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. 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 living nearby. Pesticides and other chemicals can poison plants and animals if they are not used correctly. The bald eagle is one bird that was harmed by pesticides. In the past, a pesticide called DDT was used by many farmers. Rains washed the pesticide into the lakes and streams where it poisoned fish.