Wolves - mysterious, captivating, and essential.  This case study examines the trophic cascade that occurred in Yellowstone after Gray Wolves were reintroduced...a re-entry so impactful that even the rivers of Yellowstone National Park were changed.  In my AP Environmental Science classroom, I use it as a Unit 1 opening activity after covering biotic and abiotic factors.  We watch the video “How Wolves Change Rivers” on Youtube, then students create models in collaborative groups on whiteboards.    It can also be given as an end-of-unit performance task.