What best defines a keystone species?

• A. A keystone species is an organism that helps define an entire ecosystem; without it, the ecosystem would be dramatically different or cease to exist
• B. A keystone species is the most abundant species
• C. A keystone species is always a predator
• D. A keystone species is only found in tropical ecosystems

Answer: (A)

A keystone species is one that has a disproportionately large influence on its environment compared with how common it is, helping to shape the structure and function of the whole ecosystem. Its actions regulate other populations and maintain biodiversity; removing it often leads to dramatic changes or collapse of the ecosystem’s balance. Think of sea otters keeping urchin numbers in check so kelp forests survive, or wolves controlling herbivore numbers to protect plant communities. Beavers, as ecosystem engineers, create wetlands that support many species, illustrating how a single species can underpin an entire community.

This is why the defining idea isn’t about being the most abundant or about always being a predator; keystone species can be rare or non-predatory, and they exist in many biomes, not just tropical ones. The key is the strong, wide-reaching impact their presence or absence has on the ecosystem’s structure and diversity.