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Invasive Species

Invasive Species


An invasive species is an exotic species (one that has been deliberately or accidentally introduced into a habitat it would not normally populate) that thrives in its new environment, disrupting the natural ecosystem. The majority of exotic species have been introduced unintentionally. Hitchhiking organisms such as seeds or insects attach to peoples shoes, clothes, or luggage when they travel. Most of the time, the exotic species cannot survive in its new environment. Changes in climate, resources, and competition simply do not favor survival, and the organism eventually dies out. Occasionally, the introduced species ends up being invasive, out-competing the natural habitat for resources, displacing native flora and fauna, and wreaking economic havoc on a community.

When a new species is introduced into an ecosystem, there are four different interspecific interactions that can occur. Interspecific interactions are those that occur between two different species living in the same community. These interactions can have positive, negative, or neutral effects on the involved organisms. One interspecific interaction is predation. This is when one species, the predator, feeds on the other (the prey). This interaction benefits only the predator. When an introduced species is a predator, it may become invasive if it can out-compete the native predators. This competition is the second interspecific interaction. This interaction hurts both species involved. When two species are both competing for the same resources, neither will be as successful as they would be alone. The last two possible interactions, commensalism and mutualism, would not make an introduced species invasive. Commensalism is when one species benefits and the other is unaffected, and mutualism is when both species benefit from the interaction. Most of the time, an introduced species cannot compete with native populations and does not survive. Of the species that do become successful in their new environment, the majority of these organisms have no effect on the ecosystem. For example, the pheasant is a bird that was introduced to North America from Asia. These birds have had no impact on native species. The species is considered invasive when it can out-compete and displace other species already present in the ecosystem.

An introduced species must exhibit certain characteristics in order to become invasive. The organism must be able to reproduce in their new environment, out-compete the native populations, must not be susceptible to herbivores or diseases, especially if these types of organisms characterize the ecosystem to which they have been introduced, and must be able to survive in their new climate with the available resources.

When an introduced species becomes invasive, the economic damage can be considerable. In the United States alone, invasive species cause more than $123 billion in damages per year. It has been estimated that almost 70% of the organisms listed as threatened or endangered species by the International Union for the Conservation of Nature and Natural Resources have been classified as a result of invasive species. Over 50,000 of the approximately 750,000 species in the United States are exotic, with approximately 5,000 of these exotic species considered invasive (likely an underestimate). Invasive species not only displace native flora and fauna, they homogenize existing ecosystems, greatly reducing the number of available biological resources.

There are many examples of invasive species and the problems they cause. The water hyacinth was introduced into the United States from South Africa in 1884. This was an intentional introduction; travelers brought back the flower for its ornamental beauty. These flowers grow quite rapidly, and without any natural predators in their new environments, they quickly overpopulated their new environments. As a result, they clogged waterways, out-shaded natural vegetation, and displaced several native species. A well-known example in the southern United States is kudzu. This legume was introduced from Asia, where it is considered an ornamental vine. In the United States, it has taken over the land. It grows over anything in its path, including trees, shrubs, and even houses.

Many disease-causing organisms are invasive species. For example, the fungus Ophiostoma ulmi, the pathogen that causes Dutch elm disease, and the bark beetle, which carries the pathogen, were both introduced to the United States from Europe. They were both imported on infected wood, first the beetle in 1909, and then the fungus in 1930. The combination of these two organisms has caused the destruction of millions of elm trees. The chestnut blight fungus, Cryphonectria parasitica, was introduced into the United States from Asia on nursery plants in 1900. This fungus has caused the destruction of almost all of the eastern American chestnut trees. Both of these pathogens have caused great disruptions in forest ecosystems.

Starlings and English sparrows were both introduced intentionally to the northeastern United States from Europe in the 1800s. They can now be found just about anywhere in North America, and have displaced the native birds in many communities, caused significant crop damage, and contributed to the spread of certain swine diseases. Deer were introduced to Angel Island in the San Francisco Bay from the mainland in the early 1900s. This island was a game reserve with no natural deer predators. The deer population exploded and soon outgrew the meager food supply on the small island. People who visited the island felt sorry for the starving deer and fed them bits of their picnic lunches. Therefore, the deer survived and continued to multiply, despite their limited resources. The deer were eating the native grasses, tearing the leaves off of seedlings, and killing trees by eating bark. The deer had to be removed, a project that cost the State of California over $60,000. Sea lampreys from the North Atlantic Ocean were introduced to the United States through the Erie Canal in the 1860s, and again through the Welland Canal in 1921. These organisms have displaced the lake trout and whitefish from the Great Lakes, and have cost the United States and Canada over $10 million a year.

Argentine fire ants were introduced to the southern United States from a coffee shipment from Brazil in 1891. These organisms damage crops and disrupt ecosystems. They have been spreading steadily northward since their introduction. Examples of their destruction include the reduction of native ant species in one part of Texas from 15 to 5 species, and their killing of brown pelican hatchlings (a threatened species) in wildlife refuges. The Japanese beetle was introduced to the United States from Japan on a shipment of iris or azalea flowers in 1911. These beetles have caused the destruction of over 250 native plant species. Gypsy moths were once contained in a research facility on the east coast of the United States until they escaped in 1869. These moths have caused the destruction of entire forests by eating the leaves off of the trees, with damages estimated near $5 million.

More recently, the zebra mussel was imported from the Caspian Sea to the United States via a cargo ship that emptied its ballast water into the St. Lawrence Seaway in the mid-1980s. By 1993, zebra mussels could be found as far south as New Orleans. Zebra mussels compete with native shellfish and fish for food and shelter. By attaching to water outflow channels in huge numbers, they also clog waterways.

The United States is not the only nation to suffer the effects of invasive species. Well-intentioned Europeans, for the purpose of providing food and income to natives, introduced the Nile perch into Lake Victoria in East Africa. Lake Victoria was the home of many native fish, including cichlids. These fish feed on detritus and plants at the bottom of the lake. The addition of the Nile perch introduced a new predator, which fed on the cichlids. Eventually, all of the cichlids disappeared, and once this happened, the perch had no food. The perch ended up dying off as well, leaving the native people in an even worse situation, with nothing but a lake overgrown with detritus and plants. In 1859, an Australian released two dozen English rabbits for hunting. Without any natural predators, the rabbit population grew to over 40,000 in only six years. The rabbits displaced many natural animals, including kangaroos. The Australians tried building a 2,000-mile (32.2 kilometer) long fence to contain the rabbits, but some had traveled past the fence before its completion. The myxoma virus was introduced to the rabbit population in 1951 in hopes of controlling its growth. Recently, a new population of rabbits that are resistant to this virus has begun growing, and the problem is far from being solved.

As the use of genetic engineering technologies increases, the threat of a new type of invasive species


Ballast An area of a ship filled with water to help stabilize the ship.

Biocontrol agent An organism that can itself be used to control unwanted organisms, usually by feeding on the unwanted species.

Community In ecology, a community is an assemblage of populations of different species that occur together in the same place and at the same time.

Detritus Dead organic matter.

Ecosystem All of the organisms in a biological community interacting with the physical environment.

Fauna Animals or animal life.

Flora Plants or plant life.

Homogenize To create an area made entirely of same or similar things.

Myxoma virus A fatal virus that infects rabbits.

Pathogen A disease-causing agent.

emerges. Genetically engineered organisms, if introduced into the wild, could also alter ecosystems in many ways. Genetically engineered plants have acquired such traits as herbicide resistance, pest resistance, faster growth, and tolerance of extreme climatic changes. If these engineered organisms were accidentally released, they would have a competitive advantage over native species, and could become invasive.

In 1999, then President Clinton signed an executive order to address the growing problem of invasive species in the United States. This order created an Invasive Species Council that will develop a proposal to minimize the detrimental effects caused by, as well as to prevent, the introduction of new invasive species. The agenda also included the reintroduction of native species into their original habitats.

In the United States, thousands of inspectors stationed at ports of entry examine imported agricultural products to deter the entry of insects.



Belkin, Shimshon and Rita R. Colwell. Pathogens in the Marine Environment. New York: Springer, 2005.

Boersma, P. Dee, S.H. Reichard, and A.N. Van Buren, eds. Invasive Species in the Pacific Northwest. Seattle: University of Washington Press, 2006.

May, Suellen. Invasive Terrestrial Plants. New York: Chelsea House Publications, 2006.

Jennifer McGrath

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