Why do sea urchins sometimes overrun a reef?

Sea urchins are important herbivores, but their populations can surge when natural predators are reduced by fishing or ecosystem change. In some reef systems, this can lead to intense grazing that strips away algae and, in extreme cases, damages living coral or kelp habitat. These outbreaks often follow disturbances such as overfishing, warming events, or disease in urchin predators. The result can be a shift to a less diverse ecosystem dominated by urchins or turf algae. Reef managers sometimes monitor urchin densities closely because their numbers can signal ecological imbalance.