An endangered species is an animal or plant that's considered at risk of extinction. A species can be listed as endangered at the state, federal, and international level. On the federal level, the endangered species list is managed under the Endangered Species Act.
The Endangered Species Act (ESA) was enacted by Congress in 1973. Under the ESA, the federal government has the responsibility to protect endangered species (species that are likely to become extinct throughout all or a large portion of their range), threatened species (species that are likely to become endangered in the near future), and critical habitat (areas vital to the survival of endangered or threatened species).
The Endangered Species Act has lists of protected plant and animal species both nationally and worldwide. When a species is given ESA protection, it is said to be a "listed" species. Many additional species are evaluated for possible protection under the ESA, and they are called “candidate” species. Continue reading from National Wildlife Federation
Species disappear because of changes to the earth that are caused either by nature or by the actions of people. Sometimes a natural event, like a volcano erupting, can kill an entire species. Other times, extinction will happen slowly as nature changes our world. For example, after the Ice Ages, when the glaciers melted and the earth became warmer, many species died because they could not live in a warmer climate. Newer species that could survive in a warmer environment took their places.
People can also cause the extinction of plants and animals. The main reason that many species are endangered or threatened today is because people have changed the homes or habitats upon which these species depend. A habitat includes not only the other plants and animals in an area, but all of the things needed for the species' survival -- from sunlight and wind to food and shelter. The United States has many habitats, from ocean beaches to mountain tops. Every species requires a certain habitat in order to live. A cactus, for example, needs the sunny, dry desert in order to grow. A polar bear, on the other hand, would not live in a desert, because it could not find enough food and water.
Pollution can also affect wildlife and contribute to extinction. The Nashville crayfish is endangered mainly because the creek where it lives has been polluted by people. Pesticides and other chemicals can poison plants and animals if they are not used correctly. Continue reading from Environmental Protection Agency
List of Endangered Species (World Wildlife Fund)
10 of the World's Most Endangered Animals (World Wide Fund for Nature)
The Endangered Species Act (Center for Biological Diversity)
The Difference Between a Threatened and Endangered Species (National Ocean Service)