Trophic cascades occur when apex predators indirectly benefit primary producers by suppressing mesopredators and herbivores. The classic example: wolf reintroduction in Yellowstone (1995) reduced elk browsing pressure, allowing riparian vegetation to recover, which stabilized stream banks and changed river morphology — the "ecology of fear." This model uses a 4-level Lotka-Volterra chain: wolves suppress coyotes (mesopredator release), coyotes suppress deer (herbivores), deer suppress vegetation. Remove wolves → coyote population grows → deer increase → vegetation collapses. The cascade can be surprisingly fast and far-reaching.