Invasive Species

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.