from Grolier Multimedia Encyclopedia
An invader species is any organism that is introduced accidentally or intentionally into a new habitat or enters a new habitat on its own. These species are more commonly referred to as alien, exotic, or foreign species. They include a variety of insects, birds, mammals, plants, and microorganisms. When a species enters a new environment where conditions are unsuitable, the species will perish. Favorable conditions, such as a lack of predators, suitable temperature, and abundant food, may result in successful colonization by that species.
Some invader species have little if any adverse impact. The ring-necked pheasant, for example, spread across the United States after it was introduced from Mongolia in 1790 and again in 1881. Yet it poses no serious threat to native species. The pheasant is now an important game bird in many states.
In the vast majority of cases, however, alien species cause serious ecological and economic damage. Alien species often prey on or outcompete native species. The English sparrow and the starling were deliberately introduced into the United States in the mid-1800s. Since then they have spread throughout the country. Their introduction helped cause a decline in the populations of native songbirds such as the wren and bluebird, because they compete for nesting sites and food.
A well-known example of a damaging invader species is the Africanized honeybee, commonly called the killer bee. These bees were developed in South America in 1956 by a geneticist who was attempting to develop a hardy strain of honeybee to replace European honeybees, which fare poorly in the tropical climate. In 1957, 26 queens and their entourages were accidentally freed. Their offspring spread quickly outward, moving 320–485 km (200–300 mi) a year. Africanized honeybees attacked people and livestock. Unlike other bees, they pursue their victims, often in great numbers, stinging them repeatedly, sometimes killing them. The Africanized honeybee is also a less efficient pollinator and produces less honey than the European honeybee. Killer bees arrived in Texas in 1990 and have spread to Oklahoma, New Mexico, Arizona, Nevada, California, Arkansas, Louisiana, and Florida.
Introduced insects can pose serious problems to plant life as well. The emerald ash borer (EAB), Agrilus planipennis, an exotic beetle native to Asia, is thought to have arrived in North America during the 1990s in packing material aboard cargo ships or airplanes. The small, metallic-green adult beetles cause only minor damage by eating the foliage of ash trees. But the larvae feed on the inner bark, disrupting the tree's flow of water and nutrients. The EAB was first discovered in Michigan in 2002 and has since infested Ohio, Indiana, Illinois, Pennsylvania, and Ontario, Canada. To date the beetle has killed more than 20 million ash trees. Quarantines and community preparedness plans have been imposed to prevent the unintentional distribution of infested materials.
Australia is host to a number of damaging invader species, from mammals such as the rabbit, fox, and cat, to the cane toad. These animals have no natural predators and have been extremely damaging to the continent's unique fauna.
Invader species do not have to come from another continent to have a negative impact on the ecology. One example is the barred owl, which originated in the eastern United States and then spread across the grasslands to the Pacific Northwest. Its successful emigration across this largely treeless region was facilitated by trees planted around human settlements. The trees provided shelter and nesting sites as the bird extended its range. In the Pacific Northwest the barred owl harasses and preys on endangered spotted owls and usurps their nesting sites. Barred owls may also interbreed with spotted owls.
Islands are especially vulnerable to alien species. For instance, on the islands in the state of Hawaii, many native bird species lost the ability to fly because of a lack of natural predators. This made them especially vulnerable to introduced animals such as dogs. To date, approximately 90% of all native Hawaiian bird species have been eliminated.
Introduced plants can also wreak havoc. The water hyacinth was introduced into Florida as an ornamental plant and spread throughout the waterways of the state, as well as through many other southern states. This plant clogs navigable streams and rivers. Other well-known invader plants include the purple loosestrife and kudzu.
The Great Lakes and rivers flowing into them have also been subject to numerous alien invasions, often with devastating consequences. The latest invaders causing problems are the zebra mussel and fishhook water flea. Both species were accidentally introduced in the mid-1980s by ships from Europe and Asia that released ballast water containing the organisms into the lakes. (The ships routinely pick up water for ballast at the start of their journey and dump the water when they reach port in the Great Lakes.) Both species feed on free-floating microscopic photosynthetic organisms, known as phytoplankton (see plankton), which form the base of the food chain. Their loss jeopardizes the well-being of many native fishes.
Increased worldwide travel and trade have accelerated the relocation of nonnative species in all directions. In Hungary more than 20% of protected grasslands have been infected with common ragweed and Canadian goldenrod. In Canada the giant hogweed, introduced from Asia as a garden curiosity, is crowding out native riparian species and contributing to streambank erosion. American bullfrogs have become problematic in France. European sand dune grass threatens to replace native grasses on the dunes of South Africa. The introduction of fruit flies onto the Pacific island of Nauru has had devastating effects on the fruit industry. At least six introduced species of insects—two kinds of fire ants, two kinds of wasps, a mealybug, and a parasitic fly—pose serious threats to the Galapagos Islands. In Ireland the New Zealand flatworm preys on native earthworms that farmers rely on to improve soil quality. Burmese pythons have colonized the Florida Everglades. Louisiana crayfish infest streams in China even as Asian carp have severely damaged wetlands in the United States.
Scientists estimate that there are 50,000 alien species of plants, animals, insects, arachnids, and disease organisms that have become firmly established in the United States. Of these, about 800 are considered to be invasive. Nonnative species have adversely affected more than 50% of the species listed as endangered or threatened under the Endangered Species Act (see endangered species). Annual estimated damage, as of 2006, exceeded $120 billion. The serious concerns about disruptions caused by invasives to native ecosystems have led to a new branch of study, invasive ecology.—Daniel D. Chiras