After being absent for nearly 70 years, wolves were reintroduced to the Yellowstone National Park and the most extraordinary trophic cascade occurred. George Monbiot explains what a trophic cascade is and how the wolves changed the rivers. Note: There are “elk” pictured in this video when the narrator is referring to “deer.” This is because the narrator is British and the British word for “elk” is “red deer” or “deer” for short.
Cover photo by [Dave Dance](https://www.flickr.com/photos/ddnp/10703552836/in/pool-ynpwolves).