Which statement correctly defines a keystone species and gives a representative example?

• A. A keystone species has disproportionate influence on ecosystem; example plankton; role drives food web mass.
• B. A keystone species is any predator; example shark; role reduces all species.
• C. A keystone species has disproportionate influence on ecosystem; example sea otter; controls kelp forests and biodiversity.
• D. A keystone species is a dominant primary producer; example pine tree; role supports forest structure.

Answer: (C)

Keystone species are those that have a disproportionately large effect on the structure and health of an ecosystem relative to how common they are. The example of a sea otter fits this idea well: otters keep sea urchin populations in check, which prevents overgrazing of kelp forests. When kelp forests remain, they provide habitat and food for many species, maintaining biodiversity and ecosystem function. This is the classic trophic cascade: a relatively small number of otters can sustain an entire kelp-forest ecosystem.

This interpretation makes the sea otter example the best choice, because it clearly shows how one species can control a key consumer and thereby shape the whole community. The other statements either misidentify the type of influence (not all predators, not a single dominant producer) or use an example that doesn’t illustrate how a single species governs ecosystem structure.