Which statement best describes a keystone species?

Which statement best describes a keystone species?

• A. It has a disproportionately large effect on ecosystem structure relative to its abundance.
• B. It is protected primarily to safeguard many other species and habitats.
• C. It provides most of the energy used by consumers in the ecosystem.
• D. It is always the most abundant species in the community.

Answer: (A)

A keystone species is defined by having a disproportionately large effect on ecosystem structure relative to how abundant it is. This means its presence or absence can drive major changes in the community, often by controlling other species, modifying habitats, or shaping resource availability, even if it isn’t very common. A classic example is sea otters keeping sea urchin populations in check, which allows kelp forests to flourish and support many other species. Because the impact comes from how influential the species is rather than how numerous it is, the statement captures what keystone species are all about. The other ideas miss this essential point. Being protected to safeguard many species describes umbrella/flagship concepts, not the functional role of a keystone. Providing most of the energy used by consumers would describe organisms that form the base of the food web, not keystone status. Being the most abundant is the opposite of keystone, since keystone effects are not about abundance but about the strength of influence relative to numbers.

A keystone species is defined by having a disproportionately large effect on ecosystem structure relative to how abundant it is. This means its presence or absence can drive major changes in the community, often by controlling other species, modifying habitats, or shaping resource availability, even if it isn’t very common. A classic example is sea otters keeping sea urchin populations in check, which allows kelp forests to flourish and support many other species. Because the impact comes from how influential the species is rather than how numerous it is, the statement captures what keystone species are all about.

The other ideas miss this essential point. Being protected to safeguard many species describes umbrella/flagship concepts, not the functional role of a keystone. Providing most of the energy used by consumers would describe organisms that form the base of the food web, not keystone status. Being the most abundant is the opposite of keystone, since keystone effects are not about abundance but about the strength of influence relative to numbers.