Exotic species, which are also known as alien species, invasive species, non-indigenous species, and bioinvaders, are species of plants or animals that are growing in a nonnative environment. Alien species have been moved by humans to areas outside of their native ranges. Once transported, they become removed from the predators, parasites, and diseases that kept them in balance in their native environments. As a result of the loss of these controls, they often become pests in the areas into which they are introduced.
Many plants and animals can disperse naturally into new habitats. The colonization of North America by cattle egrets from Africa, and the slow spread of the nine-banded armadillo into Texas and Louisiana occurred without human intervention. But the most destructive invasions are invariably those caused by human activity, whether deliberate or inadvertent.
The introduction of exotic species into the United States probably began with the first colonists that came ashore. When the Pilgrims landed at Plymouth in 1620, various non-native rodents, such as Rattus, and the house mouse, Mus musculus, almost certainly disembarked right along with them. The problem of exotic species became even more acute in the eighteenth and nineteenth centuries, as the United States entered world trade.
Nonnative species are not always harmful. Ninety-eight percent of the food grown in the United States come from nonnative species of wheat, barley, rice, cattle, and poultry. The nonnative honeybee is essential in growing plant crops, as well as generally benefiting flower pollination. Non-native species add $500 billion a year to the United States economy.
However, many nonnative species do enormous environmental damage. Research has shown that more than 40 percent of species on the U.S. Department of the Interior's lists of endangered or threatened species are at risk primarily because of nonindigenous species.
The economic damage caused by rats is huge. Rats alone do more than $19 billion of damage per year. Damage caused by alien insects cost $20 billion. Altogether, the more than 30,000 nonnative species in the United States cost the country $123 billion a year in economic losses, according to a June 12, 1999 report by Cornell University ecologists. In that report, David Pimentel of Cornell said that the United States has become the land of a billion rats.
Invading species can cause complex changes within the structure and function of their new ecosystem. Their presence can lead to the restructuring of established food webs, the importation of new diseases to the new surroundings, and competition with indigenous organisms for space and food. Other ecological changes may occur when the invading organisms reproduce with native species, possibly altering the gene pool. This may lead to hybridization and homogeneity, which reduces biodiversity, the primary element associated with an ecosystem's ability to adapt to natural or human-induced changes.
How Do They Get Here?
Introductions of nonnative species can be planned, incidental, accidental, or unintentional. They can also be caused by a natural disaster. Scientists have made several attempts to identify the possible pathways of introduction, with varied success.
The most common method of introduction into marine environments is through the ballast water of shipping vessels. A cargo ship floats high in the water and is very unstable when it is empty. To stabilize the ship, the crew fills the ballast tanks with water. When the tanks are filled, marine organisms are pumped in along with the water. Then, when the ballast water is discharged at the next port of call, exotic species can be introduced. Scientists estimate that as many as 3,000 alien species per day are transported around the world in the ballast water of ships.
Aquaculture, the cultivation of natural products of water such as fish, also introduces invading organisms. Although nonnative species can provide inexpensive food and sources of recreation for human communities, these same species can cause environmental damage if they are released or escape.
Extent of the Problem
Every state in the United States and nearly all communities have been affected by bioinvaders. However, two states have been especially hard hit— Hawaii and Florida—and for similar reasons. Both states are geographically isolated and both have a semitropical-to-tropical climate.
Hawaii has been geographically isolated from the rest of the world for millions of years. Because of this isolation, Hawaii originally had thousands of species that existed nowhere else on Earth. But it has suffered the highest rate of extinctions of any area of the United States and one of the highest rates anywhere in the world, with hundreds and possibly thousands of unique species already extinct. The tropical climate of Hawaii allows invasive plants and animals to thrive. Nonnative plants and animals frequently displace native species. Predation by nonnative rats, feral cats, dogs, and mongooses has led to the extinction of many species of birds. Habitat destruction by feral pigs has altered landscapes. To compound the problem, nonnative species are usually more aggressive at colonizing disturbed ground left behind by the feral pigs.
Like Hawaii, Florida has a subtropical-to-tropical climate that allows many plants and animals to thrive. The state is protected by ocean on three sides. On the fourth side, it is geographically isolated from the states further north by differences in climate. Because of this isolation, Florida is considered to have been somewhat species-poor, with many niches available for invasive species to colonize.
Florida now lays claim to 63 percent of the nonindigenous bird species, 25 percent of nonindigenous plants, 25 percent of land mammal species, and the largest number of established nonindigenous amphibian and reptilian species in the United States. Overall, approximately 42 percent of Florida's reptiles, 23 percent of its mammals, 22 percent of its amphibians, 16 percent of its fishes, 15 percent of its flora, and 5 percent of its birds are naturalized nonindigenous species.
Florida's nonindigenous species cause severe problems for the state's ecology, economy, and resource management. This is largely because of their impact on fishing and water sports, the degradation of wildlife habitat, the reduction of biological diversity, and the alteration of natural ecosystems.
Well Known Invaders
Thousands of invasive species worldwide are notorious for their distinctive habits, destructive potential, or ecological damage. Other invaders seem to be having little environmental impact. A few of the more well-known exotic species in the United States are discussed here.
African Clawed Frog.
The African Clawed frog, Xenopus laevis, was widely used in human pregnancy testing in the 1940s and 1950s, and as a result was shipped all over the world. The frog is native to southern Africa, but is now found around the world in suitable habitats, probably due to accidental or deliberate releases. It is voracious and prolific, preying on insect larvae, small fish, and tadpoles. It is a completely aquatic frog. The state of Washington prohibits importation of Xenopus, and other states require a permit for possessing it. Xenopus remains a popular laboratory animal and is still available as a pet in some areas. The environmental damage caused by Xenopus is due to its voracious appetite and fecundity. The frog competes with native species for small fish, insect larvae, amphibians, and other prey. However, researchers disagree as to the extent of environmental damage it causes.
This small gecko (Hemidactylus turcicus ) is a native of the Mediterranean, and apparently first arrived in the United States on cargo ships unloading in New Orleans. Some areas may also have been colonized by escaped pets. The lizard is primarily nocturnal, preferring rocky walls near bright lights. It is found all along the Gulf Coast of the United States and as far west as Arizona. Because there are no other nocturnal, insectivorous lizards in areas the gecko have colonized, it does not appear to be causing any environmental damage.
Zebra Mussels (Dreissena polymorpha ) originated in the Balkans, Poland, and areas within the former Soviet Union. The species was introduced into the Great Lakes in the ballast water of ships in 1988. It has been spread by barge traffic into all the major East Coast rivers of the United States that are connected through canals to the Great Lakes. At first, the zebra mussel was believed to be intolerant of the warm water in the southern parts of the United States, but it is now established in the lower Mississippi River. Many of the small lakes near the Great Lakes are not connected to the Great Lakes by waterways, but they still have zebra mussels. In these cases, the mussels were probably transported on boats moved from lake to lake on trailers. They would not necessarily have to be moved from lake to lake on the same day, because in cool, humid conditions, zebra mussels can stay alive out of water for several days.
The economic impact of zebra mussels is due to their habit of colonizing the pipes that supply water to electric power plants and public water supplies. The colonies can become so dense that flow through the pipes is restricted. At one power plant in Michigan, zebra mussel densities were as high as 700,000 individuals per square meter (80,000 per square foot), and the diameters of pipes had been reduced by two-thirds at some Michigan water-treatment facilities.
Imported Fire Ants.
There are two species of imported fire ants, Solenopsis invicta, the red fire ant, and Solenopsis richteri, the black fire ant. S rich-teri was introduced first, but the much more aggressive red fire ant has displaced it and the native fire ant species across most of the south. Currently, S richteri is found only in a few areas of northeast Mississippi, northwest Alabama and southern Tennessee. The attempts to control these invaders have been controversial. Early efforts to eradicate the ants with the widespread application of pesticides severely damaged the environment and may have contributed to the spread of the insect. Recently, a small parasitic fly (Psuedacteon ) which offers promise as a fire ant control has been successfully bred, and test releases are underway. Techniques are now being developed to breed large numbers of the tiny flies for more widespread release.
The red fire ant is well established from North Carolina to eastern Texas, although the further extension of its range may be limited by geographical factors such as dry summers or cold winters. The two fire ant species inhabit approximately 93,120,000 hectares (23,010,4531 acres) in nine southern states, making them a familiar feature of life in these areas. There are probably about 10 billion colonies. The ants are feared because, when a nest is disturbed, the ants swarm over any nearby object, delivering multiple, painful stings to the intruder. However, the greatest economic impact of the imported fire ant comes from their attraction to electrical equipment. Short circuits, fires, and other damage can occur after ants colonize the equipment.
Reptiles and amphibians.
The native range of the giant toad (Bufo marinus ) extends from southern Texas, through Mexico and Central America, to Brazil in South America. This marine toad is widespread, occurring outside its natural range in places such as Australia, Fiji, Guam, Hawaii, Japan, New Guinea, the Philippines, the Solomon Islands, Tonga, several islands in the West Indies, and southern Florida. In 1955 an accidental release of 100 frogs led to an established population around Miami International Airport. This population has now spread throughout southern Florida and into the fringes of Everglades National Park through an extensive system of canals and drainage ditches.
Giant toads have replaced the native toad Bufo terrestris in much of its range. Marine toads have voracious appetites and eat small, moving or non-moving objects such as other toads, insects, snails, snakes, garbage, and dog food. If bitten by a pet, the toads release a milky bufotoxin from their parotid glands. Bufotoxin causes profuse salvation, twitching; vomiting; shallow breathing and collapse of the hind limbs. The toxin has been known to cause death in small mammals. The long-term environmental impact of this animal is unknown.
The parakeet or budgerigar (Melopsittacus undulatus ), commonly known as the budgie, is indigenous to interior Australia. Budgerigars are popular as caged birds throughout the world, but escaped or released birds have become established as wildlife in Florida. Another small parrot, the Monk parakeet (Myopsitta monachus ), is native to South America. It has established colonies in several cities around the United States, including one in Austin, Texas. This parrot is considered a pest in its native territory, causing substantial damage to grains and fruit crops. It is also a very aggressive bird, competing with other species for food sources. There are several reports of Monk parakeets attacking and killing other birds. The overall environmental impact of these birds is unknown at this time.
In the 1850s and 1860s, the weaver finch, Passer domesticus (also called the house sparrow) was deliberately introduced into North America at several different times and places. In 1853, a group of 100 birds from England were released in Brooklyn, New York, in a misguided attempt to control canker worms. Since its introduction, it has rapidly and aggressively colonized almost all of North America, displacing native birds by competing for nest sites and food. It is also hardy and fecund.
Where do we go from here?
Many scientists think that the spread of exotic species is one of the most serious, yet largely unrecognized, threats to our environment. Nonnative animal species cause enormous economic each year to crops, waterways, and natural environments in the United States. Safeguarding our natural heritage from alien and exotic species involves stopping additional introductions, the early detection and quick eradication of pests, integrated systems for the control and management of existing pests, and the restoration of native species and ecosystems.
Allen, Craig R., R. Scott Lutz, and Stephen Demarais. "Red Imported Fire Ant Impacts on Northern Bobwhite Populations." Ecological Applications 5, no. 3 (1995): 632-638.
Billington, Elizabeth T. Understanding Ecology. New York: Penguin Putnam Books for Young Readers, 1971.
Holt, Alan. "Hawaii's Reptilian Nightmare." World Conservation 28, no. 4 (1997): 31-32.
Kurdila, Julianne. "The Introduction of Exotic Species Into the United States: There Goes the Neighborhood." Boston College Environmental Affairs Law Review 16 (1995):95-118.
Lafferty, Kevin D., and Armand M. Kuris. "Biological Control of Marine Pests." Ecology 77, no. 7 (1996):1989-2000.
Laycock, George. The Alien Animals. New York: Natural History Press, 1966.
Long, John L. Introduced Birds of the World: The Worldwide History, Distribution, and Influence of Birds Introduced to New Environments. New York: Universe Books, 1981.
Nature Conservancy. America's Least Wanted: Alien Species Invasions of U. S. Ecosystems. Arlington, VA: The Nature Conservancy, 1996.
Roots, Clive. Animal Invaders. New York: Universe Books, 1976.
Silverstein, Alvin, and Virginia Silverstein. Animal Invaders. New York: Atheneum, 1974.
Simberlof, Daniel, Don C. Schmitz, and Tom C. Brown. Strangers in Paradise: Impact and Management of Nonindigenous Species in Florida. Washington, D.C.: Island Press, 1997.
Wachtel, S. P, and J. A. McNeely. "Oh Rats." International Wildlife 15, no. 1 (1985): 20-24.
Wilcove, David S., and M. J. Bean. The Big Kill: Declining Biodiversity in America's Lakes and Rivers. Washington, D.C.: Environmental Defense Fund, 1994.
Williamson, Mark H., and Bryan Griffiths. Biological Invasions. Norwell, MA: Kluwer Academic Publishers, 1997.
Pimentel, David, Lori Lach, Rodolfo Zuniga, and Doug Morrison. Environmental and Economic Costs Associated with Non-Indigenous Species in the United States. Cornell University. <http://www.news.cornell.edu/releases/Jan99/species_costs.html>.
Exotic species are organisms that are introduced to a region or ecosystem , often unintentionally, through human migration or trade. Some exotic species are useful to man, such as horses, goats, pigs, and edible plants including wheat and oats. These are examples of species that were brought to the Americas intentionally by European colonists. Other exotic species that were introduced accidentally such as the Mediterranean fruit fly (Ceratitis capitata ), Japanese beetle (Popillia japonica ), Africanized bees (Apis mellifera scutellata )(sometimes called killer bees), and Norway rat (Rattus norvegicus ) have become pests. Many exotic species, including most tropical fish, birds, and houseplants brought to colder climates, can survive only under continuous care. A few prove extremely adaptable and thrive in their new environment , sometime becoming invasive and out competing native species.
The federal government's Office of Technology Assessment has estimated that more than 2,000 plant species introduced from around the world currently live and thrive in the United States, and that 15 of these have caused more than $500 million worth of damage. Economic costs associated with exotic species include agricultural losses, damage to infrastructure, as when aquatic plants clog water intakes, and the costs of attempts to restore native species whose survival is endangered by introduced species .
Exotic species are most are most destructive when they adapt readily to their new environment and compete successfully with native species. Unfortunately there is no sure way to know which introduced species will become invasive. Often these plant and animals turn out to be better competitors because, unlike native species, they have no natural pests, parasites , diseases, or predators in their new home.
Purple loosestrife (Lythrum salicaria ) is an example of a plant that is kept in check in its native habitat , but is invasive in North America. This showy wetland plant with tall purple flowers may have arrived accidentally or been intentionally imported to North America as a garden ornamental. In its native northern Europe, resident beetles feed on its roots and leaves, keeping the loosestrife in check, so that it only appears occasionally and temporarily in disturbed sites. When loosestrife arrived in North America, the beetles, along with other pests and diseases, were left behind. Loosestrife has proven to be extremely adaptable and has become an aggressive weed across much of the American East and Midwest, often taking over an entire wetland and choking out other plants, eliminating much of the wetland biodiversity .
In addition to the competitive advantage of having few predators, invasive exotic plants and animals may have ecological characteristics that make them especially competitive. They can be hardy, adaptable to diverse habitat conditions, and able to thrive on a variety of food sources. They may reproduce rapidly, generating large numbers of seeds or young that spread quickly. If they are aggressive colonizers adapted to living in marginal habitat, introduced species can drive resident natives from their established sites and food sources, especially around the disturbed environments of human settlement.
This competitiveness has been a problem, for example, with house sparrows (Passer domesticus ). These birds were intentionally introduced from Europe to North America in 1850 to control insect pests. Their aggressive foraging and breeding habits often drive native sparrows, martins, and bluebirds from their nests, and today they are one of the most common birds in North America. Exotic plants can also become nuisance species when they crowd, shade, or out-propagate their native competitors. They can be extraordinarily effective colonists, spreading quickly and eliminating competition as they become established.
The list of species introduced to the Americas from Europe, Asia, and Africa is immense, as is the list of species that have made the reverse trip from the Americas to Europe, Asia, and Africa. Some notable examples are kudzu (Pueraria lobata ), the zebra mussel (Dreissena polymorpha ), Africanized bees Apis mellifera scutellata ), and Eurasian milfoil (Myriophyllum spicatum ).
Kudzu is a cultivated legume in Japan. It was intentionally brought to the southern United States for ground cover and erosion control. Fast growing and tenacious, kudzu quickly overwhelms houses, tangles in electric lines, and chokes out native vegetation.
Africanized "killer" bees were accidentally released in Brazil by a beekeeper in 1957. These aggressive insects have no more venom than standard honey bees (also an Old World import), but they attack more quickly and in great numbers. Breeding with resident bees and sometimes traveling with cargo shipments, Africanized bees have spread north from Brazil at a rate of up to 200 miles (322 km) each year and now threaten to invade commercially valuable fruit orchards and domestic bee hives in Texas and California.
The zebra mussel, accidentally introduced to the Great Lakes around 1985 presumably in ballast water dumped by ships arriving from Europe, colonizes any hard surface, including docks, industrial water intake pipes, and the shells of native bivalves. Each female zebra mussel can produce 50,000 eggs a year. Growing in masses with up to 70,000 individuals per square foot, these mussels clog pipes, suffocate native clams, and destroy breeding grounds for other aquatic animals. They are also voracious feeders, competing with fish and native mollusks for plankton and microscopic plants. The economy and environment of the Great Lakes now pay the price of zebra mussel infestations. Area industries spend hundreds of millions of dollars annually unclogging pipes and equipment, and commercial fishermen complain of decreased catches.
Eurasian milfoil is a common aquarium plant that can propagate from seeds or cuttings. A tiny section of stem and leaves accidentally introduced into a lake by a boat or boat trailer can grow into a huge mat covering an entire lake. When these mats have consumed all available nutrients in the lake, they die and rot. The rotting process robs fish and other aquatic animals of oxygen, causing them to die.
Exotic species have brought ecological disasters to every continent, but some of the most extreme cases have occurred on isolated islands where resident species have lost their defensive strategies. For example, rats, cats, dogs, and mongooses introduced by eighteenth century sailors have devastated populations of ground-breeding birds on Pacific islands. Rare flowers in Hawaii suffer from grazing goats and rooting pigs, both of which were brought to the island for food, but have escaped and established wild populations. Grazing sheep threaten delicate plants on ecologically fragile North Atlantic islands, while rats, cats, and dogs endanger northern seabird breeding colonies. Rabbits introduced into Australia overran parts of the island and wiped out hundreds of acres of grassland.
Humans have always carried plants and animals as they migrated from one region to another with little regard to the effects of these introductions might have on their new habitat. Many introduced species seem benign, useful, or pleasing to have around, making it difficult to predict which imports will become nuisance species. When an exotic plant or animal threatens human livelihoods or economic activity, as do kudzu, zebra mussels, and "killer" bees, people begin to seek ways to control these invaders.
Control efforts include using pesticides and herbicides, and introducing natural predators and parasites from the home range of the exotic plant or animal. For example, beetles that naturally prey on purple loosestrife have been experimentally introduced in American loosestrife populations. This deliberate introduction requires a great deal of care, research, and monitoring, however, to ensure that an even worse problem does not result, as happened with the house sparrow. Such solutions, and the time and money to develop them, are usually elusive and politically controversial, so in many cases effective control methods remain unavailable.
In 1999, President Bill Clinton signed the Executive Order on Invasive Species. This order established the Invasive Species Council to coordinate the activities of federal agencies, such as the Aquatic Nuisance Species Task Force, the Federal Interagency Committee for the Management of Noxious and Exotic Weeds, and the Committee on Environment and Natural Resources . The Invasive Species Council is responsible for the development of a National Invasive Species Management Plan. This plan is intended to be updated very two years to provide guidance and recommendations about the identification of pathways by which invasive species are introduced, and measures that can be taken for their control.
Non-profit environmental organizations across the globe are leading the effort for control of exotic species. For example, The Nature Conservancy has established Landscape Conservation Networks to address issues of land conservation that include invasive species management. These networks bring in outside experts and land conservation partners to develop innovative and cost effective means of controlling exotic species. The Great Lakes information Network, managed by the Great Lakes Commission based in Ann Arbor, Michigan, provides online access about environmental issues, including exotics species, in the Great Lakes region. The Florida Exotic Pest Plant Council, founded in 1984, provides funding to organizations that educate the public about the impacts of exotic invasive plants in the State of Florida.
[Mary Ann Cunningham and Marie H. Bundy ]
Cunningham, W. Understanding Our Environment: An Introduction. Dubuque, IA: William C. Brown, 1993.
Barrett, S. C. H. "Waterweed Invasions." Scientific American 261 (October 1989): 90–7.
Rendall, Jay. " Invasive Species". Imprint 7, no. 4 (1990): 1–8, 1990.
Walker, T. "Dreissena Disaster—Scientists Battle an Invasion of Zebra Mussels." Science News 139 (May 4, 1991): 282–84.