A keystone species in an ecosystem is a species that has a disproportionately large impact on its environment relative to its abundance. The presence or absence of a keystone species can significantly alter the structure and function of an ecosystem. Examples include the sea otter in coastal ecosystems, which helps control sea urchin populations, or the wolf in terrestrial ecosystems, influencing herbivore populations. Keystone species are vital for maintaining biodiversity and overall ecological balance.