Invasive species

An exotic species is one that has been introduced into a habitat it would not normally populate. This introduction can be intentional or unintentional. Exotic species have also been called introduced, nonnative, nonindigenous, or alien species. An invasive species is an exotic species that thrives in its new environment, disrupting the natural ecosystem . The majority of exotic species have been introduced unintentionally. "Hitchhiker" organisms such as seeds or insects attach to people's shoes, clothes, or luggage when they travel. When the humans return to their native land, they arrive bearing these nonnative species. Sometimes people bring beautiful plants and flowers home with them for ornamental purposes. These intentional introductions occur less frequently, but can have the same disastrous effects. 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.

Survival of exotic species

When a new species is introduced into an ecosystem, there are four different interspecific interactions that can occur. These are interactions that occur between two different species living in the same community. These interactions could have positive, negative , or neutral effects on the involved organisms. One such interaction is predation. This is when one species, the predator , uses the other, the prey , for food. This interaction obviously benefits the predator, but not the prey. 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. For one, the organism must be able to reproduce in their new environment. They must also be able to out-compete the native populations. They must not be susceptible to herbivores or diseases, especially if these types of organisms characterize the ecosystem to which they have been introduced. Lastly, they must be able to survive in their new climate with the available resources.

Effects of invasive species

When an introduced species becomes invasive, the effects can be terrible. In the United States alone, invasive species cause more than $123 billion in damages per year. It has been estimated that over 68% 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 750,000 species in the United States are exotic. Approximately 5,000 of these are considered invasive. Because it is difficult to gather these data, these are probably underestimations of the actual effects of invasive species. Invasive species not only displace native flora and fauna, they homogenize existing ecosystems, greatly reducing the number of available biological resources.

Examples of invasions

There are countless 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 over-populated 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.

A recent invasive species that has received much media attention is the zebra mussel. This organism 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. They also clog waterways. It is not known what effects these organisms will eventually have on the ecosystems they have invaded, but it is likely that the shellfish and native fish will suffer. It has been estimated that if the zebra mussel population is not controlled, damages will reach $5 billion by the year 2002.

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-mi (322-km) 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 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.

Management

There are many more examples of the destruction invasive species can cause. The problem facing environmentalists and naturalists is management of these organisms. Management is important because invasive species can disrupt entire ecosystems, reduce biodiversity , endanger plants and animals, destroy landscapes and habitats, and transmit diseases. The United States National Park Service has proposed over 500 projects to eradicate invasive species in over 150 different parks. The cost of these projects would be over $80 million. The Park Service has also established an Integrated Pest Management Program with the agenda of controlling the introduction of new invasive species. Some National Park Service sites are being used as insect nurseries, where insects that could be used as biocontrol agents are harvested and distributed. In 1993, the Bureau of Land Management developed the Federal Interagency Committee for Management of Noxious and Exotic Weeds to eradicate invasive plants on federal lands and to provide help to similar projects on public lands. In 1998, the Fish and Wildlife Service started a program in North and South Dakota, Nebraska, Kansas, Oklahoma, and Texas to halt the spread of zebra mussels into these states.

In 1999, 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. This council will work with groups at the state, university, and local levels to solve the problems these organisms can cause. The Council's budget for the year 2000 was approximately $30 million, to be used for program implementation as well as research. The agenda also includes the reintroduction of native species into their original habitats. Federal legislation has already been created to begin the process of restoration. The USDA now has over 1,300 inspectors at 90 ports of entry, assisted by the "Beagle Brigade," beagles trained to smell agricultural products being transported into the country. The USDA has also prohibited importing untreated wood packing material from China, which can carry the Asian long-horned beetle. A proposal has been made to other countries to enact this ban as well. A barrier is being built in the Chicago Ship Canal to stop the spread of invasive species from Great Lakes to the Mississippi River. The National Oceanic and Atmospheric Administration is funding research into possible ballast water treatments that could eliminate the introduction of invasive species from cargo ships. These measures will help eliminate some invasive species, but not all. More research is needed, and international programs need to be implemented in order to completely eradicate the problem of invasive species.

Resources

books

Campell, Neil A. Biology. Menlo Park, CA: The Benjamin/Cummings Publishing Company, 1996.

Starr, Cecie. Biology: Concepts and Applications. Belmont, CA: Wadsworth Publishing Company, 1997.

periodicals

"Costly Interlopers." Scientific American (February 15, 1999).

Gordon, Doria R. "Effects of Invasive, Non-indigenous Plant Species on Ecosystem Processes: Lessons from Florida." Ecological Concepts in Conservation Ecology no. 84, (1998).

Jennifer McGrath

KEY TERMS

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.

Invasive Species

Resources

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 people’s 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

KEY TERMS

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.

Resources

BOOKS

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

Invasive Species

Plants that grow aggressively and outcompete other species are called invasive species. Invasive plants are usually those that were introduced, either intentionally or unintentionally, into a locality where they previously did not grow. Introduced plants, also called exotics or alien species, form an important part of our environment, contributing immensely to agriculture, horticulture, landscaping, and soil stabilization. But among the thousands of plant species introduced to North America, approximately 10 percent display the aggressive growth tendencies of invasive species. Although the terms exotic, alien, and invasive are sometimes used interchangeably, not all exotic plants are invasive. In addition, some native species, those plants that grew in an area prior to European settlement, can be invasive, especially as natural landscapes are altered.

Characteristics of Invasive Species

Invasive species are not a separate biological category, and all types of plants, including vines, trees, shrubs, ferns, and herbs, are represented by invasive species. They do, however, share certain characteristics that help them rapidly grow and invade new areas. Invasive plants typically exhibit at least some of the following:

• production of many seeds
• highly successful seed dispersal
• no special seed germination requirements
• grow in disturbed ground
• high photosynthetic rates
• thrive in high-nutrient conditions
• rapid growth and maturity
• early maturation
• reproduction by both seeds and vegetative means
• long flowering and fruiting periods

Most exotic plants do not pose an obvious threat to native plants when they are first introduced, but we do not fully understand the dynamics of what makes plants invasive. The same plant species can be invasive in one habitat or area and not aggressive in another. Sometimes many years separate the first introduction of a plant and its later spread as an invasive species. For example, Atlantic cord grass (Spartina alterniflora ) was present in small areas on the Pacific coast for more than fifty years before it became invasive.

Often by the time a plant is recognized as being a major problem it has become so well established that eradication is difficult or impossible. Even when plants are recognized as a potential problem, finding the money and manpower needed to eliminate them may not be easy. For example, leafy spurge (Euphorbia esula ), which forms dense stands that cattle refuse to graze, was seen as a potential problem in Ward County, North Dakota, in the 1950s. By the time funding was available to deal with the problem on both public and private lands, leafy spurge was present in all townships in the county and had increased from one small patch to about 12,000 acres.

Spread of Invasive Species

People have been the major factor in the spread of invasive species. Humans have always carried plants with them for food, medicine, fiber, ornament, or just curiosity. As human population has increased, so has the demand for food, housing, transportation, and other necessities of life. More and more land is disturbed to provide people with what they need and want, and disturbed land is where invasive species get their footholds. Increased international travel and global world trade also contribute to the problem. Invasive species have arrived in North America in the cargo holds of airplanes, as seeds in grain shipments, in the soil of ornamental plants, and as ship ballast. Improvements in transportation technology allow both people and plants to travel thousands of miles in just a few hours.

New environments provide an ideal place for invasive plants. These species leave behind the natural controls (usually insects) that kept them under control in their native habitats and can often spread unchecked. Some, such as the common dandelion (Taraxacum officinale ), ox-eye daisy (Chrysanthemum leucanthemum ) or tree-of-heaven (Ailanthus altissima ), have become integrated over time into the flora of urban areas and are the dominant and familiar vegetation.

Most of the invasive species in North America are originally from Europe or Asia, areas with very similar climate. Many of these species were first introduced as ornamental plants. An excellent example is honeysuckle (Lonicera spp.), which was introduced in the late 1890s as horticultural shrubs and vines and for wildlife habitat improvement. Honeysuckle often out-competes native plants due to earlier leaf expansion and later fall leaf retention. Large thickets of honeysuckle interfere with the life cycles of many native shrubs and herbs. These stands alter habitats by decreasing light and depleting soil moisture and nutrients. Some honeysuckle species also release chemicals into the soil that inhibit the growth of other plants. Fruits are consumed and passed by birds, which makes effective control difficult.

Another ornamental that turned invasive is kudzu (Pueraria lobata ), a vine with attractive purple flowers that was first exhibited in the United States at the Philadelphia Centennial Exposition in 1876. It is now listed as a noxious weed in many states, especially in the South, where it smothers large trees as it clambers for light.

Accidental introduction is also a common way for invasive species to become established. Mile-a-minute weed (Polygonum perfoliatum ), an Asian vine named for its fast growth rate, appeared in rhododendron nurseries in Pennsylvania in 1946, presumably the result of seeds mixed with imported plants. Since then it has spread to other areas in Pennsylvania as well as to surrounding states and is rapidly becoming a major problem along roadsides and other disturbed areas.

Impact and Eradication

The economic impact of invasive plants is staggering. They affect agriculture, the environment, and health. Invasive plants cause reductions in crop harvests as well as increased production costs. Farmers worldwide spend billions of dollars annually on chemicals and other methods to control weeds. The toll in human time is enormous, as hand-weeding of crops is the number one work task of 80 percent of people in the world. Some invasive species that contaminate harvested crops or pastures are toxic and pose a threat to both people and animals ingesting that food or milk.

Invasive plants are also a major threat to native plants and animals, including rare and endangered species. In fact, alien species are considered by some experts to be second only to habitat destruction as a threat to biodiversity . In the United States, for every acre of federal land destroyed by fire in 1995, two acres were lost to invasive plants. Two-thirds of all endangered species are impacted by invasive plants. Wetlands, home to many endangered plants, are especially susceptible to invasive species, such as purple loosestrife (Lythrum salicaria ), which has taken over thousands of acres in at least forty-two states.

The problem of invasive species affects all fifty states. Introduced species make up 8 to more than 50 percent of the total plant species of most states. Nowhere is the problem more serious than in Hawaii, where exotic species now outnumber native species. In Florida, at least 1.5 million acres of natural areas are infested with nonnative plants. Of mainland states, New York and Pennsylvania have the highest ratio of introduced-to-native species.

Methods for eradicating invasive plants range from hand-pulling to chemical controls. When weeding plants, it is important to disturb the soil as little as possible because disturbed areas are where invasive species can grow well. Other mechanical means include mulching soil to prevent or reduce seed germination, applying heat to seedlings, mowing, and girdling trees (pulling a strip of bark off all the way around the trunk to prevent the flow of nutrients). As more and more noxious weeds become resistant to chemical treatments, attempts at biocontrol (using natural predators) are increasing. Researchers have identified thirteen different insect species that may potentially control leafy spurge, and a beetle that eats the leaves of purple loosestrife has already been released in some areas.

Perhaps most important is public awareness and participation in the problem. People should avoid using invasive plants in their yards and gardens. This can be a complicated task as some invasive species, such as purple loosestrife, are sold in garden stores and catalogs. Beware of any plants described as "spreading rapidly." Another important defense is being on the lookout for alien plants and removing them before they become a problem. Organized efforts at invasive plant removal are a major weapon in preventing their spread. In Utah, middle and high school students who participate in a Scotch Thistle Day each spring have significantly reduced the amount of this noxious weed in their area.

Although most invasive species have been introduced from other areas of the world, native plants can become aggressive, especially as habitats are altered or destroyed. Boxelder (Acer negundo ) and wild grapes (Vitis spp.) as well as other native species can form fairly exclusive monocultures that thrive in disturbed environments. On the other hand, some otherwise invasive species can be useful in heavily disturbed sites. For example, tree-of-heaven grows where other plants cannot, thus providing just the foothold needed by other species to colonize .

The problem of invasive species is a costly one in terms of time, money, and loss of native habitats and species. Since the 1950s, weed-associated losses and costs worldwide have increased exponentially and are continuing to spiral upward. Of the more than sixty-seven hundred plants worldwide that are considered to be invasive, only about two thousand presently occur in North America. This leaves more than four thousand invasive plants now growing in other countries that could in the future become a problem in the United States.

see also Endangered Species; Human Impacts; Kudzu; Seed Dispersal; Wetlands.

Sue A. Thompson

Bibliography

Collins, Tim, David Dzomback, John E. Rawlins, Ken Tamminga, and Sue A. Thompson. Nine Mile Run Watershed Rivers Conservation and Natural Resources. Harrisburg, PA: Pennsylvania Department of Conservation and Natural Resources, 1998.

Randall, John M., and Janet Marinelli, eds. Invasive Plants: Weeds of the Global Garden. Brooklyn, NY: Brooklyn Botanic Garden, 1996.

Westbrooks, R. Invasive Plants, Changing the Landscape of America: Fact Book. Washington, DC: Federal Interagency Committee for the Management of Noxious and Exotic Weeds, 1998.

Invasive Species

Introduction

Invasive species, also called invasive alien (non-native) species, are plants, animals, or microbes that damage ecosystems in which they do not occur naturally. The introduction of a new species into an ecosystem may be deliberate or accidental: When the introduction is delib-rate, some benefit is expected, such as the elimination of an existing pest. In most cases, introduced species cannot survive or spread widely in an alien environment and so do little or no damage; for example, in the United States the common goldfish is a widely distributed introduced species, but rarely achieves high population densities and so does not displace native species.

However, some species that are introduced into environments where they did not evolve are suddenly freed from predators or other factors that kept its numbers in check. In these cases the species becomes invasive, that, is, tends to spread widely, breed in large numbers, and displace native species. So many species are being transported across ecosystem borders by humans today that the few that become invasive are causing immense damage. Losses due to invasive species are estimated at $400 billion per year worldwide, about 5% of global gross domestic product (total economic output).

Historical Background and Scientific Foundations

Human cultures have been colonizing the globe in stages ever since a group of modern humans first emigrated from Africa about 60,000 years ago. Settlers have often carried crop plants, microorganisms, food animals, parasites, and other creatures with them, deliberately or accidentally. In some cases, these species have multiplied greatly, destroying native species by filling up their habitat, preying on them, eating up their food, and the like. Invasive alien species have caused the extinction of thousands of native species throughout history.

For example, the common rat accompanied human settlers of the Pacific islands as they spread from island to island, arriving in the western Pacific about 40,000 years ago and in the remote islands of the southern and eastern Pacific a little over 1,000 years ago. The rat has established itself on hundreds of Pacific islands and caused the extinction of hundreds of native species, especially birds (for example, in New Zealand). It continues to be a threat to native species in many places. For example, in the Galapagos Islands in the early 2000s, rats were endangering the last nesting sites of the indigenous mangrove finch, with fewer than 100 finches surviving as of 2006. At that time, rats and feral cats (wild cats descended from pets) were also threatening a subspecies of marine iguana found only on one island of the group, Isabela.

During the last few centuries, the spread of nonnative species accelerated due to the increasing number of ships traversing the world. Species were sometimes deliberately introduced to islands, as when whalers and pirates deliberately stocked the Galápagos with pigs, goats, and other animals in the 1700s to be hunted for food on future trips. After centuries of fairly harmless residence, the goats of Isabela in the Galápagos began to multiply in the late 1980s, possibly because changing weather patterns increased their food supply. The goats stripped the vegetation, causing erosion and threatening to starve out the giant, land-dwelling Galápagos tortoises.

In the last century, the rate of species invasion has increased still further as globalization results in increased trade, air travel, and increased shipping volumes and variety of products being shipped to every continent. Invasive species, along with habitat destruction, are now one of the largest threats to global biodiversity. Islands and aquatic environments are particularly vulnerable; for

WORDS TO KNOW

BIODIVERSITY: Literally, “life diversity”: the wide range of plants and animals that exist within any given geographical region.

ECOSYSTEM: The community of individuals and the physical components of the environment in a certain area.

GLOBALIZATION: The integration of national and local systems into a global economy through increased trade, manufacturing, communications, and migration.

INDIGENOUS SPECIES: A species that is native to its region, but may occur in other regions as well.

example, the deliberate introduction of the Nile perch to Lake Victoria in Africa in the 1950s, to serve as a food fish, led to the extinction of about 200 native species of cichlid fish. Zebra mussels, a type of shellfish, began to spread from their native Russian lakes in the 1700s and had colonized most of Europe and Britain by the mid-twentieth century; in 1988 they first appeared in American fresh waters and rapidly spread throughout the ecosystems of the Great Lakes. Besides causing ecological damage, they are also an expensive nuisance to industry, clogging the intake pipes of power plants and water-treatment plants.

Although species have been introduced by humans for thousands of years, the scientific study of invasive species, the field termed invasion ecology, began with the publication in 1958 of a single textbook, The Ecology of Invasions by Animals and Plants by Charles S. Elton (1900–1991). Elton, a British ecologist, was the first scientist to analyze the effect of human actions on species invasions. (Ecology is the study of the interactions of living things with each other and their environments.) Among Elton’s early insights that have been confirmed by later work was the claim that simpler food webs such as those found on islands and in temperate climates are likely to be more vulnerable to invasion than complex food webs, since the latter contain a wider variety of predators that might constrain an introduced species. On this theory, more-diverse ecosystems are more stable in the face of challenge by species introduction.

Impacts and Issues

Invasive alien species can alter entire ecosystems. Invasive insects, by threatening native species, can impact species higher up the food chain, such as birds and fish, as well as plants that may be pollinated or dispersed by the

native insects. Invasive species also cause economic harm: Weedlike invasive plants may interfere with farming, insects may destroy entire forests, invasive marine plants and animals may destroy entire fisheries.

Nor are these effects merely hypothetical. In the United States in 2008, invasive species were directly helping to reduce the populations of 42% of the country’s endangered and threatened species, and were causing between $120 billion and $140 billion of economic loss every year. In Brazil, South African lovegrass has spoiled about 10% of southern grazing lands for the purposes of raising cattle, causing about $30 million of loss to the industry every year.

In Africa, alien species were arriving in luggage, aircraft, imported soil and plants, and by other means, including deliberate introduction (as in the case of the Nile perch, now 80% of the food-fish catch in Lake Victoria). A 2003 report by the World Conservation Union on the effect of alien invasive species in Africa’s wetlands found that economic damage was in the billions and that the effect on human health was severe. For example, the red water fern (azolla), an Asian plant, has invaded some African rivers and lakes. The plant can double its biomass every two to three days, carpeting the surface with mats that harbor mosquitoes and snails. The mosquitoes carry malaria, which kills 1 to 3 million people worldwide every year, while the snails carry bilharzia (schistosomiasis), a rarely fatal but debilitating (weakening) disease that afflicts about 300 million people. Another invasive species, the water hyacinth, which was brought from South America as an ornamental plant, can double its biomass in 12 days. Water-covering plants of this type deprive the waters beneath them of light and oxygen, killing off some fish species. The Louisiana crayfish, an import from the United States, attacks native species of plants, crustaceans, and snails, and its burrows damage earthen dams.

In some cases, measures can be taken to manage or even eliminate particular invasive species. For example, in the Galapagos Islands starting in 2001, the government of Chile and international science organizations undertook a six-year effort to root out invasive species of plants and animals. Botanists armed with herbicides(chemicals that kill unwanted plants) have fought back invasions by such plants as blackberry, quinine bush, and passionfruit in order to preserve stands of rare native trees. Rangers shot 140,000 goats on the island of Isabela; sharpshooters in helicopters, tracking dogs, and sterile female goats that would waste the mating efforts of males were all used. Pigs were purged from Santiago Island and wild donkeys from both Isabela and Santiago. Eradication efforts are ongoing in Galapagos National Park. Although the islands are more vulnerable to invasion initially, they are also more amenable to eradication, because re-colonization by eradicated species is not as likely.

See Also Ecosystem Diversity; Human Impacts

BIBLIOGRAPHY

Books

McNeely, J. A., ed. The Great Reshuffling: Human Dimensions of Invasive Alien Species. Cambridge, MA: World Conservation Union, 2001.

Mooney, H. A., and R. J. Hobbs, eds. Invasive Species in a Changing World. Washington, DC: Island Press, 2000.

Perrings, C., et al., eds. The Economics of Biological Invasions. Northampton, MA: Edwar Elgar, 2000.

Periodicals

Guo, Jerry. “The Galapagos Islands Kiss Their Goat Problem Goodbye.” Science 313 (2006): 1567.

Raghu, S., et al. “Adding Biofuels to the Invasive Species Fire?” Science3131 (2006): 1742.

Stokstad, Erik. “Feared Quagga Mussel Turns Up in Western United States.” Science 315 (2007): 453.

Withgott, Jay. “Are Invasive Species Born Bad?” Science 305 (2004): 1,100-1,101.

Web Sites

Charles Darwin Research Station. “Invasive Rat Eradication.” http://www.darwinfoundation.org/files/species/pdf/rats-en.pdf (accessed May 5, 2008).

Global Invasive Species Program. “Global Strategy on Invasive Alien Species.” http://www.gisp.org/publications/brochures/globalstrategy.pdf (accessed May 5, 2008).

U.S. Department of Agriculture. “National Invasive Species Information Center.” http://www.invasivespeciesinfo.gov/ (accessed May 5, 2008).

Larry Gilman

Invasive Species

Animals, plants, and other organisms that are newly introduced into an area from another part of the world are sometimes referred to as "alien" or "exotic" species. These words are used to distinguish newly arrived species from the native species that have lived in the environment for very long periods of time. Although some people refer to all exotic species as invaders, some scientists believe it makes more sense to use the term "invasive species" only when referring to new species that are spreading rapidly and having a large negative impact on the environment, economic activities, or human health.

Many of these invasive species have been introduced into new environments by human activities. Sometimes they are introduced intentionally, such as European starlings, kudzu, and purple loosestrife, three species that spread very rapidly across the United States beyond their initial range of introduction and are believed to have reduced the abundance of native bird and plant species in many areas. However, most species introductions probably occur inadvertently by humans, a byproduct of frequent movements around the globe. For example, small ocean organisms are commonly picked up in the ballast water of ocean ships. When the ships release their ballast water at a port in another part of the world, these organisms are introduced into a new environment. Logs and other wood and fiber products imported into the United States sometimes contain insects from their country of origin, which accounts for the introduction of Chestnut Blight fungus in the United States.

In many cases, the new species do not spread very much nor do they have a large impact. However, many of these new species have created huge problems. Zebra mussels are reducing populations of native mussels in many areas of the United States, and they are so numerous in places that they are clogging up water intake pipes of power plants and municipal water supplies. Leafy spurge, an introduced poisonous plant of grassland, has covered large regions of the northern Great Plains and threatens many of the livestock operations in these areas.

One of the most famous ecological disasters associated with invasive species is the brown tree snake that was accidentally introduced on to the Pacific island of Guam. In just a few decades, through its hunting habits, the snake was responsible for the extinction of several of the island's bird species that were found nowhere else on Earth. Problems produced by invasive species are believed to cost billions of dollars every year.

Scientists are working very hard to find out what factors facilitate these biological invasions in hopes of providing some help to those trying to control their negative effects. It is clear that trying to prevent the introduction of new species into an area in the first place is the primary step to take. Some scientists are also trying to determine why some environments seem to be invaded more easily than others, or why some environments are invaded only at certain times. Some think that environments that have a high diversity of native species may be more resistant to invasions by new species, while others believe that disturbances and other factors that free up new resources are more important to opening an environment to invaders. There is still much work that needs to be done to increase scientists' understanding of the causes and effects of invasive species.

see also Biodiversity; Conservation; Extinction; Global Climate Change

Mark A. Davis

Bibliography

Elton, Charles S. The Ecology of Invasions by Animals and Plants. Chicago: University of Chicago Press, 2000.

Pimental, D., L. Lach, R. Zuniga, and D. Morrison. "Environmental and Economic Costs of Nonindigenous Species in the United States." Bioscience 50: 53–65.