The food web of the ecosystems within Yellowstone National Park are very complex and varied, and provide a terrific opportunity to teach the concept within your classroom. This is an activity written for high school environmental science students.  They will take a small sampling of animals and plants from Yellowstone and draw in the arrows to show the flow of energy.  They are then presented with a series of questions that drive at the important concepts that are revealed by a food web. Finally, they watch the excellent "How Wolves Change Rivers" video and relate that to the idea of keystone species. Food Web Analysis Worksheet and Lesson