Trophic Cascades

When a change at one level of the food web ripples through several levels, it is called a trophic cascade. The Yellowstone wolf story is a top-down cascade — predator affects herbivore affects vegetation. Bottom-up cascades work in reverse — changes in nutrients or primary producers ripple upward.

Ocean cascades can be just as dramatic. Where sea otters are abundant, they eat sea urchins, the urchins stay in check, and kelp forests flourish. Remove the otters, and exploding urchin populations can graze the kelp down to bare rock.

Trophic cascades demonstrate that ecosystems are not collections of independent species. They are integrated systems where changes propagate through connections in ways that are difficult to predict but follow discoverable patterns.