What is a keystone species?

A keystone species plays a critical role in maintaining the structure and diversity of an ecosystem. This species can have a disproportionately large effect on its environment relative to its abundance. For example, a predator that helps control the population of other species can prevent any one species from becoming overly dominant, which would otherwise disrupt the balance of the ecosystem. The presence and activity of a keystone species are often essential for other species' survival, thus sustaining biodiversity and the overall health of the ecosystem.

The other options present different concepts that do not accurately describe a keystone species. While a common species might dominate an ecosystem, it does not inherently mean it is crucial for maintaining ecological stability. A predator without a significant impact fails to fulfill the role of influencing the ecosystem dynamics that a keystone species does. Similarly, a species that migrates seasonally may contribute to ecosystem function, but it doesn’t define the critical role played by a keystone species in preserving structural integrity and diversity. Therefore, identifying option C as the definition of a keystone species is crucial for understanding its ecological importance.