Keystone species

A keystone species is a species that holds its ecological community together, influencing the structure and diversity of that community far more than its abundance would suggest. The name comes from architecture: the keystone is the wedge-shaped stone at the top of an arch, and if it is pulled out, the arch collapses. In the same way, losing a keystone species can set off a chain of changes that leaves an ecosystem very different.

The ecologist Robert Paine introduced the idea in the 1960s after removing the predatory sea star Pisaster ochraceus from rocky shores in Washington State. Without the sea stars, mussels took over the rocks and crowded out many other species, and diversity fell sharply. Keystone species are often predators that keep prey in check, as sea otters do by eating the sea urchins that would otherwise destroy kelp forests. Others are ecosystem engineers, like beavers whose dams create wetlands, or mutualists, like pollinating bees.

A keystone species differs from a foundation species, such as kelp or reef-building coral, which shapes a community mainly through its sheer bulk. The label is informal, and ecologists sometimes debate which species deserve it.