Illustration of sea otters playing an essential role in a healthy kelp forest ecosystem, as found on the Pacific coasts of the USA and Canada. This is a classic keystone species example. The otters feed on sea urchins (diving and on surface at left). Without this predation, the sea urchins would kill the kelp. In the Aleutian Islands, orcas (killer whales, one at left) appear to be largely responsible for hunting sea otters. Also shown are a seal (bottom left), a sea eagle (bald eagle, centre) and sea lions (on rocks, upper right).