Wolves were reintroduced to Yellowstone National Park in 1995. They began to hunt the abundant elk population, so elk numbers decreased. The elk carcasses provided food for other predators, like foxes, coyotes, and bears, so all of their populations increased. Elk used to overgraze on willow plants; now that willows are growing back, beaver numbers are increasing, because they prefer the slow-moving water that willows cause.

In the context you've described, the wolves are considered a keystone species. Keystone species have a disproportionately large impact on their environment relative to their abundance. The reintroduction of wolves to Yellowstone helped regulate the elk population, which in turn allowed willows and other plants to recover. This chain of events affected various other species, including beavers and various predators. The presence of wolves has thus had a significant and cascading impact on the ecosystem, making them a classic example of a keystone species.