Keystone interdependence: Sea otter responses to a prey surplus following the collapse of a rocky intertidal predator

The sea star <i>Pisaster ochraceus</i> and sea otters (<i>Enhydra lutris</i>) are two predators capable of shaping rocky intertidal and kelp forest community structure and functioning. In 2013, a sea star wasting event decimated populations of <i>Pisaster</i> along the west coast of North America. The collapse of this species in the rocky intertidal revealed an unexpected relationship between two keystone predators. In this study, we show how the loss of <i>Pisaster</i> along the Monterey Peninsula, CA, USA led to an increase in mussel (<i>Mytilus californianus</i>) size and expansion into lower tidal zones. Before the sea star wasting event, the local sea otter population fluctuated around a near equilibrium. However, in the absence of <i>Pisaster</i>, sea otters increased their dietary intake on mussels, which contributed in part to a local population-level rise. These results demonstrate how the loss of a keystone predator in one ecosystem may impart population-level changes to another.