Named after the wedge-shaped stone at the top of an arch, which locks the other stones into position and prevents collapse, a keystone species is one that plays a central role in the structure of an ecosystem.
Beavers, for example, physically change the landscape through their dam-building, creating habitats used by other species. And some top predators enable diverse vegetation to flourish by controlling the number of grazing animals.
The term does not apply to just any influential organisms. Crucially, keystone species are those that have a disproportionately large effect relative to their abundance. It would not apply to, say, trees or reef-building corals, which have a huge effect on ecosystems because of their abundance.
According to Robert Paine, the zoologist who coined the phrase 'keystone species' in the 1960s, it “was intended to convey a sense of nature’s dynamic fragility and the unsuspected consequences of removing (or adding) species”.
Paine had experimentally removed ochre starfish from areas of rocky shore, which led to mussels – the starfish’s prey – crowding out other encrusting organisms.
Other keystone species include sea otters, which are highly influential in the functioning of kelp forests, and wolves, whose loss from Yellowstone – and subsequent reintroduction – had dramatic effects on the national park’s ecology.