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.
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