Answer: Wolves

By the early 1990s, Yellowstone National Park, one of America’s most iconic parks, was in trouble. Herds of deer and elk, unchecked by natural predators, were stripping the forest bare. The huge herds were eating tree saplings before they had a chance to mature, consuming the heavy foliage that protected the lake shores and river banks from erosion, and otherwise enjoying their free reign like they owned the place.

And own the place they did: the predators that could keep them in check had been gone from the park since the 1920s. In 1926, the last known wolf pack in Yellowstone was killed, and the huge park was apex-predator free for the intervening seven decades. All that changed in 1995 when, after years of encouragement by biologists, park rangers introduced wolves back into Yellowstone.

The introduction of the wolves completely transformed the park. Not only did the wolves begin to cull the deer and elk herds, their presence also changed the movement patterns of the herds. Deer lingered less in any one area of the park, moved more frequently, and the vegetation thickened up, encouraging other animals (such as birds and beavers) to repopulate the area. The thicker vegetation, more abundant saplings (which are now maturing into larger trees), and other secondary changes brought about by the presence of the wolves has stopped river erosion and changed the very landscape of the park.