Alien species

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Yellow star thistle, widespread alien species in the USA. @ Peggy Greb
Ecology Theory (main)

An alien species is an organism that finds itself in a new geographic location or habitat. Many of these species arrive in the new location due to inadvertent human activities such as shipping or agriculture, although many are purposefully introduced for food cultivation or for attempts (usually misguided) at ecological intervention . Some of these species reproduce and flourish in their new environment. This success is due largely to the absence of natural predators and parasites, and this may allow alien species to out-compete native species, usurping a habitat previously populated by native species within their niches. Alien species that have negative effects on native species, or cause economic or environmental problems are generally referred to as "invasive" species. An alien species is most commonly thought of as a macroscopic or obviously visible plant or animal; however, some of the most significant alien species are micro-organisms which have conveyed diseases or parasites to distant lands, where there may be insufficient endemic defenses or immunities.

Flora examples

Invasive plants such as Parthenium hysterophorus
can reduce agricultural yields and alter ecosystems

Garlic Mustard, Alliaria petiolata, is a widespread invasive herb in the eastern forests of North America; this plant has a had a widespread historic distribution in the Old World, ranging from the British Isles eastward through continental Europe, Russia and Asia Minor and southward to North Africa. (Cox, 2004) This North American taxon bears closest DNA relationship to its relatives identified in native Scottish forms, and is thought to have been transported to North America in the 1860s. (Meekins et al., 2001) In the North American venue, there are fewer natural insect and fungal consumers of A. petiolata, so that this invader has flourished; correspondingly, its presence in the new ecosystem is deleterious to various Pieris butterfly species, who mistakenly lay eggs on the invading plant, rather than its native hosts.A petiolata, which has toxins that are damaging to the larval stage of the butterflies.

In many cases the conduct of humanitarian programmes lead to introduction of alien species which in turn can depress the long term capability of a region to sustain its own agricultural productivity. For example the United Nations has documented that certain food aid tactics in Ethiopia have led to wide scale introductions of the alien species Congress Weed, Parthenium hysterophorus, imported as seed admixed with food grain; in turn the propagation of this invasive species has led not only to massive ecosystem disturbance but also to reduced Ethiopian agricultural productivity..(United Nations Environment Programme. 2006) Congress Weed is an invasive grass that is toxic to most livestock, and has also led to epidemic level reduction of crop productivity and adverse human health impacts. (Horn of Africa Regional Environmental Centre and Network. 2008) Alien species in Africa have led to significant declines in agricultural productivity and been contributory to food shortages in many parts of Africa.

Fauna examples

In certain cases faunal species have been introduced to increase agricultural or fishery productivity. In the case of the Signal Crayfish, Pacifastacus leniusculus,an aquatic feeding machine has been unleashed in western Europe and many parts of Asia, under the purpose of increasing the crayfish harvest beyond that available from native species biomass formation. The experiment was wildly successful from a food production perspective; however, the outcome has been an ecological disaster from the standpoint of numerous prey species, whose populations have been severely diminished by the presence of the aggressive Signal Crayfish. (Nystrom. 1999)

The Eastern Gray Squirrel, Sciurus carolinensis, has few natural predators within the United Kingdom, so that its population expanded swiftly after introduction to the British Isles. This species has out-competed the native Red Squirrel which is smaller and lest athletic; hence the Red Squirrel population is now limited to narrow geographic areas that include the Isle of Anglesey and parts of northern Scotland.

The Eastern Gray Squirrel also has a higher capability for fat storage, which aids it in survival relative to the Red Squirrel. An ancillary issue is that the Eastern Gray Squirrel is resistant to Parapoxvirus, which is typically fatal to the Red Squirrel, even though the Eastern Gray Squirrel is a carrier of this disease. Finally the native Red Squirrel is less tolerant of the extensive broadleaf woodland habitat loss and habitat fragmentation that has occurred in the United Kingdom than the introduced Eastern Gray Squirrel.

Regulatory framework

A number of national governments have enacted legislation or executive governance orders to respond to ecological threats posed by alien species. For example in the USA, Federal Executive Order 13112 issued in the year 1999 addresses the need to control "an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health" (U.S. Council on Environmental Quality. 1999) Individual states of the USA, notably California, have also enacted rules to regulate interstate and intrastate transport of certain alien species, major motivations being the protection of agricultural productivity as well as ecosystem integrity.

In the United Kingdom parliament has passed a law that seeks to prevent the introduction of any faunal species not naturally occurring in the natural environment, and also establishes a list of both flora and fauna, that have been introduced previously and have shown to be invasive. (United Kingdom Parliament. 1981)

Genetic pollution

Painted hunting dog, endangered by genetic pollution and habitat fragmentation. Source: C.Michael Hogan

A special case of harm to wild species arises from the introduction of domesticated or introduced taxa that can interbreed with with their wild counterparts. (Ellstrand. 2001) An example of this phenomenon is the Painted Hunting Dog, Lycaon pictus, on the African continent, where introduction of domesticated dogs into the habitat of L. pictus, combined with extensive habitat fragmentation have rendered this canid endangered. (Hogan. 2009)


  • George W. Cox. 2004. Alien species and evolution, Island Press, 377 pages
  • J.E. Meekins, H.E. Ballard Jr., and B.C. McCarthy. 2001. Genetic variation and molecular biogeography of a North American invasive plant (Alliaria petiolaria, Brasicaceae). International Journal of Plant Science 162:161-169
  • P.Nystrom. 1999. Ecological impact of introduced and native crayfish on freshwater communities: European perspectives. In Gherardi, F. and Holdich, D.M. (eds.) Crustacean Issues 11: Crayfish in Europe as Alien Species (How to make the best of a bad situation?) A.A. Balkema, Rotterdam, Netherlands: 63-85
  • United States Council on Environmental Quality, Executive Order 13112, signed by William J. Clinton, Washington DC
  • United Kingdom Wildlife and Countryside Act. 1981, London, England
  • Norman C. Ellstrand, 2001. "When Transgenes Wander, Should We Worry?" Plant Physiol, Vol.125, pp.1543-1545
  • C. Michael Hogan. 2009. Painted Hunting Dog: Lycaon pictus,, ed. N. Stromberg
  • United Nations Environment Programme. 2006. Africa environment outlook 2: our environment, our wealth. 542 pages
  • Horn of Africa Regional Environmental Centre and Network. 2008. [ The distributions of parthenium weed (Parthenium hysterophorus L. Asteraceae) and some of its socio-economic and ecological impacts in the Central Rift Valley, Adami Tulu-Jido Kombolcha Woreda; Ethiopia (accessed Feb. 4, 2010)


C. Michael Hogan (2011). Alien species. ed. Mark MacGinley. Encyclopedia of Earth. National Council for Science and the Environment. Washington DC Retrieved from