A keystone species is best described as which of the following?

• A. A species that migrates seasonally but has minimal ecological impact.
• B. The most abundant species in a habitat.
• C. A species that is always the top predator in every ecosystem.
• D. A species whose ecological role has a disproportionately large effect on ecosystem structure; example: sea otter maintaining kelp forests by controlling urchin populations.

Answer: (B)

Keystone species are those whose ecological role has a disproportionately large effect on ecosystem structure relative to how common they are. The sea otter is a classic example: by preying on sea urchins, otters keep urchin numbers in check, which allows kelp forests to flourish. If otters are removed, urchins overgraze kelp, leading to loss of habitat and ripple effects for many other species. This shows that impact isn’t about being the most abundant or always being at the top of a food chain; it’s about the big influence a species has on the whole community. The idea being tested is that a keystone species has a large effect on ecosystem structure, not simply that it is abundant or a top predator in every situation.