The Sea Star That Reshaped a Shore

On a rocky Pacific shore, mussels can crowd together so tightly that little bare stone remains. They are excellent competitors for space. Yet a familiar predator can keep that strength from turning into total control: a sea star that pulls open mussel shells and feeds on the animals inside.

Ecologist Robert Paine tested this relationship on a section of coast. He repeatedly removed the predatory sea stars from one area while leaving another area alone for comparison. Without the predator, mussels spread. Several other species lost places to attach, feed, or shelter, and the community became less diverse. The sea star was not the most numerous animal on the rocks. Its influence came from which animal it ate.

This experiment helped shape the idea of a keystone species. In an arch, a keystone is one stone, but its position helps support the whole structure. In an ecosystem, a keystone species has an effect much larger than its abundance might suggest. Removing it can reorganize relationships among many other species.

The label does not mean every predator is a keystone in every location. Food webs change across habitats and seasons, and later studies may reveal additional causes. Paine's result mattered because it joined a clear intervention to a broad ecological change. Instead of merely noticing that sea stars and diversity occurred together, he changed one factor and watched the shore respond.