A keystone species is a species that has a disproportionately large impact on its ecosystem relative to its abundance. The presence or absence of a keystone species can significantly affect the structure and diversity of the community in which it exists. These species can influence the types and numbers of other species in the habitat, often playing a critical role in maintaining the equilibrium of the ecosystem.
An example of a keystone species is the sea otter (Enhydra lutris). Sea otters help control sea urchin populations in coastal kelp forests. Without sea otters, sea urchin populations can explode, leading to overgrazing of kelp. This results in the destruction of the kelp forest ecosystem, which provides habitat and food for a variety of marine species. Thus, the presence of sea otters is crucial for maintaining the health and biodiversity of kelp forest ecosystems.