Sea   otters   as   keystone
   predators  in  the  North Pacific. Sea otters (Enhydra lutris) feed on
   sea urchins, which feed mainly on kelp.

   In  areas where sea otters are abundant, sea urchins are rare and kelp
   forests are well developed.

   Over  the last 20 years, killer whales have been preying on sea otters
   in Alaska as the whales' usual prey has declined.

   The decline of sea otters as a keystone species has allowed sea urchin
   populations to increase, resulting in the destruction of kelp forests.