Over the last three decades, Yellowstone National Park has undergone an ecological cascade. As elk numbers fell, aspen and willow trees thrived. This, in turn, allowed beaver numbers to increase, creating new habitats for fish and birds.
The shift has largely been attributed to the reintroduction of wolves to the park — as predators, they helped control the elk numbers. But their return may not have reshaped the entire ecosystem in the way that scientists thought, and has sparked a fierce debate among scientists over exactly why and how Yellowstone has rebounded.
