Wolves were once the top predator in America’s world-famous Yellowstone National Park. But the population was eradicated in the 1920s, leaving the wilderness wolf-free for seven decades.

In 1995, however, wolves were reintroduced to Yellowstone; this gave biologists a unique opportunity to study what happens when a top predator returns to an ecosystem.

They were brought in to manage the rising elk population, which had been overgrazing much of the park, but their effect went far beyond that. In this film, The Nature Conservancy’s lead scientist Dr M Sanjayan, Dr Valerie Kapos of the UN Environment Programme and animal behaviourist Kirsty Peake describe how the returning wolves dramatically changed the park’s rivers, forests – and the landscape itself.

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