When sea stars decline, otter numbers climb

When sea stars have major die-offs on the west coast of North America, sea otter numbers increase. When sea star wasting syndrome, a disease that kills echinoderms, caused ochre sea stars (Pisaster ochraceus) to decline dramatically on the west coast of North America and along the Monterey Peninsula in California, mussels that the stars typically eat increased. Sea otters (Enhydra lutris) were met with a massive buffet now that they didn’t have to compete with the sea stars. “Our research shows that the loss of a predator in one ecosystem can send shockwaves into another,” said Joshua Smith, a research scientist at Monterey Bay Aquarium. “It’s an example of how ecosystems are connected through keystone predation in ways that we haven’t understood before.” Smith led a study published in Science Advances on the phenomenon. Using long-term monitoring data on sea otters, sea stars and mussels in the area, the researchers found that after the sea star decline, sea otters more than doubled their mussel consumption from less than 7% to almost 18%. In addition, the number of sea otters increased from an average of 373 from 2000 to 2012 to an average of 535 from 2014 to 2024. “Predators like sea stars and sea otters are essential to maintaining resilient ecosystems,” Smith said. When these predators are removed, everything crumbles. “Our findings reveal what we are calling ‘keystone interdependence’—where the loss of a predator in one ecosystem doesn’t just affect that particular ecosystem; it also extends to adjacent ecosystems, even benefiting other predators.”

Read the study in Science Advance.