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Listen to part of a lecture in an environmental science class. Professor: Today we're going to discuss a fascinating example of ecological restoration: the reintroduction of wolves to Yellowstone National Park in the 1990s. For decades, the absence of wolves had allowed elk populations to grow unchecked, which led to overgrazing of young trees and shrubs along riverbanks. This, in turn, caused soil erosion and changed the course of streams. In 1995, wildlife managers reintroduced wolves from Canada. The immediate effect was a reduction in elk numbers, but the more interesting effect was behavioral. Elk began avoiding open river valleys, where they were vulnerable to attack, and this allowed vegetation to regenerate. As trees grew, beavers returned, and their dams created habitats for fish and amphibians. Even the physical landscape changed—streams became more stable and meandered less. This process is called a trophic cascade, where predators at the top of the food chain indirectly benefit plants and animals at lower levels. So, the wolves didn't just change the elk population; they reshaped the entire ecosystem.