In the late 1990s scientists began to notice changes in Yellowstone National Park, in Wyoming. Trees that had stopped growing decades ago began to grow again. Animals started behaving differently. What caused these mysterious changes?
Scientists believe they have found the answerÂ—wolves.
Wolves used to live in Yellowstone, which is in the northern U.S. Rocky Mountains. But many people thought wolves were pests that killed livestock and harmed crops. In 1926 the last wolf in Yellowstone was shot and killed.
For the next 70 years Yellowstone did not have any wolves. Scientists decided to bring them back to the area in 1995. The U.S. Fish and Wildlife Service brought in 15 gray wolves from Canada.