The Problem of Exotic Species

The introduction of a foreign, or exotic, species into an area where it is not native often upsets the balance among the organisms living in that area. The foreign species may compete with native spe­cies for food or habitat or may prey on them. Gen­erally, an introduced competitor or predator has a greater negative effect on local organisms than do native competitors or predators. Although exotic species may be introduced into new areas by natural means, humans are usually responsible for such in­troductions, either knowingly or unknowingly. The blue water hyacinth, for example, was deliberately brought from South America to the United States because it has lovely flowers. Today it has become a nuisance in Florida waterways, clogging them so that boats cannot easily move and crowding out native species. Another exotic species, the zebra mussel, is a small mollusk that was accidentally in­troduced into the United States from Europe dur­ing the 1980s when it hitched a ride on a ship. Since that time, it has been growing out of control in U.S. waterways.

In 1977 a carnivorous snail was introduced in Moorea, an island in French Polynesia, as a way to control another snail species that had been intro­duced by humans and had become a pest. The newly introduced species unex­pectedly started consuming native snail species in large numbers. As a result, six of the seven native species originally present in Moorea are no longer found in the wild and exist only as small captive populations. Likewise, herbivorous mammals such as goats and sheep were introduced to Santa Catalina Island, California, where they overgrazed the Catalina mahogany (Cercocarpus traskiae) until only seven trees remained.

islands are particularly susceptible to the intro­duction of exotic species. Fur example, Abingdon Island, one of the Galapagos Islands off the coast of South America, was home to an endemic giant tor­toise. In 1957 several fishermen introduced goats to Abingdon Island, and within five years the Abingdon tortoise was extinct. The goats, with no natural predators on the island, had greatly increased in number and had eaten the tortoises' food. In Ha­waii, the introduction of mouplan sheep has imper­iled the mamane tree—because the sheep eat it— and the honeycreeper, an endemic bird that relics on the tree for food.

Hunting

Sometimes species become endangered or extinct as a result of deliberate efforts to eradicate or con­trol their numbers. Many of these species prey on of large predators such as the wolf, mountain lion, and grizzly bear have been decimated by rancher, hunters, and government agents. Predators of .mime animals and livestock are not the only animals vul­nerable to human control efforts. Some animals are killed because their life styles cause problems for humans. The Carolina parakeet, a beautiful green, red, and yellow bird endemic to the southeastern United States, was extinct by 1920, exterminated by farmers because it ate fruit. Prairie dogs and pocket gophers, two more examples of animals killed by humans because of their life styles, have been poisoned and trapped because their burrows weaken, the ground on which unwary cattle graze, and if the cattle step into the burrows, they may be crippled. As a result of sharply decreased numbers of prairie dogs and pocket gophers, the black-footed ferret, the natural predator of these animals, has not been found in the wild in the United States since 1986. {A successful captive breeding program enabled scientists to release 50 black-footed ferrets to the Wyoming prairie in September 1991.)

In addition to predator and pest control, hunt­ing is done for three other reasons: commercial hunters kill animals for profit—for example, by selling their fur; (2) sport hunters kill animals for recreation; and (3) subsistence hunters kill ani­mals for food. Subsistence hunting has caused the extinction of certain species in the past but is not a few human groups still rely on hunting for their food supply. Sport hunting, also a major factor in the extinction (in the case of the passenger pigeon) or near extinction (in the case of the American bison) of animals in the past, is now strictly controlled in most countries.

Commercial hunting, however, continues to endanger a number of larger animals including the tiger, cheetah, and snow leopard, whose beautiful furs are quite valuable. Rhinoceroses are slaughtered for their horns, (used for dagger handle in the Middle East and for the purported medicinal purposes and as an aphrodisiac in Asia and bear, for their gallbladders (used in Asian medicine purportedly to treat ailments from indigestion to hemorrhoids). Although these animals are legally protected, the demand for their products on the black market inn caused them to be hunted illegally. The American black bear's gallbladder, for exam­ple, can fetch thousands of dollars on the Hack market.

In contrast to commercial hunting, in which the target organism is killed, commercial harvest is the removal of the live organism from the wild.

Commercially harvested organisms end up in zoos, aquaria, research laboratories, arid pet stores. Sev­eral million birds are commercially harvested each year for the pet trade, but unfortunately many of them die in transit, and many more die from im­proper treatment after they are in their owners' homes. At least nine bird species are now threat­ened or endangered because of commercial harvest. Although it is illegal to capture endangered ani­mals from the wild, there is a thriving black market, mainly because collectors in the United States, Europe, and Japan are willing to pay extremely large amounts to obtain rare tropical birds. Imperial Amazon macaws, for example, fetch up to $30,000 each.

Animals are not the only organisms threatened by commercial harvest. Many unique and rare plants have been collected from the wild to the point that they are classed as endangered. These include carnivorous plants, certain cacti and orchids.

Where Is Declining Biological Diversity the Greatest Problem?

As many as 40 percent of the world's species are concentrated in tropical forests, areas that are in­creasingly threatened by habitat destruction (see Focus On: Vanishing Tropical Forests). This means that a few countries, primarily developing nations,

hold most of the biological diversity that is so eco­logically and economically important to the entire world. The situation is complicated by the tact that these countries are least able to afford the protec­tive measures needed to maintain biological diver­sity. International cooperation will clearly be needed to preserve our biological heritage.