What defines a keystone species?

A keystone species is defined as a species that has a disproportionately large effect on its environment relative to its abundance. This means that the presence or absence of a keystone species significantly influences the structure and dynamics of the entire ecosystem. For example, the removal of a keystone species can lead to dramatic changes in population sizes and interactions among other species, ultimately affecting the overall biodiversity and health of the ecosystem.

This concept highlights the importance of certain species in maintaining the ecological balance, often due to their role in predator-prey relationships, competition, or as ecosystem engineers. Unlike mere abundance, it's the unique contribution to ecosystem functioning that characterizes a keystone species, making it critical for maintaining ecological stability.