People do not even have to directly destroy habitats to make them unsuitable for animals. Sometimes a few changes simply make life too difficult for a species. Dams on rivers, for instance, stop fish from moving up and downstream to breeding areas. The draining of a wetland might wipe out a key resting and feeding stop for migratory birds. The building of a highway might cut across the territory of a wide ranging herd of elk or a panther.
Pollution is a serious threat to wild life, especially in areas where lots of people live.
Carelessly thrown away garbage, sewage, and chemical containers contaminate the soil, rivers, and sea. Gases from factories and cars pollute the air. Too much pollution can ruin an animal's habitat. Animals also die directly at the hands of people through hunting and fishing. Wild animals are killed for food, for sport, and for their fur, skin, horns, or shells.