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

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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.