SPECIES, INTRODUCED. Introduced species have become a significant ecological problem in the United States and elsewhere. As humans settled in North America over the past fifteen thousand years, they brought with them a variety of species novel to the environment. They intentionally brought many introduced, or exotic, species, such as wheat, potatoes, cattle, pigs, and horses. Many species, however, came uninvited, traveling in the holds of ships or planes, hitching rides on imported produce, or escaping from captivity. Both intentional and unintentional introductions can leave unforeseen and destructive economic and ecological effects. Although most introduced species are poorly adapted to a novel ecosystem and die out, some thrive. Arriving in an ecosystem that evolved without them, introduced species frequently have no predators. Native species may not have the defenses necessary to ward off a novel predator, and introduced species can out compete native species and drive them to extinction or change an ecosystem by altering relationships within it. At the end of the twentieth century, the United States had thousands of introduced species, some of which the Department of Agriculture estimated were the primary factor in the population decline of approximately 225 of the nation's 660 endangered or threatened species.
The results of species introductions are often dramatic. Introduced species have devastated the ecosystems of Hawaii, which evolved in isolation. Since the arrival of Polynesians one thousand years ago and Europeans two hundred years ago, many of the islands' native plant, bird, and insect species have become extinct because of competition and predation by rats, cats, sheep, goats, and other introduced organisms. In the 1990s the number of introduced species of plants on the islands was triple the number of native plants.
Many introduced species, intentional and not, have been insects that wrought large-scale havoc. In the nineteenth century, merchants imported the gypsy moth caterpillar to the United States in an effort to produce domestic silk. Moths escaped in 1869, spread throughout the country, and continue to cause widespread damage to hardwood forests throughout North America. In 1956 an African species of honeybee that had been imported into Brazil escaped from captivity, established colonies, and hybridized with the European species of honeybee (itself introduced). In 1990 Africanized honeybees began migrating north and crossed into Texas, prompting the Department of Agriculture to predict substantial negative effects on U.S. agriculture. One of the most celebrated cases of an introduced species causing economic damage was the unintentional introduction of the Mediterranean fruit fly, or Medfly, in the 1970s in California. Imported on foreign produce, these small flies caused widespread fruit and vegetable damage.
Other types of species can also be quite disruptive. In the 1990s the zebra mussel began causing widespread economic and ecological disruption. Believed to have been released from the ballast of a European tanker in the Great Lakes in 1988, the mussel population exploded, displacing native mollusks, blocking water pipes and dam outlets, disrupting fisheries, and destroying native aquatic ecosystems. The cost to control them has run well into the billions of dollars, and the urgency of the situation provoked Congress to pass the Nonindigenous Aquatic Nuisance Prevention and Control Act of 1990.
Drewett, John. "Aliens Not Wanted Here." New Scientist 151, 2047 (14 September 1996): 48.
Jones, Clive G., and John H. Lawton, eds. Linking Species and Ecosystems. New York: Chapman and Hall, 1995.
Simberloff, Daniel, and Strong, Donald R. "Exotic Species Seriously Threaten Our Environment." Chronicle of Higher Education 47 (8 September 2000): B20.
David W. Cash / d. b.