Bringing back one predator reshaped an entire national park's rivers

Wolves were hunted out of Yellowstone by 1926; without them, elk grazed river-bank willows and aspens unchecked for decades. When 31 wolves were reintroduced in 1995–96, elk began avoiding exposed riverbanks again, letting vegetation recover — changes researchers linked to shifts in beaver populations and river-channel stability. The scale of the ripple effects is still debated, but the reintroduction remains a closely studied example of a top predator reshaping an ecosystem.

— Wikipedia contributors, History of Wolves in Yellowstone — Yellowstone National Park, 1995–1996