Endangered species are living things whose population is so reduced that they are threatened with extinction. Thousands of species are included in this category. The International Union for the Conservation of nature and Natural Resources publishes a list of threatened mammals, birds, reptiles, amphibians, and (many people donÕt consider them) plants.
             Millions of years before humans, extinction of living things was linked to geological and climate, the effects of which were translated into major alternation of the environment. Environmental change is still the primary cause of the extinction of animals, but now the changes are greatly accelerated by humans activity. Clearing land for farms and towns, lumbering, mining, building dams, and draining wetlands all alter the environments so extensively that ecosystems may be completely destroyed. With a burgeoning human population requiring food, shelter, and clothing and constantly demanding more energy-using devices, the temperation to exploit land for human use without regard for consequences is great.
             Frequently, several forms of environmental change are responsible for the disappearance of species. For example, as tropical forests are cut down, primates have progressively smaller feeding and living spaces. They also become more accessible to hunters, who kill monkeys for food and trap many primates for sale as pets, research animals, and zoo specimens. Some animal species may move into human communities when their own are destroyed. Extermination of marauding monkeys, roaming tigers, or foraging deer is easy to justify by people whose livelihood is threatened.
             Pollution is another form of environments change. Forty species of birds in the United States, including peregrine hawk, bald eagle, pelicans, and roseate terns, lay thin-shelled as a result of ingesting degradation products of and some other chlorination hydrocarbon insecticides that make
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