George Monbiot explains that when wolves were reintroduced to Yellowstone National Park in the United States after being absent nearly 70 years, the most remarkable 'trophic cascade' occurred. What is a trophic cascade and how exactly do wolves change rivers? Find out in this beautiful little film.

"When we try to pick out anything by itself, we find it hitched to everything else in the Universe." - John Muir

Visit http://sustainableman.org/ to explore the world of sustainability. For more from George Monbiot, visit http://www.monbiot.com/

To read more about wolves see Eleanor O'Hanlon's wonderful book, The Eyes of the Wild.

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