What best defines a keystone species?

• A. A species with a disproportionately large effect on ecosystem structure and function relative to its abundance; its presence maintains community diversity and function. Example: African elephants shaping habitats.
• B. The most abundant species in an ecosystem.
• C. A predator that hunts only at night.
• D. A species that lives exclusively in water.

Answer: (A)

Keystone species are defined by having a disproportionately large effect on ecosystem structure and function relative to their abundance, with their presence helping maintain community diversity and functioning. This means a species doesn’t have to be the most common to shape how energy moves through the system, control competitive dynamics, or create habitats that support many other organisms. African elephants illustrate this idea: their feeding, movement, and seed dispersal alter vegetation structure, create open space, and foster a variety of microhabitats, which in turn support a wide array of other species and help maintain the ecosystem’s overall balance. If a species is merely the most abundant, that doesn’t automatically mean it governs the ecosystem’s structure. Similarly, a predator that hunts only at night describes a timing pattern, not the influence on community organization, and a species that lives exclusively in water focuses on habitat preference rather than its role in shaping the broader community.