A species that has a disproportionately large impact on its ecosystem relative to its abundance, playing a critical role in maintaining the structure and function of the ecological community.
Metaphorically named after the keystone in an arch—the central stone that holds the structure together. Coined by ecologist Robert Paine in 1969 after studying how removing sea stars dramatically altered rocky shore communities.
Remove a keystone species and the whole ecosystem can collapse like a house of cards! Sea otters are classic examples—when they disappear, sea urchins explode in number and devour kelp forests, transforming underwater cathedrals into barren seafloor.
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