Off the coast of California, Kelp fronds  are surrounded by sardines and provide food and cover for a wide range of marine animals. 

Sea urchins are abundant bottom-dwelling grazers and scavengers. Kelp are tough and fast-growing, but urchins (animals related to starfish) can eat them with their hard rasp-like mouth parts. Urchins are protected by their prickly spines and a hard outer shell. In the absence of factors controlling their abundance, urchins can increase to very high densities, producing what are sometimes called "urchin barrens" devoid of kelp. Unlike kelp forests, urchin barrens do not support much life.

Sea otters are uniquely suited to eating the urchins. With their dexterous paws and long incisors, they can crack open the urchins, eating the soft flesh inside. Sometimes they even use rocks as tools to crush tough prey.

Identify the foundation species and the keystone species.