Which term describes a species whose impact on its community or ecosystem is larger than expected from its abundance?

• A. Trophic Level
• B. Keystone Species
• C. Biomass
• D. Phase Shift

Answer: (B)

A keystone species is one whose impact on its ecosystem is disproportionately large compared with its abundance. This means that even in small numbers, its actions—such as predation, competition, or habitat modification—shape which species thrive and how energy moves through the community. For example, sea otters keep sea urchin populations in check, which allows kelp forests to flourish; removing otters leads to urchin overgrazing and a major shift in the community. Beavers, by building dams, create wetlands that support a wide range of species, altering habitat structure. If the keystone species is removed, the ecosystem often undergoes cascading changes and can shift to a very different state.

By contrast, trophic level simply describes a species’ position in the food web, biomass refers to the total mass of living material present, and a phase shift is a large, abrupt change in the ecosystem’s state—not a description of a species’ relative influence.