More recently, the dodo bird and the sea mink also have disappeared. Extinction has been going on since life began on earth. But today, extinction is happening faster than ever before. There are more than 600 endangered or threatened species in the United States today. Endangered species are those plants and animals that are so rare they are in danger of becoming extinct.
Threatened species are plants and animals whose numbers are very low or decreasing rapidly.
Threatened species are not endangered yet, but are likely to become endangered in the future. Species disappear because of changes to the earth that are caused either by nature or by the actions of people. Sometimes a terrible 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 great glaciers melted and the earth became warmer, many species died because they could not live in a warmer climate.