Keystone species are those species _____.

• A. whose absence would have little impact on the ecosystem.
• B. that are the most abundant in the ecosystem.
• C. that dominate a trophic level.
• D. whose absence would cause major disruption in a community.

Answer: (D)

Keystone species have a disproportionately large influence on the structure and function of an ecosystem relative to how abundant they are. Their actions help regulate populations, create or maintain habitats, or otherwise shape community dynamics, so removing them causes major changes in the ecosystem. That’s why the option describing species whose absence would cause major disruption in a community best captures what a keystone species is.

The other ideas miss this key point: being the most abundant or dominating a trophic level doesn’t by itself ensure a large regulatory effect, and lacking disruption when removed wouldn’t earn the keystone label.