A species whose impact on its community is much larger than would be expected from its abundance.

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Keystone species are organisms whose impact on their ecosystem is disproportionately large compared to their abundance. This means that even a small number of individuals can regulate predator–prey dynamics, modify habitats, or shape energy flow in ways that support many other species. For example, sea otters keep sea urchin populations in check, allowing kelp forests to flourish; without otters, urchins can overgraze kelp and drastically alter the habitat. Similarly, predators like wolves can curb herbivore numbers and trigger cascading effects through the ecosystem, while beavers engineer habitats by building dams that create wetlands used by numerous organisms. The defining idea is that the ecological influence is much greater than what their population size would suggest. This isn’t describing a phase shift (a broad change in ecosystem structure after disturbance), primary succession (the formation of a community on new land), or biomass (the total amount of living matter), which are about processes or measurements rather than the outsized role of a particular species.