What is a keystone species?

A keystone species is defined as a species that has a disproportionate effect on the structure and functioning of its ecosystem, relative to its abundance. This concept was introduced to convey the idea that some species play a crucial role in maintaining the integrity of their ecological community. These species can influence various ecological processes, such as competition, predation, and community dynamics, which can ultimately impact species diversity and the overall health of the ecosystem.

For example, a keystone predator might regulate the populations of prey species, which in turn affects the composition of the entire community. If a keystone species were to be removed, it could lead to dramatic changes in the ecosystem, often resulting in a decrease in biodiversity.

In contrast, population size alone does not determine a keystone species; a species could be abundant yet have minimal ecological impact. Additionally, a rare species may be important, but its rarity does not automatically classify it as a keystone. Finally, generalist species, which can thrive in a wide variety of environments, may not exert the same significant influence that keystone species do. Hence, the defining characteristic of a keystone species is its substantial impact on community structure, aligning perfectly with the correct answer.