Sea Otters (Enhydra lutris) are considered keystone species of marine ecosystems due to their role as predators at the top of the trophic cascade. Sea Otters eat sea urchins, keeping the sea urchin population low enough for kelp forests to thrive. Healthy kelp forests support the ecosystem biodiversity. Sea Otters are responsible for maintaining the balance within these symbiotic relationships. 

Source: Monterey Bay Aquarium