Sea otter
Without the sea otters to eat them, there were too many sea urchins.  The sea urchins then began to eat large amounts of kelp.  They ate and killed so many kelp plants that they turn kelp forests into desert-like places called urchin barrens. The fish, krill and other animals that lived in the kelp either died or left.  In some places where sea otters have returned, like Juan de Fuca Strait, they ate enough sea urchins that the kelp forests grew back.