I heard a very inserting TED talk[0] about the wolves in Yellowstone, actually indirectly changing the course of the river, and returning much of Yellowstone to its previous glory. The speaker wants to reintroduce these mega faunae all over the world. I don't remember if he brings up the elk or not.
[0]
https://www.youtube.com/watch?v=kbrq08p7zfM