Endangered species are still alive today but exist in numbers so low or are decreasing so rapidly that they are in immediate danger of extinction. The Endangered Species Act of 1973 defines an endangered species as any plant or animal species that is in danger of extinction throughout all or a significant portion of its range. Extinct species are species that are gone forever. They once lived on this planet but have died out. Extinction has been occurring since life began on Earth. However, today extinction is occurring at a faster rate than ever. Species disappear because of either changes in nature or the actions of people. In some instances a major natural event, such as a volcanic eruption, can kill an entire isolated species. In other cases, extinction will happen slowly as nature changes the world. For example, the dinosaurs disappeared because of a change in Earth's climate, and they could not survive the changing temperatures.
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