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An introduced species is a plant or animal) that is not native to the place or area where it is considered introduced and instead has been accidentally or deliberately transported to the new location by human activity. This article discusses the concept and gives numerous examples of introduced species. A list of introduced species is given in a separate article.
The terminology associated with introduced species is presently in flux for a variety of reasons. Other terms that are used sometimes interchangeably with introduced are: acclimatized, adventive, alien, bioinvasive, exotic, escaped, foreign, injurious, invasive, non-native, naturalized, immigrant, non-indigenous, or xenobiotic. Nonetheless, distinctions can and should be made between some of these terms. In the broadest sense, an introduced species is synonymous with non-native and therefore applies as well to most garden and farm organisms—these adequately fit the basic definition given above. However, most sources add to that basic definition: "...and are now reproducing in the wild" (Carlton, 2002), which removes from consideration as introduced all of those species raised or grown in gardens or farms that do not survive without tending by humans. With respect to plants, these latter are perhaps best defined as either ornamental or cultivated plants. The following definition from the U.S. Environmental Protection Agency, although perhaps lacking ecological sophistication, is more typical: introduced species are .."[s]pecies that have become able to survive and reproduce outside the habitats where they evolved or spread naturally" (EPA, 2003). And at IUFRO (FAO, undated): "An established species not native to the ecosystem, region or country" — wherein 'established' conveys the ability to reproduce in the wild.
There is valid disagreement as to whether the term invasive species is exactly synonymous with introduced species. A species that is invasive is one that has been introduced and become a pest in its new location, spreading (invading) by natural means. The term is used to imply both a sense of urgency and actual or potential harm. For example, U.S. Executive Order 13112 (1999) defines "invasive species" as "an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health" (CEQ, 1999). Although some argue that "invasive" is a loaded word and harm is difficult to define (Carlton, 2002), the fact of the matter is that organisms have and continue to be introduced to areas where they are not native, sometimes with, usually without, much regard to the harm that could result. Ecological purists would argue that all non-natives capable of becoming established in the wild are harmful where they are introduced. From a regulatory perspective, it is neither desirable nor practical to simply list as undesirable or outright ban all non-native species (although the State of Hawaii has adopted an approach that comes close to this). Regulations require a definitional distinction between non-natives that are deemed especially onerous and all others. Introduced pest species that are officially listed as invasive, best fit the definition of an invasive species.
| NATIVE | NON-NATIVE "Introduced" in broadest sense |
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| INDIGENOUS or ENDEMIC |
CULTIVATED ORNAMENTAL LIVESTOCK |
Established in the wild INTRODUCED (common definition) |
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| INVASIVE (pest) |
All others not listed* |
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