How Wolves Change Rivers [video]
I think this is just one example of how little we understand nature's systems. The food chain effect is well-documented in terms of numbers. For example, we already know that if you decrease the number of predators in an ecosystem, the species they normally prey on grow out of control. But the video shows that there's a massive amount of complexity under the surface of this model. For example, the wolves in Yellowstone changed not just the number of deer (their prey) but also their behaviors, which is one of the factors responsible for the cascade effect. Similarly, the increase in beaver numbers resulted in the creation of more niche ecosystems for other species.