You must watch this fascinating four-minute video on the effects of the re-introduction of wolves in Yellowstone National Park. Not only they affected the entire live of the park—increasing the number of species in it!—but actually changed the geography of the park itself, affecting the rivers in a way that positively affected everything.
The narrator is incredibly excited about this whole phenomenon, but that only serves to make US more interested. If he's this ecstatic about something, it must be worth watching, right? Lo and behold, "worth watching" is a gross understatement. This is easily one of the best, most informative and even mesmerizing videos I've seen in quite some time. Kudos, times two.