Robert Paine cleared ochre sea stars from marked stretches of rocky intertidal at Mukkaw Bay and kept them out, comparing the result with untouched controls. Without predation, California mussels outcompeted everything else for space. Barnacles, algae, limpets and anemones disappeared from the cleared plots, and species richness fell sharply.
The insight was that a species' influence on community structure need not scale with its abundance. Pisaster was not numerically dominant, but its feeding freed the limiting resource, primary rock space, for everything else. Paine named this role after the wedge-shaped stone that holds an arch together.
The concept has since been stretched well beyond what the original experiment supports, applied to charismatic animals with no comparable removal evidence. Since 2013 the species has itself been hit hard by sea star wasting disease, with populations dropping steeply in many areas, and how completely the intertidal community will revert without it is still being tracked.