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. Newer species that could survive a warmer environment took their places.