Plants and animals sometimes disperse naturally into new habitats , either by natural migrations or via floods, storms, and other events. Species also can be transported by humans, either deliberately or accidentally. When species are introduced into areas outside their natural ranges, they become known as nonindigenous (i.e., not originating from that geographic area), alien, or exotic.
Once introduced, exotic or nonindigenous species are separated from the predators, parasites , and diseases that kept them in balance in their native environments. With such controls lost or diminished, the species often become pests, some to the extent that they injure ecosystems and cause economic damage. In addition, some species have adaptations that allow them to overtake and possibly displace resident species. Exotic species that cause these negative impacts can be described as nuisance, injurious, or invasive, depending on the general degree of harm.
It should be remembered that nonindigenous species can be native to a larger geographic region: for example, nonindigenous to the Ohio River Valley, yet native to North America. Further, some native species can be invasive. However, this article focuses on invasive species that generally are nonindigenous, and that are introduced by human activities.
A common method for accidental introduction of nonindigenous species in marine environments is through the ballast water of oceangoing vessels. To stabilize a ship, the crew fills the ballast tanks with water, along with any organisms in the water. When the ballast water is later discharged at a different port, possibly in another part of the world, these organisms are introduced into a new environment. Scientists estimate that up to 10,000 alien species per day are transported around the world in the ballast water of ships.
Aquaculture of fish, shellfish, and other aquatic organisms can accidentally or intentionally introduce cultivated species into natural habitats, where the nonnative species may harm indigenous populations. For example,
Accidental or deliberate release of unwanted pets can be another introduction method. For example, aquatic reptiles such as the spectacled caiman (a small crocodile) and the red-eared slider (a turtle) both occur in Florida waters, and probably originated from pet releases. Exotic amphibians and tropical fish also are found throughout Florida.
Every state in the United States has been affected by aquatic nuisance species. For example, the nutria (a semi-aquatic rodent native to South America) has become particularly abundant in the southern United States, where it damages vegetation in waterways. Mosquito fish introduced in Arizona to control mosquito larvae now compete with and prey on native fish. Purple loosestrife grows wild in at least 42 of the 50 states. Eurasian watermilfoil has been recorded in 46 states.
In Lake Erie, a relatively new invader surfaced in the 1990s—the quagga mussel. This thumbnail-sized mollusk was introduced into the Great Lakes in the early 1990s, probably from ballast water. By 2002, quagga mussels were found in much of Lake Erie, Lake Ontario, the Erie Canal, the upper St. Lawrence River, and parts of Lake Huron. This mussel has the potential of outpacing the highly invasive zebra mussel (whose population exploded after 1988) and causing even more damage.
Two states—Hawaii and Florida—have been especially hard hit by nonindigenous species because both are geographically isolated and have a semitropical to tropical climate. For example, Florida has the largest total of established nonindigenous amphibian and reptilian species. Twenty-eight exotic fish species are reproducing in Florida's fresh-water lakes, rivers, and canals. Twenty-one of those are permanent residents. Overall, approximately 42 percent of Florida's reptile species, 22 percent of its amphibian species, 16 percent of its fish species, and 5 percent of its bird species are naturalized nonindigenous species.
Numerous species of fish, oysters, clams, mussels, other shellfish, crustaceans, aquatic plants, and algae are native to the United States but have been moved to areas outside of their native range, often to improve sport fishing and to support aquaculture operations (see box on this page). Many of these species are highly valued, commercially cultured, and economically important.
However, many nonnative species can cause enormous ecological damage if released or transported to a different environment. The presence of bioinvaders 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 occur when the nonnative species interbreed with native species, likely altering the gene pool. This may lead to hybridization and homogeneity, which reduces biodiversity and the ecosystem's ability to adapt to natural or human-induced changes. More than 40 percent of species on the federal list of endangered and threatened species are at risk primarily because of nonnative species.
Some invaders possess survival mechanisms that can give them an advantage over indigenous species. Although similar survival tools often can be found in indigenous species, the ability of bioinvaders to survive and adapt to harsh or unusual conditions compounds the problem already created by their introduction.
For example, the northern snakehead fish can survive in a moist, out-of-water environment for up to 3 days, and can even use its fins to push itself or "walk" short distances on land. The zebra mussel can survive out of the water for several days if harbored in a cool, moist environment, such as water in the bottom of sport fishing boats. The cane toad, if attacked by another animal, releases a milky bufotoxin that causes neurological impacts and even death in small animals. Thus the toad causes declines in predator populations because these predators have no natural immunity to the bufotoxin it secretes.
Economic consequences of nonnative species arise from impacts on commercial fisheries and water-based recreation, the degradation of wildlife habitat, the reduction of biodiversity, and the alteration of natural ecosystems. Natural resource, environmental, and fisheries managers must take action when invasive species threaten ecosystems, local and regional economies, and even the human environment.
The zebra mussel, for example, colonizes not only bottom substrates , but also a wide array of surfaces and structures, including pipes that supply water to power plants and public water supplies. Colonies can become so dense that the flow through the pipes is restricted. Zebra mussels also may attach themselves to other aquatic animals with hard shells, such as crustaceans. The zebra mussel's ecological and economic impacts factored heavily into passage of the U.S. Nonindigenous Aquatic Nuisance Prevention and Control Act of 1990 and its amendments via the National Invasive Species Act of 1996.
Thousands of invasive species worldwide are notorious for their distinctive habits, destructive potential, and ecological damage. Other invaders seem to have little environmental impact, but it can be many years after a species is introduced before adverse consequences appear. A few examples of bioinvaders threatening U.S. fresh-water ecosystems are listed below.
The northern snakehead ( Channa micropeltes ) is a popular food item in parts of Asia, where some people believe it has medicinal benefits. Yet in the United States, this fish is a potentially devastating bioinvader that had been documented in more than eight states as of 2002. In Maryland, for example, if these aggressive and predacious northern snakeheads were to escape into the Little Patuxent River, just 75 meters (245 feet) away from a pond in which reproducing snakeheads were found, significant environmental impacts could result. Many states have banned the import or possession of northern snakeheads.
Grass, bighead, silver, and black carp are imported by some U.S. catfish farmers. Some of these nonnative carp, which may reach up to 1.5 meters (5 feet) long and 68 kilograms (150 pounds) as adults, have escaped into natural waterways. Carp have been found dangerously close to the Chicago Sanitary Canal, which could serve as a gateway to the Great Lakes. In spring 2002, the U.S. Army Corps of Engineers installed an electric barrier in the canal to discourage the northward spread of carp and other undesirable fish, such as round gobies.
But carp have other ways of reaching the Great Lakes. Bighead carp have been found in Lake Erie, where they probably were directly introduced by an admirer of the nonindigenous species (e.g., a carp hobbyist or perhaps certain Asian immigrants for whom carp is a dietary staple or for whom it may have religious significance).
The African clawed frog ( Xenopus laevis ) was widely used in human pregnancy testing in the 1940s and 1950s, and was shipped throughout the world. This completely aquatic frog is native to southern Africa, but is now found worldwide in suitable habitats. It is prolific and voracious, competing with native species for food such as insect larvae, small fish, and tadpoles. Although Xenopus remains a popular laboratory animal and is still available as a pet in many areas, some states require a permit for possessing it, and Washington state prohibits its importation.
The common bullfrog ( Rana catesbeiana ) is native to the eastern half of the United States. It has been accidentally or deliberately introduced to several western states, where it has been implicated in declines of native frogs and toads. The bullfrog is large and voracious, and will eat a surprising array of plants and animals.
The giant toad or cane toad ( Bufo marinus ) occurs naturally from the lower Rio Grande valley in Texas south to the Amazon basin. This fertile and highly productive toad is probably the most widely introduced amphibian in the world. It has been introduced to control insects in sugar
In Maryland's Chesapeake Bay and along the northeast Atlantic Coast southward to Virginia, mute swans ( Cygnus olor ) are spreading rapidly. Mute swans are much larger and more aggressive than native waterfowl. They drive off and even kill native birds, and can eat up to 4.5 kilograms (10 pounds) of aquatic vegetation each day. Their copious droppings foul ponds and lawns.
The zebra mussel ( Dreissena polymorpha ) originated in the Balkans, Poland, and areas within the former Soviet Union. The species was accidentally introduced into the Great Lakes in 1988 via the ballast water of ships. By 1990, it had spread to all the Great Lakes. In 1991, zebra mussels escaped the Great Lakes Basin and found their way into the Illinois River, giving them access to the entire Mississippi River Basin. As of 2002, they had expanded further, and were found from Virginia to New York, bringing the total number of states with documented occurrences to twentythree.
The quagga mussel ( Dreissena bugensis ) looks much like the now-infamous zebra mussel, and lives in many of the same habitats. The quagga mussel could potentially cause more economic and ecological damage than the zebra mussel. Scientists studying Lake Erie in 2002 found that the quagga outnumbered the zebra mussel 10-to-1 in some areas.
The sea lamprey ( Petromyzon marinus ) is a primitive, jawless vertebrate with a slender, eel-like body. It attaches its circular mouth to its prey (e.g., lake trout), tearing the flesh and blood vessels to obtain blood and body fluids. After the lamprey detaches, the host usually dies from direct predatory attack, or the host may succumb to bacterial and fungal parasites that enter through the open wound.
The sea lamprey originally was an anadromous fish. The anadromous form was found in much of eastern North America, including Lake Ontario. In 1833, the Welland Canal was completed, connecting Lake Ontario with Lake Erie. Sea lamprey migrated through the Welland Canal, eventually reaching the other four Great Lakes. Once trapped in the Great Lakes, sea lamprey adapted and became land-locked, fresh-water fish. Their predatory activity devastated the Great Lakes trout and whitefish fishing industry by the 1950s.
While it is impossible to completely rid the Great Lakes of sea lampreys, controlling them is vital to the health of the Great Lakes fisheries. The most effective treatment technique is applying a poison that kills sea lamprey larva but that is harmless to other fish and mammals. This has been an extremely successful yet expensive treatment. Less expensive alternatives include various kinds of barriers, trapping, and the release of sterile males. With effective control of the sea lamprey, the populations of commercially important fish in the Great Lakes have gradually recovered.
Purple loosestrife ( Lythrum salicaria ) is a wetland plant from Europe and Asia. It was introduced into the East Coast of North America in the 1800s as an ornamental, and to provide nectar for honeybees. It escaped cultivation and spread along roadsides, canals, and drainage ditches. The plant is now found in 40 states and all Canadian border provinces.
Purple loosestrife forms dense, impenetrable stands that are unsuitable as cover, food, or nesting sites. It also has a dense mat of tangled roots that are difficult to pull up. By displacing native vegetation, purple loosestrife has adversely affected a wide range of wetlands as well as plants and animals that rely partly or totally on wetlands. Many rare and endangered wetland species are at risk because of purple loosestrife.
The best way to reduce ecological impacts of nonnative species is to prevent them from invading and becoming established. This often entails education campaigns to increase public awareness.
For example, although the zebra mussel's introduction to the Great Lakes was via ballast water from a commercial ship, the mussel is easily transported between lakes and rivers by recreational boaters and sport fishers. When a boat is pulled from the water, mussels may be trapped in the bilge water or inside the water intakes of the engine. Then, when the boat is launched into a different lake of river, the zebra mussel is flushed out. Eurasian watermilfoil ( Myriophyllum spicatum ) can be spread by similar means. Hence, many state natural resources agencies conduct public education programs that give practical guidance on how to help prevent the further spread of these bioinvaders.
Complete removal of an invasive species may be possible only early in an invasion, or if the invasion is limited to a restricted area. However, once an aquatic nuisance species becomes established in a new habitat, removal
There are four basic techniques that have been used with some success: physical removal of the invading species; ecological control; chemical control; or biological control.
Physical removal of the invading species is usually possible only if the invasion is in its early stages or is limited to a small area. Physical removal is expensive and labor-intensive.
Ecological control consists of manipulating environmental factors such as presence of fire (burning off a marsh), or changing water levels or water flow to disrupt the life cycle of an invasive species. Obviously the environmental changes must be compatible with the life cycle of native species, but, if carefully chosen, ecological controls can provide native species an advantage in competing with invasive species. Unfortunately, nuisance species often are more tolerant of adverse conditions, such as drought and fire, than are native species.
Chemical control often is an effective method of controlling invasive species. However, chemical control has many adverse side effects. It is difficult to avoid harming other organisms. Chemical control also is expensive. However, chemical control is currently the most widely successful means of control and remains the technique of choice for most nuisance species.
Biological control may be the most environmentally sound way to control invasive species with minimal expense. However, selecting the appropriate biological control agent is a daunting task. On the one hand, biological control agents may not survive or thrive in the new environmental conditions; conversely, control agents can themselves become invasive. For example, the round goby ( Neogobius melanostomus ), a fish which was accidentally introduced into the Great Lakes, has potential to be an effective biological control for zebra mussels. Yet the goby takes over prime spawning sites traditionally used by native fish species, competes with them for habitat, and changes the balance of the ecosystem.
Control of nuisance species is often made more complicated by personal preference, values, opinions, and even religious beliefs. For example, there is an Asian cultural belief that the northern snakehead fish has curative powers. A related belief is that a live fish should be released for each fish consumed.
Control measures receive broad public support if the species causes a clearly harmful environmental or economic impact (e.g., zebra mussels and Asian carp), or if it has visibly "unpopular" habits (e.g., noisy and voracious bullfrogs that snatch ducklings from private ponds). But attractive species such as mute swans enjoy public popularity that can hinder the control efforts of resource managers.
In Maryland, for example, the Department of Natural Resources and some federal agencies have controlled mute swan populations to prevent their further spread. However, public opinion about the swans is mixed. Many people enjoy seeing the large, graceful birds, and do not want to see them harmed. Also, animal rights activists object to lethal control methods. Mute swan control activities are coupled with outreach activities to heighten public awareness to the problems these animals create.
Many scientists think the spread of exotic species is one of the most serious, yet largely unrecognized, threats to natural ecosystems and the overall environmental balance. Further, it is a permanent, nonreversible form of pollution. Safeguarding natural systems from bioinvaders involves preventing additional introductions; detecting new threats before major damage is done; controlling and managing existing invaders; and restoring native habitats.
SEE ALSO Amphibian Population Declines ; Birds, Aquatic ; Bivalves ; Canals ; Chesapeake Bay ; Crustaceans ; Ecology, Fresh-Water ; Globalization and Water ; Great Lakes ; Human Health and the Ocean ; Lake Health, Assessing ; Mississippi River Basin ; Pollution of Lakes and Streams ; Ports and Harbors ; Reptiles ; Stream Health, Assessing ; Transportation ; Wetlands .
and Elliot Richmond
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Some exotic species do not seem to cause any particular harm. The Mediterranean gecko is now common across the southern United States. This small lizard prefers rock walls under lights, a niche that was not occupied by any native species. So the environmental impact of this exotic is minimal. Such exotic species are more a nuisance than an environmental threat. In contrast, exotic wildlife species such as the zebra mussel that pose an environmental threat are called "injurious."
The U.S. Geological Survey maintains a comprehensive list of over 500 nonindigenous fish species in the United States. Of those, 317 are native to other parts of the United States, such as rainbow trout, brook trout, arctic char, channel catfish, striped bass, Pacific salmon, Atlantic salmon, and ornamental fish.
About 185 of the nonindigenous fish species were brought in from foreign countries. The species brought in from foreign countries include 71 that either have already established self-sustaining populations in open waters or are likely to do so.
Lake trout illegally introduced to Wyoming's Yellowstone Lake in the 1990s are imperiling the native cutthroat trout population. The cutthroat is a major food source for birds and mammals, including dozens that are either endangered or "of special concern." The region's cutthroat trout fishery is world-famous, historically significant, and an economic strength. Biologists and managers are developing an aggressive lake trout control program designed to protect the cutthroat population and thus the ecological character of the entire Yellowstone Lake basin.