When wolves were reintroduced into Yellowstone National Park after a 70-year absence they had a remarkably wide-ranging set of effects. Most immediately they helped control the elk and coyotes, not just by killing some but by altering their behavior. As a result vegetation recovered, and that led to the return of various other wildlife and even to the stabilization of rivers.
The following brief video describes this. The narration is by George Monbiot from a longer TED talk he gave in June 2013 in Edinburgh Scotland. (He refers to the elk as "deer" because in the UK elk are known as red deer.)
(Thanks to my brother Mark for pointing out this video.)