Which statement correctly defines a keystone species?

• A. Species with the largest population size.
• B. Species found only in one location.
• C. Species that are always the most abundant in the ecosystem.
• D. Species with a disproportionately large ecological role relative to its abundance.

Answer: (D)

A keystone species is defined by its disproportionately large ecological role relative to its abundance. This means that even if there aren’t many individuals, their actions have a big influence on the structure and function of the ecosystem. Removing such a species often triggers large changes in which other species can survive, how energy and nutrients flow, and the overall habitat. That’s why the statement that describes a species with a disproportionately large ecological role relative to its abundance is the best definition.

Context helps: sea stars can keep mussel populations in check, allowing more species to share the space; wolves in some ecosystems help regulate herbivore numbers and promote plant diversity; beavers—often not the most numerous—engineer habitats by building dams that create wetlands used by many organisms. These examples show that impact isn’t about how many there are, but about the size of their effect on the ecosystem.