Final answer:
Sea otters are a keystone species as they prevent a herbivorous species from destroying a plant species in their ecosystem.
Step-by-step explanation:
Sea otters can be considered a keystone species because they play a critical role in their ecosystem by preventing sea urchins from destroying kelp vegetation. Kelp is the keystone species in this context because its survival is crucial to the survival of the entire ecosystem. When sea otters are present, they prey on sea urchins, controlling their population and preventing them from overgrazing on kelp. Without sea otters, sea urchins multiply rapidly and destroy much of the natural kelp vegetation.