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4 Major Reasons Why Animal Species Become Endangered

Species become endangered for various reasons. But today almost all of them can be related directly or indirectly to human activity. Endangered status most frequently results from overhunting, loss of habitat and food, low population, or poisoning of the environment.

1. Hunting

Some animals are hunted as food, some as trophies; others are hunted commercially. The polar bear (Ursus maritimus) of the Arctic has long been hunted for subsistence by Inuit (Eskimo) and other native peoples

Until recently it was also a popular sport-hunting trophy. As long as the bears were chased by hunters on foot, the prey had a reasonable chance to escape. Their numbers declined precipitously once hunters began using airplanes and helicopters. Today, polar bears are protected throughout their range. They are hunted only in limited numbers by indigenous peoples.

Trophy hunters once killed hundreds of tigers (Panthera tigris). But the greatest threat to these endangered cats now comes from poachers -hunters who supply an illegal trade in body parts used for Asian folk medicines. Another threat to tigers comes from human encroachment.

As the human population of southern Asia increases, the amount of wilderness will diminish, and the cat may disappear. Of the nine tiger subspecies that once roamed Earth, the Caspian, Bali, and Javan tigers became extinct in the 20th century, leaving only six subspecies. As of 2010, the population of wild tigers was estimated at 3,200. Poaching and habitat loss remain serious threats.

Other animal species are hunted commercially. The spotted cats are all threatened. This is especially true for the cheetah (Acinonyx jubatus); leopard (Panthera pardus); jaguar (Panthera onca); ocelot (Leopardus pardalis); and margay (Leopardus wiedii).

Spotted cats are usually hunted for their fur, for sport, and to protect livestock. Hunting for sport is most easily controlled. Commercial and protective hunting are difficult to regulate and pose the greatest threats.

Cheetahs are extinct in India, and their range is much reduced in the Middle East and in Africa. The number of leopards has also been greatly reduced. Until the Convention on International Trade in Endangered Species (CITES) in 1973, many of these cats were killed each year for their fur.

Populations of South American wildcats -jaguars, ocelots, and margays- have also been impacted by the illegal fur trade. Although these cats have disappeared from many areas where they once lived, sufficient numbers probably still exist for each species to survive if the hunting is stopped.

Commercial hunting is also directly responsible for the endangered status of many species of whales. The cause of the population reduction in this species is understood and reversible. These mammals are killed mainly for the oil extracted from their blubber, for human and animal food, and for fertilizer.

One hopeful example is the endangered blue whale (Balaenoptera musculus). In the mid-1950s, there were 30,000 to 40,000 of these huge mammals. Some individuals reached more than 100 feet (30 meters) in length. Soon after, the whales were widely hunted using modern deck-mounted harpoon cannons. Each year more were killed than were born. In 1965, a moratorium was placed on taking blue whales.

The current blue-whale population is estimated to be fewer than 3,000 animals. International agreements in the 1980s and early 1990s have limited the hunting of larger whales.

2. Loss of Habitat and Food Supply

Indirect threats to animals and plants are often more dangerous, more widespread, and less obvious than such direct threats as poaching and commercial hunting. Most threatened species are not hunted. Instead, they are endangered by the loss of their habitat or food supply.

For example, the polar bear is protected from hunting. But a 2005 study determined that climate change seriously threatens the species. Melting sea ice hampers the animals' ability to hunt and reproduce. The bear is also threatened by recent decisions to allow drilling for oil and gas in their habitat areas. In 2008, the U.S. Fish and Wildlife Service added the species to the Endangered Species List.

Another example is the ivory-billed woodpecker (Campephilus principalis). This large bird once inhabited the great river-bottom forests from the Mississippi River to North Carolina to the swamps of Florida. As the forests were cut down, the woodpecker became scarce and was ultimately thought to be extinct in this country.

In 2005, a team of scientists reported that at least one ivory bill lives in the remote Big Woods region of southeastern Arkansas. They backed up their claim with audio recordings of the bird's call. Despite the lack of conclusive evidence, conservation efforts are attempting to protect the bird's habitat.

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The American black-footed ferret (Mustela nigripes), a member of the weasel family, has always lived in close association with the black-tailed prairie dog (Cynomys ludovicianus), a grassland rodent.

The ferrets not only feed almost exclusively on the prairie dogs, but also live in their burrows. But prairie dogs have been exterminated by ranchers throughout most of their range. The rodents eat the grass desired for grazing livestock, and their burrows are sometimes responsible for injuring cattle and horses.

As the prairie dogs gradually disappeared, so did the ferrets. In the 1980s, the few remaining wild ferrets, threatened by disease, were removed to be bred in captivity. In the 1990s and early 2000s, efforts to replenish the species involved releasing groups of ferrets into protected areas where the animals could establish small, stable populations. There are currently about 300 ferrets in captivity. The additional 500 individuals in the wild were all reintroduced.

The loss of habitat threatens many forest species, especially on the island of Madagascar. The golden bamboo lemurs, which live only on this island, may soon become extinct because there is little forest left. The same is true of another native of Madagascar, the aye-aye (Daubentonia madagascariensis).

The aye-ayes, whose current population is unknown, are victims of logging and local superstition -they are considered bad luck. These specialized primates feed on wood-boring insects, which they detect with their excellent hearing. They gnaw the wood with their strong front teeth and remove the grubs or adult insects with a highly specialized, long, thin middle finger. Without large expanses of forests, the aye-aye cannot survive.

Other primates endangered by the destruction of forests include the orangutan (Pongo pygmaeus) and the mountain gorilla (Gorilla gorilla beringei). Both species have also been threatened by wars waged near their habitats.

3. Population Levels

Some species are threatened with extinction because their numbers, while relatively stable, are so small that they may never be able to increase to a safe level. Any small population can easily be wiped out by one catastrophic occurrence, such as a hurricane, a flood, or a fire. This is especially true when all the animals are found within a single region.

An example of such a species is the American whooping crane (Grus americana), whose numbers were down to only 15 birds in 1941. Even after intense conservation efforts, there were still only 30 birds in 1963. The number in the wild has increased in recent years. In 2010 there were approximately 540 wild and captive birds.

4. Poisoning of the Environment

Another threat to wildlife has increased since the middle of the 20th century: the poisoning of the environment. As the human population has grown, vast amounts of wild land have been cleared for agriculture. This deprives wildlife of crucial habitat. But new agricultural techniques to make the land more productive have also included the widespread use of pesticides.

Herbicides that kill weeds and insecticides that destroy insects are dumped on the land in huge quantities. Such chemicals are beneficial in increasing crop yields and controlling disease-carrying insects. However, many of the poisons undergo chemical breakdown very slowly, allowing them to accumulate in the soil and wash into streams, lakes, and oceans.

Once they enter water, the chemicals are eaten or absorbed by microscopic organisms. As these tiny animals and plants are eaten by larger animals, the poisons accumulate in the bodies of the animals in increasingly high concentrations. Thus, the largest animals -those at the top of the so-called food chain- suffer the greatest damage because they take in the highest concentrations of poisons.

For example, contaminated algae may be eaten by a small crustacean, which in turn is eaten by a small fish. The small fish may then be eaten by a larger fish, and the larger fish eaten by a grizzly bear. This progression makes up a food chain. The bear at the top of the chain receives the most poison.

The insecticide DDT moves along the food chain in this manner. Beginning in 1946, DDT was widely used as an effective weapon against agricultural pests. But DDT does not break down easily in nature, causing it to remain in the environment for many years.

DDT can cause physiological and genetic changes in animals and humans. For example, it disrupts the calcium-producing mechanism in birds. This means that their eggs are laid with an inadequate, thin, flaky shell that usually breaks before the embryos reach maturity.

As a result, populations of the American bald eagle (Haliaeëtus leucocephalus), the osprey (Pandion haliaëtus), the peregrine falcon (Falco peregrinus), the brown pelican (Pelecanus occidentalis), and the Bermuda petrel (Pterodroma cahow) declined sharply. DDT also reaches humans—as analysis of the milk of some nursing mothers shows.

The governments of several countries, including the United States, Canada, and Sweden, restricted the use of DDT in the late 1960s and early 1970s. Since then, many of the previously declining bird species have made a dramatic recovery. However, there are many other defoliant and pesticide substances still in use whose long-range effects need to be carefully evaluated.

Mercury, for instance, is used by agriculture and industry to kill slime mold and fungus. When this poisonous heavy metal finds its way into the sea, it accumulates in living organisms. In some areas, certain fish have been rendered at least temporarily inedible by the high concentrations of mercury in their bodies. Some of the sea mammals that feed heavily on fish are beginning to show high concentrations of mercury and insecticides.

California sea lions (Zalophus californianus), Northern fur seals (Callorhinus ursinus), and even polar bears in the Arctic and penguins in the Antarctic have been affected by poisonous heavy metals. The poisons circulate throughout the world and are present in the cells of marine plants and animals in all the oceans. A recent Canadian study suggests that a toxic mercury rain falls on the Arctic each spring.

Polychlorinated biphenyls (PCBs) -chemicals that are used in insulating fluids, paints, plastics, and rubbers- have also found their way into the sea.

The indiscriminate use of poisonous chemicals may now represent the greatest threat to plant and animal life, including humans. The most serious aspect of this situation is that even if all use of these poisons were to cease today, they would continue to pollute the environment for many years to come.

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