In the 1800’s, wolves were top predators in Yellowstone National Park. As humans moved westward, wolf populations were negatively impacted. Human expansion decreased their available prey. Human agricultural practices introduced livestock, which were easy prey for wolves. Humans protected their livestock and intentionally eliminated the wolves. By the mid-1900’s, no wolf packs lived in Yellowstone National Park. This left the park with few top predators. As a result, elk overgrazed young willow and cottonwood trees that stabilized riverbanks. This caused increased erosion, which filled rivers with silt. Fish populations declined as a result of the additional silt in the rivers and populations of animals that depended upon fish also declined. With a drop in cottonwood and willow trees, songbird and beaver populations lost potential habitat and materials needed to construct habitat. Fewer beaver dams resulted in fewer small, still ponds which fish use to reproduce and grow to adulthood. In 1995, wolves were reintroduced to Yellowstone National Park.