Keystone species

A keystone species is a species that has a disproportionately large effect on its ecosystem relative to its abundance. The term was coined by Robert Paine in 1969, who observed that removing the predatory sea star Pisaster ochraceus from a rocky intertidal community caused a dramatic increase in mussel populations, which outcompeted other species and collapsed community diversity.

In network ecology, keystone species are identified not by their biomass but by their topological position: they are often highly connected nodes that bridge otherwise disconnected modules. Their removal can trigger trophic cascades or fragment the interaction network.

The concept has been extended to include ecosystem engineers (species that physically modify habitats) and mobile links (species that connect spatially separated ecosystems through migration). What unifies these categories is that the species' influence is structural: it shapes the network within which other interactions occur.