In the biodiversity passage, what is the best definition of a keystone species?

• A. A species that is most abundant.
• B. A species whose extinction would dramatically alter the ecosystem.
• C. A species with the widest geographic range.
• D. A species with strong morphological defenses.

Answer: (B)

The main idea here is that a keystone species has a crucial ecological role that keeps the ecosystem functioning, and its loss would cause big changes in the community. This kind of species shapes interactions, resources, or habitat in a way that supports many other species. When such a species goes extinct, the ecosystem can shift dramatically, sometimes losing diversity and altering structure.

That’s why the best definition is a species whose extinction would dramatically alter the ecosystem. It emphasizes the big ripple effect on the whole community that keystone species have, beyond just being common, widespread, or well-defended. The other ideas focus on abundance, geographic range, or defenses, none of which by themselves capture the idea of causing a major ecosystem-wide change when removed.