Introduced Species | Research & Encyclopedia Articles

Introduced species (also called invasive species) are those that have been released by humans into an area to which they are not native. These releases can occur accidently, from places such as the cargo holds of ships. They can also occur intentionally, and species have been introduced for a range of ornamental and recreational uses, as well as for agricultural, medicinal, and pest control purposes.

Introduced species can have dramatically unpredictable effects on the environment and native species. Such effects can include overabundance of the introduced species, competitive displacement, and disease-caused mortality of the native species. Numerous examples of adverse consequences associated with the accidental release of species or the long term effects of deliberately introduced species exist in the United States and around the world. Introduced species can be beneficial as long as they are carefully regulated. Almost all the major varieties of grain and vegetables used in the United States originated in other parts of the world. This includes corn, rice, wheat, tomatoes, and potatoes.

The kudzu vine, which is native to Japan, was deliberately introduced into the southern United States for erosion control and to shade and feed livestock. It is, however, an extremely aggressive and fast-growing species, and it can form continuous blankets of foliage that cover forested hillsides, resulting in malformed and dead trees. Other species introduced as ornamentals have spread into the wild, displacing or outcompeting native species. Several varieties of cultivated roses, such as the multiflora rose, are serious pests and nuisance shrubs in field and pastures. The purple loosestrife, with its beautiful purple flowers, was originally brought from Europe as a garden ornamental. It has spread rapidly in freshwater wetlands in the northern United States, displacing other plants such as cattails. This is viewed with concern by ecologists and wildlife biologists since the food value of loosestrife is minimal, while the roots and starchy tubes of cattails are an important food source to muskrats. Common ragweed was accidently introduced to North America, and it is now a major health irritant for many people.

Introduced species are sometimes so successful because human activity has changed the conditions of a particular environment. The Pine Barrens of southern New Jersey form an ecosystem that is naturally acidic and low in nutrients. Bogs in this area support a number of slow-growing plant species that are adapted to these conditions, including peat moss, sundews, and pitcher plants. But urban runoff, which contain fertilizers, and wastewater effluent, which is high in both nitrogen and phosphorus, have enriched the bogs; the waters there have become less acidic and shown a gradual elevation in the concentration of nutrients. These changes in aquatic chemistry have resulted in changes in plant species, and the acidophilus mosses and herbs are being replaced by fast-growing plants that are not native to the Pine Barrens.

Zebra mussels were transported by accident from Europe to the United States, and they are causing severe problems in the Great Lakes. They proliferate at a prodigious rate, crowding out native species and clogging industrial and municipal water-intake pipes. Many ecologists fear that shipping traffic will transport the zebra mussel to harbors all over the country. Scattered observations of this tiny crustacean have already been made in the lower Hudson River in New York.

Although introduced species are usually regarded with concern, they can occasionally be used to some benefit. The water hyacinth is an aquatic plant of tropical origin that has become a serious clogging nuisance in lakes, streams, and waterways in the southern United States. Numerous methods of physical and chemical removal have been attempted to eradicate or control it, but research has also established that the plant can improve water quality. The water hyacinth has proved useful in the withdrawal of nutrients from sewage and other wastewater. Many constructed wetlands, polishing ponds, and waste lagoons in waste treatment plants now take advantage of this fact by routing wastewater through floating beds of water hyacinth.

The reintroduction of native species is extremely difficult, and it is an endeavor that has had low rates of success. Efforts by the Fish and Wildlife Service to reintroduce the endangered whooping crane into native habitat in the southwestern United States were initially unsuccessful because of the fragility of the eggs, as well as the poor parenting skills of birds raised in captivity. The service then devised a strategy of allowing the more common sandhill crane to incubate the eggs of captive whooping cranes in wilderness nests, and the fledglings were then taught survival skills by their surrogate parents. Such projects, however, are extremely time and labor intensive; they are also costly and difficult to implement for large numbers of most species.

Due to the difficulties and expense required to protect native species and to eradicate introduced species, there are not many international laws and policies that seek to prevent these problems before they begin. Thus customs agents at ports and airports routinely check luggage and cargo for live plant and animal materials to prevent the accidental or deliberate transport of non-native species. Quarantine policies are also designed to reduce the probability of spreading introduced species, particularly diseases, from one country to another.

There are similar concerns about genetically engineered organisms, and many have argued that their creation and release could have the same devastating environmental consequences as some introduced species. For this reason, the use of bioengineered organisms is highly regulated; both the Food and Drug Administration and the Environmental Protection Agency (EPA) impose strict controls on the field testing of bioengineered products, as well as on their cultivation and use.

Conservation policies for the protection of native species are now focused on habitats and ecosystems rather than single species. It is easier to prevent the encroachment of introduced species by protecting an entire ecosystem from disturbance, and this is increasingly well recognized both inside and outside the conservation community.

Bioremediation; Endangered Species; Fire Ants; Gypsy Moth; Rabbits in Australia; Wildlife Management

Common Weeds of the United States. United States Department of Agriculture. New York: Dover Publications, 1971.

Forman, R. T. T., ed. Pine Barrens: Ecology and Landscape. New York: Academic Press, 1979.

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