7/9. The Yellowstone Wolves. It's been ten years since wolves were reintroduced to Yellowstone National Park in Wyoming, USA. They are now very much a part of the ecosystem and their effect on their prey species, mainly elk, is giving wolf biologists a wonderful opportunity to witness the role of a keystone species establishing itself in its natural environment. Howard Stableford heads off into wolf country to find out how the population has developed. Producer Sheena Duncan Repeated tomorrow at 11am