Final answer:
Sea otters are classified as keystone species because they maintain a wide variety of other populations in the community along the Pacific coast of North America. Their removal would result in changes to other species' populations and reduced biodiversity.
Step-by-step explanation:
A keystone species is one whose presence has inordinate influence in maintaining the prevalence of various species in an ecosystem. The sea otter is classified as a keystone species along the Pacific coast of North America because it acts to maintain a wide variety of other populations in the community. Removing sea otters from the community would result in major changes in the populations of many other species, leading to an altered species composition and reduced biodiversity.