An organism that plays a critical role in its ecosystem is termed a keystone species. Keystone species have a disproportionately large impact on their environment relative to their abundance, shaping community structure and maintaining biodiversity. Their presence or absence can significantly affect other species and the ecosystem's resilience. Examples include predators, herbivores, and certain plants that influence nutrient cycling and habitat availability. Understanding the role of keystone species is crucial for conservation efforts and ecosystem management.