the Mustard family, Brassicaceae. It is native to Europe, western and central Asia, and northwestern Africa, from Morocco, Iberia and the British Isles, north to northern Scandinavia, and east to northern India and western China...

 and kudzu
Kudzu
Kudzu is a plant in the genus Pueraria in the pea family Fabaceae, subfamily Faboideae. It is a climbing, coiling, and trailing vine native to southern Japan and southeast China. Its name comes from the Japanese name for the plant, . It is a weed that climbs over trees or shrubs and grows so...

 have been introduced as a means of erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

 control. Other species have been introduced as biological control agents to control invasive species
Invasive species
"Invasive species", or invasive exotics, is a nomenclature term and categorization phrase used for flora and fauna, and for specific restoration-preservation processes in native habitats, with several definitions....

 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
Yellowstone National Park
Yellowstone National Park, established by the U.S. Congress and signed into law by President Ulysses S. Grant on March 1, 1872, is a national park located primarily in the U.S. state of Wyoming, although it also extends into Montana and Idaho...

 in the U.S., and the Red kite
Red Kite
The Red Kite is a medium-large bird of prey in the family Accipitridae, which also includes many other diurnal raptors such as eagles, buzzards, and harriers. The species is currently endemic to the Western Palearctic region in Europe and northwest Africa, though formerly also occurred just...

 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.

Non-native species can become such a common part of an environment, culture, and even diet that little thought is given to their geographic origin. For example, soybeans, kiwi fruit, wheat and all livestock except the llama
Llama
The llama is a South American camelid, widely used as a meat and pack animal by Andean cultures since pre-Hispanic times....

, American Bison and the turkey
Turkey (bird)
A turkey is a large bird in the genus Meleagris. One species, Meleagris gallopavo, commonly known as the Wild Turkey, is native to the forests of North America. The domestic turkey is a descendant of this species...

 are non-native species to North America. Collectively, non-native crops and livestock comprise 98% of US food. These and other benefits from non-natives are so vast that, according to the Congressional Research Service, they probably exceed the costs.

Accidental introductions

Unintentional introductions occur when species are transported by human vectors. For example, three species of rat
Rat
Rats are various medium-sized, long-tailed rodents of the superfamily Muroidea. "True rats" are members of the genus Rattus, the most important of which to humans are the black rat, Rattus rattus, and the brown rat, Rattus norvegicus...

 (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
Zebra mussel
The zebra mussel, Dreissena polymorpha, is a small freshwater mussel. This species was originally native to the lakes of southeast Russia being first described in 1769 by a German zoologist Peter Simon Pallas in the Ural, Volga and Dnieper rivers. They are still found nearby, as Pontic and Caspian...

. Over 200 species have been introduced to the San Francisco Bay
San Francisco Bay
San Francisco Bay is a shallow, productive estuary through which water draining from approximately forty percent of California, flowing in the Sacramento and San Joaquin rivers from the Sierra Nevada mountains, enters the Pacific Ocean...

 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 are not considered native. There is also the accidental release of the Africanized honey bees (AHB), known colloquially as "killer bees" or Africanized bee
Africanized bee
Africanized honey bees, known colloquially as "killer bees", are a hybrid variety of the European honeybee , generated by a man-made breeding of the African honey bee, A. m. scutellata, with various European honey bees such as the Italian bee A. m. ligustica and A. m. iberiensis. These bees are far...

 to Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...

 in 1957 and the Asian carps to the United States.

Introduced plants

Many non-native plants have been introduced into new territories, initially as either ornamental plant
Ornamental plant
Ornamental plants are plants that are grown for decorative purposes in gardens and landscape design projects, as house plants, for cut flowers and specimen display...

s or for erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

 control, stock feed, or forestry. Whether an exotic will become an invasive species
Invasive species
"Invasive species", or invasive exotics, is a nomenclature term and categorization phrase used for flora and fauna, and for specific restoration-preservation processes in native habitats, with several definitions....

 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 China, and have been carried to much of the populated world. Tomatoes are native to the Andes. Squash (pumpkins), maize(corn), 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
Interspecific competition
Interspecific competition, in ecology, is a form of competition in which individuals of different species compete for the same resource in an ecosystem...

 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
Seaweed
Seaweed is a loose, colloquial term encompassing macroscopic, multicellular, benthic marine algae. The term includes some members of the red, brown and green algae...

 Caulerpa taxifolia
Caulerpa taxifolia
Caulerpa taxifolia is a species of seaweed, an alga of the genus Caulerpa. Native to the Indian Ocean, it has been widely used ornamentally in aquariums. The alga has a stem which spreads horizontally just above the seafloor, and from this stem grow vertical fern-like pinnae, whose blades are flat...

. Caulerpa was first observed in the Mediterranean Sea
Mediterranean Sea
The Mediterranean Sea is a sea connected to the Atlantic Ocean surrounded by the Mediterranean region and almost completely enclosed by land: on the north by Anatolia and Europe, on the south by North Africa, and on the east by the Levant...

 in 1984, off the coast of Monaco
Monaco
Monaco , officially the Principality of Monaco , is a sovereign city state on the French Riviera. It is bordered on three sides by its neighbour, France, and its centre is about from Italy. Its area is with a population of 35,986 as of 2011 and is the most densely populated country in the...

. By 1997, it had covered some 50 km². It has a strong potential to overgrow natural biotope
Biotope
Biotope is an area of uniform environmental conditions providing a living place for a specific assemblage of plants and animals. Biotope is almost synonymous with the term habitat, but while the subject of a habitat is a species or a population, the subject of a biotope is a biological community.It...

s, and represents a major risk for sublittoral ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

s. The origin of the alga in the Mediterranean was thought to be either as a migration through the Suez Canal
Suez Canal
The Suez Canal , also known by the nickname "The Highway to India", is an artificial sea-level waterway in Egypt, connecting the Mediterranean Sea and the Red Sea. Opened in November 1869 after 10 years of construction work, it allows water transportation between Europe and Asia without navigation...

 from the Red Sea, or as an accidental introduction from an aquarium.

Japanese knotweed
Japanese knotweed
Japanese Knotweed is a large, herbaceous perennial plant, native to eastern Asia in Japan, China and Korea...

 grows profusely in many nations. Human beings introduced it into many places in the 19th century. It is a source of resveratrol
Resveratrol
Resveratrol is a stilbenoid, a type of natural phenol, and a phytoalexin produced naturally by several plants when under attack by pathogens such as bacteria or fungi....

, a dietary supplement.

Introduced animals

Examples of introduced animals include the gypsy moth
Gypsy moth
The gypsy moth, Lymantria dispar, is a moth in the family Lymantriidae of Eurasian origin. Originally ranging from Europe to Asia, it was introduced to North America in the late 1860s and has been expanding its range ever since...

 in eastern North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

, the zebra mussel
Zebra mussel
The zebra mussel, Dreissena polymorpha, is a small freshwater mussel. This species was originally native to the lakes of southeast Russia being first described in 1769 by a German zoologist Peter Simon Pallas in the Ural, Volga and Dnieper rivers. They are still found nearby, as Pontic and Caspian...

 and alewife
Alewife
The alewife is a species of herring. There are anadromous and landlocked forms. The landlocked form is also called a sawbelly or mooneye...

 in the Great Lakes
Great Lakes
The Great Lakes are a collection of freshwater lakes located in northeastern North America, on the Canada – United States border. Consisting of Lakes Superior, Michigan, Huron, Erie, and Ontario, they form the largest group of freshwater lakes on Earth by total surface, coming in second by volume...

, the Canada Goose
Canada Goose
The Canada Goose is a wild goose belonging to the genus Branta, which is native to arctic and temperate regions of North America, having a black head and neck, white patches on the face, and a brownish-gray body....

 and Gray Squirrel
Eastern Gray Squirrel
The eastern gray squirrel is a tree squirrel in the genus Sciurus native to the eastern and midwestern United States, and to the southerly portions of the eastern provinces of Canada...

 in Europe, the Muskrat
Muskrat
The muskrat , the only species in genus Ondatra, is a medium-sized semi-aquatic rodent native to North America, and introduced in parts of Europe, Asia, and South America. The muskrat is found in wetlands and is a very successful animal over a wide range of climates and habitats...

 in Europe and Asia, the Cane Toad
Cane Toad
The Cane Toad , also known as the Giant Neotropical Toad or Marine Toad, is a large, terrestrial true toad which is native to Central and South America, but has been introduced to various islands throughout Oceania and the Caribbean...

 and Red fox
Red Fox
The red fox is the largest of the true foxes, as well as being the most geographically spread member of the Carnivora, being distributed across the entire northern hemisphere from the Arctic Circle to North Africa, Central America, and the steppes of Asia...

 in Australia, Nutria in North America, Eurasia, and Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...

, and the Common Brushtail Possum
Common Brushtail Possum
The Common Brushtail Possum is a nocturnal, semi-arboreal marsupial of the family Phalangeridae, it is native to Australia, and the largest of the possums.Like most possums, the Common Brushtail is nocturnal...

 in New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

.

Most commonly introduced species

Some species, such as the Brown Rat
Brown Rat
The brown rat, common rat, sewer rat, Hanover rat, Norway rat, Brown Norway rat, Norwegian rat, or wharf rat is one of the best known and most common rats....

, House Sparrow
House Sparrow
The House Sparrow is a bird of the sparrow family Passeridae, found in most parts of the world. One of about 25 species in the genus Passer, the House Sparrow occurs naturally in most of Europe, the Mediterranean region, and much of Asia...

, Ring-necked Pheasant
Common Pheasant
The Common Pheasant , is a bird in the pheasant family . It is native to Georgia and has been widely introduced elsewhere as a game bird. In parts of its range, namely in places where none of its relatives occur such as in Europe , it is simply known as the "pheasant"...

 and European Starling
European Starling
The Common Starling , also known as the European Starling or just Starling, is a passerine bird in the family Sturnidae.This species of starling is native to most of temperate Europe and western Asia...

, have been introduced very widely. In addition there are some agricultural
Agriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...

 and pet
Pet
A pet is a household animal kept for companionship and a person's enjoyment, as opposed to wild animals or to livestock, laboratory animals, working animals or sport animals, which are kept for economic or productive reasons. The most popular pets are noted for their loyal or playful...

 species that frequently become feral
Feral
A feral organism is one that has changed from being domesticated to being wild or untamed. In the case of plants it is a movement from cultivated to uncultivated or controlled to volunteer. The introduction of feral animals or plants to their non-native regions, like any introduced species, may...

; these include rabbit
Rabbit
Rabbits are small mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world...

s, dog
Dog
The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...

s, goat
Goat
The domestic goat is a subspecies of goat domesticated from the wild goat of southwest Asia and Eastern Europe. The goat is a member of the Bovidae family and is closely related to the sheep as both are in the goat-antelope subfamily Caprinae. There are over three hundred distinct breeds of...

s, fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

, pig
Pig
A pig is any of the animals in the genus Sus, within the Suidae family of even-toed ungulates. Pigs include the domestic pig, its ancestor the wild boar, and several other wild relatives...

s and cat
Cat
The cat , also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests...

s.

Invasive exotic diseases

History is rife with the spread of exotic diseases, such as the introduction of smallpox
Smallpox
Smallpox was an infectious disease unique to humans, caused by either of two virus variants, Variola major and Variola minor. The disease is also known by the Latin names Variola or Variola vera, which is a derivative of the Latin varius, meaning "spotted", or varus, meaning "pimple"...

 into the indigenous peoples of the Americas
Indigenous peoples of the Americas
The indigenous peoples of the Americas are the pre-Columbian inhabitants of North and South America, their descendants and other ethnic groups who are identified with those peoples. Indigenous peoples are known in Canada as Aboriginal peoples, and in the United States as Native Americans...

 by the Spanish, where it obliterated entire populations of indigenous civilizations before they were ever even seen by Europeans.

Problematic exotic disease introductions in the past century or so include the chestnut blight
Chestnut blight
The pathogenic fungus Cryphonectria parasitica is a member of the ascomycota category, and is the main cause of chestnut blight, a devastating disease of the American chestnut tree that caused a mass extinction in the early 1900s of this once plentiful tree from its historic range in the eastern...

 which has almost eliminated the American Chestnut
American Chestnut
The American Chestnut is a large, deciduous tree of the beech family native to eastern North America. Before the species was devastated by the chestnut blight, a fungal disease, it was one of the most important forest trees throughout its range...

 tree from its forest habitat, and Dutch elm disease
Dutch elm disease
Dutch elm disease is a disease caused by a member of the sac fungi category, affecting elm trees which is spread by the elm bark beetle. Although believed to be originally native to Asia, the disease has been accidentally introduced into America and Europe, where it has devastated native...

, which has severely reduced the American Elm
American Elm
Ulmus americana, generally known as the American Elm or, less commonly, as the White Elm or Water Elm, is a species native to eastern North America, occurring from Nova Scotia west to Alberta and Montana, and south to Florida and central Texas. The American elm is an extremely hardy tree that can...

 trees in forests and cities.

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
Ecological niche
In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...

.

An additional problem is that birds native to small islands may have become flightless because of 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
Island restoration
The ecological restoration of islands, or island restoration, is the application of the principles of ecological restoration to islands and island groups. Islands, due to their isolation, are home to many of the world's endemic species, as well as important breeding grounds for seabirds and some...

 has developed as a field of conservation biology
Conservation biology
Conservation biology is the scientific study of the nature and status of Earth's biodiversity with the aim of protecting species, their habitats, and ecosystems from excessive rates of extinction...

 and ecological restoration, a great deal of which deals with the eradication of introduced species.

New Zealand

In New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

 the largest commercial crop is Pinus radiata, the native Californian
California native plants
California native plants are plants that existed in California prior to the arrival of European explorers and colonists in the late 18th century...

 Monterey Pine tree, which grows as well in New Zealand as in California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

. However, the pine forests are also occupied by deer
Deer
Deer are the ruminant mammals forming the family Cervidae. Species in the Cervidae family include white-tailed deer, elk, moose, red deer, reindeer, fallow deer, roe deer and chital. Male deer of all species and female reindeer grow and shed new antlers each year...

 from North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

 and Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

 and by possum
Possum
A possum is any of about 70 small to medium-sized arboreal marsupial species native to Australia, New Guinea, and Sulawesi .Possums are quadrupedal diprotodont marsupials with long tails...

s from Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

. 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
Gorse
Gorse, furze, furse or whin is a genus of about 20 plant species of thorny evergreen shrubs in the subfamily Faboideae of the pea family Fabaceae, native to western Europe and northwest Africa, with the majority of species in Iberia.Gorse is closely related to the brooms, and like them, has green...

, originally a hedge plant in Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

, was introduced to New Zealand for the same purpose. Like the Monterey pine, it has shown a favour to its new climate. It is, however, 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.

Rabbit
Rabbit
Rabbits are small mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world...

s, introduced as a food source by sailors in the 1800s, have become a severe nuisance to farmers, notably in South Island. The myxomatosis
Myxomatosis
Myxomatosis is a disease that affects rabbits and is caused by the Myxoma virus. It was first observed in Uruguay in laboratory rabbits in the late 19th century. It was introduced into Australia in 1950 in an attempt to control the rabbit population...

 virus
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

 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 Māori (the second humans to arrive in New Zealand) or later by Europeans, have had a devastating effect upon the 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
GRAIN
GRAIN is a small international non-profit organisation that works to support small farmers and social movements in their struggles for community-controlled and biodiversity-based food systems. Our support takes the form of independent research and analysis, networking at local, regional and...

 crops, have displaced native birds as have Rainbow Lorikeet
Rainbow Lorikeet
The Rainbow Lorikeet, is a species of Australasian parrot found in Australia, eastern Indonesia , Papua New Guinea, New Caledonia, Solomon Islands and Vanuatu. In Australia, it is common along the eastern seaboard, from Queensland to South Australia and northwest Tasmania...

s and cockatoos (both from Australia) which fly free around areas west of Auckland
Auckland
The Auckland metropolitan area , in the North Island of New Zealand, is the largest and most populous urban area in the country with residents, percent of the country's population. Auckland also has the largest Polynesian population of any city in the world...

 City such as the Waitakere Ranges
Waitakere Ranges
The Waitakere Ranges are a chain of hills in the Auckland metropolitan area, generally running approximately 25 km from north to south, 25 km west of central Auckland, New Zealand. The maximum elevation within the ranges is 474 m...

.

In much of the New Zealand the Australian black swan
Black Swan
The Black Swan is a large waterbird, a species of swan, which breeds mainly in the southeast and southwest regions of Australia. The species was hunted to extinction in New Zealand, but later reintroduced. Within Australia they are nomadic, with erratic migration patterns dependent upon climatic...

 has effectively eliminated the existence of the previously introduced mute swan
Mute Swan
The Mute Swan is a species of swan, and thus a member of the duck, goose and swan family Anatidae. It is native to much of Europe and Asia, and the far north of Africa. It is also an introduced species in North America, Australasia and southern Africa. The name 'mute' derives from it being less...

.

Two notable varieties of spiders have also been introduced: the white tail spider and the Red back 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 katipo
Katipo
Latrodectus katipo, the katipo, is an endangered species of spider native to New Zealand. A member of the genus Latrodectus, it is related to the Australian redback spider, and the North American black widow spiders. The species is venomous to humans, capable of delivering a comparatively dangerous...

 which is very similar to the Red Back and which is known to successfully interbreed with the more aggressive Australian variety.

Genetic pollution

Purebred naturally evolved region specific wild species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 can be seriously threatened with extinction
Extinction
In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...

  through the process of genetic pollution
Genetic pollution
Genetic pollution is a controversial term for uncontrolled gene flow into wild populations. This gene flow is undesirable according to some environmentalists and conservationists, including groups such as Greenpeace, TRAFFIC, and GeneWatch UK.-Usage:...

 i.e. uncontrolled hybridization, introgression
Introgression
Introgression, also known as introgressive hybridization, in genetics is the movement of a gene from one species into the gene pool of another by the repeated backcrossing of an interspecific hybrid with one of its parent species...

 and genetic swamping which leads to homogenization or replacement of local genotypes as a result of either a numerical and/or fitness
Fitness (biology)
Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment...

 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 is not always apparent from morphological
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....

 (outward appearance) observations alone. Some degree of gene flow
Gene flow
In population genetics, gene flow is the transfer of alleles of genes from one population to another.Migration into or out of a population may be responsible for a marked change in allele frequencies...

 may be a normal, evolutionarily constructive process, and all constellations of gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

s and genotypes cannot be preserved however, hybridization with or without introgression may, nevertheless, threaten a rare species' existence.

See also

  • Avian range expansion
    Avian range expansion
    Avian range expansion describes how birds expand their habitat. Because of the activities of birdwatchers these range expansions are well documented. Throughout the last century a number of birds have expanded their range...

  • Biodiversity
    Biodiversity
    Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...

  • Biological dispersal
    Biological dispersal
    Biological dispersal refers to species movement away from an existing population or away from the parent organism. Through simply moving from one habitat patch to another, the dispersal of an individual has consequences not only for individual fitness, but also for population dynamics, population...

  • Burmese Pythons in Florida
    Burmese Pythons in Florida
    The Burmese python, a large, non-venomous constrictor native to Southeast Asia, has been sighted and captured with increasing frequency in the Everglades and is generally labeled an invasive species...

  • Genetic pollution
    Genetic pollution
    Genetic pollution is a controversial term for uncontrolled gene flow into wild populations. This gene flow is undesirable according to some environmentalists and conservationists, including groups such as Greenpeace, TRAFFIC, and GeneWatch UK.-Usage:...

  • Invasion biology terminology
    Invasion biology terminology
    The need for a clearly defined and consistent invasion biology terminology has been acknowledged by many sources. Invasive species, or invasive exotics, is a nomenclature term and categorization phrase used for flora and fauna, and for specific restoration-preservation processes in native habitats...

  • Invasive species
    Invasive species
    "Invasive species", or invasive exotics, is a nomenclature term and categorization phrase used for flora and fauna, and for specific restoration-preservation processes in native habitats, with several definitions....

  • List of introduced bird species
  • List of introduced species
  • Introduced mammals on seabird breeding islands
    Introduced mammals on seabird breeding islands
    Seabirds include some of the most threatened taxa anywhere in the world. For example, of extant albatross species, 82% are listed as threatened, endangered, or critically endangered by the International Union for Conservation of Nature. The two leading threats to seabirds are accidental bycatch by...


External links

 
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