Sea otters 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, orcas have been preying on sea otters in the North Pacific 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.