What Is a Keystone Species?

The concept was introduced by ecologist Robert Paine in 1969 after his experiments on the Pacific coast. He removed the predatory starfish Pisaster ochraceus from tide pools and found that without it, mussels took over and outcompeted other species, reducing biodiversity from 15 to 8 species. The starfish, though not particularly abundant, was keeping the ecosystem diverse by controlling mussel populations — it was a "keystone" holding the ecological arch together. Classic keystone species include: sea otters (controlling sea urchins that would otherwise destroy kelp forests), wolves in Yellowstone (controlling elk that were overgrazing riverbanks, causing erosion), beavers (creating wetland habitats through dam-building), and African elephants (knocking down trees to maintain savanna grasslands). Keystone species can be predators, herbivores, pollinators (like fig wasps), or ecosystem engineers (like coral polyps). Their identification is crucial for conservation prioritisation.