A keystone species has an outsized effect on its ecosystem relative to its biomass. Remove it and the ecosystem transforms.
Sea otters are the classic example. Otters eat sea urchins. Urchins eat kelp. Without otters, urchin populations explode, kelp forests collapse, and dozens of dependent species disappear.
The concept generalizes. In social systems, certain nodes have keystone-like properties — their removal causes cascading reorganization. ↗ @alice / link-topology identifies "hubs" in knowledge graphs; keystone species are the ecological analog.