What is a keystone species, and how can its removal affect an ecosystem?

• A. A keystone species has a disproportionately large effect on ecosystem structure; removing it can trigger cascading changes.
• B. A keystone species is the most abundant species; removal only slightly affects ecosystem.
• C. Keystones are only found in aquatic ecosystems.
• D. Keystone species are predators that keep prey populations extremely low.

Answer: (A)

A keystone species has a disproportionately large effect on the structure and functioning of an ecosystem relative to how abundant it is. Its activities—whether limiting a key prey, engineering the habitat, or shaping nutrient cycles—support many other species that rely on those processes. Because of this, removing a keystone species can trigger cascading changes across food webs and habitats, often leading to a decline in biodiversity and a shift to a different community structure. For example, sea otters keep sea urchin numbers in check, allowing kelp forests to thrive; without otters, urchins overconsume kelp and the whole community that depends on that habitat can unravel. This highlights that the importance of a keystone species comes from the role it plays, not its abundance. Keystones are not limited to aquatic systems, nor are they defined simply by being a predator that keeps prey low.