Which marine mammal serves as a keystone species by maintaining kelp forests through urchin predation?

• A. Sea Otter
• B. Walrus
• C. Manatee
• D. Sea Lion

Answer: (A)

Sea otters keep kelp forests healthy by preying on sea urchins, the herbivores that graze kelp holdfasts. With otters controlling urchin numbers, kelp can grow undisturbed, creating dense kelp forests that provide habitat, food, and shelter for many species. This top-down control—where a predator regulates the abundance of a key herbivore to maintain a whole habitat—is what makes sea otters such a keystone species in these ecosystems. The other mammals listed don’t provide this critical urchin-predation role: walruses focus on shellfish, manatees graze seagrass, and sea lions mainly hunt fish and squid, without maintaining the kelp-urchin balance.