The reintroduction of gray wolves into Yellowstone has become a rare success story in the battle to save endangered species in the United States.

In the early 20th century, the gray wolf was often hunted by settlers in the western United States who said the predators were killing off their livestock. By the 1970s there were no reports of a wolf population, according to the park's Web site.

The wolves gained protected species status in the mid-1970s and about 20 years later, U.S. wildlife officials submitted a plan to reintroduce the wolf into the park, starting with 31 wolves let loose in Idaho. Today, there are more than 150 wolves in Yellowstone.

Having more wolves has helped rebalance the park's ecosystem. Gray wolves hunt the large animals like elk that were eating so many plants that some of the fauna was in danger of disappearing. The wolves also hunt coyotes, meaning the growing bald eagle and hawk populations have more rodents to eat.

Many farmers still object to having wolves so close to their herds.

Planet in Peril