Define keystone species and its importance.

• A. A keystone species has a disproportionately large effect on the ecosystem relative to its abundance; removing it can cause cascading changes
• B. A keystone species is the most abundant organism in the ecosystem
• C. A keystone species is always a top predator with no effect
• D. A keystone species only exists in aquatic systems

Answer: (A)

A keystone species is one that has a disproportionately large effect on the structure and function of its ecosystem relative to how abundant it is. This means its actions help regulate populations, create or maintain habitats, and support other species. Because of this strong influence, removing a keystone species can trigger cascading changes across the community, often reducing biodiversity and altering how the ecosystem works.

For example, sea otters keep sea urchin numbers in check, which lets kelp forests thrive and support a wide array of marine life. In terrestrial systems, wolves in Yellowstone helped control elk populations, allowing vegetation to recover and benefiting many other species. Beavers also act as engineers, building dams that create wetlands used by numerous organisms. These illustrate how a species can shape an ecosystem far beyond what its abundance might suggest.

Statements that say a keystone species is the most abundant, always a top predator with no effect, or only exists in aquatic systems don’t fit, because keystone roles come from functional impact rather than numbers, can be non-predators or engineers, and occur in many different environments.