The Grey Wolf is widely credited as the animal that "saved" the Yellowstone National Park ecosystem through a process known as a trophic cascade. After being eradicated from the park in the 1920s, wolves were reintroduced in 1995. Before their return, the elk population had exploded, overgrazing the park’s willow and aspen trees along riverbanks. This led to massive erosion and the disappearance of songbirds and beavers, who lacked the wood needed for their dams. Once the wolves returned, they naturally thinned the elk herds and, more importantly, changed the elk’s behavior (the "ecology of fear"), forcing them to avoid open valleys where they were vulnerable. This allowed the vegetation to recover, which stabilized the riverbanks, reduced erosion, and brought back the beavers. The beavers' dams created new habitats for fish, otters, and amphibians, while the wolves' leftover kills provided food for bears, ravens, and eagles. This restoration of balance is one of the most famous examples in modern biology of how a single "apex predator" can revitalize an entire landscape and repair a broken environment.