Which term describes a species with a disproportionately large effect on ecosystem structure?

• A. Keystone predator
• B. Keystone species
• C. Biological magnification
• D. Energy pyramid

Answer: (B)

A keystone species is one whose influence on an ecosystem is disproportionately large compared to its abundance, shaping community structure, interactions, and even habitat. This kind of species helps regulate populations of other species, maintain habitat features, or drive crucial processes like nutrient cycling, so its presence keeps the ecosystem balanced. It isn’t limited to predators—some keystone species are engineers or mutualists that create or modify habitats, while others help sustain biodiversity in indirect ways. For example, beavers build dams that create new wetland habitats, and sea otters keep kelp forests healthy by controlling sea urchin populations. The other terms don’t fit this idea: a keystone predator is a type of keystone species but not all keystone species are predators; biological magnification describes toxin buildup in food chains, not ecosystem structure; an energy pyramid diagram shows energy flow, not a species’ role.