When sea stars fall, sea otters rise: Monterey Bay Aquarium research reveals sea otters benefit from prey boom triggered by loss of ochre sea stars

New research led by Monterey Bay Aquarium reveals a surprising ripple effect in coastal ecosystems: the collapse of one marine predator can benefit another. Published today in the journal Science Advances, the study shows that a mass die-off of ochre sea stars (often called Pisaster ochraceus) due to a sea star wasting event created a mussel boom – offering sea otters an unexpected prey buffet.

“Our research shows that the loss of a predator in one ecosystem can send shockwaves into another,” said lead author Dr. 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.”

In 2013, a sea star wasting syndrome decimated populations of Pisaster along the west coast of North America and along the Monterey Peninsula in California, where this study was conducted. The orange and purple stars have a hungry appetite for mussels in the rocky intertidal. Without the voracious sea stars lurking around, mussel populations exploded, expanding in cover from around five percent to more than 18 percent within three years. In the wake of the sea star die-off, mussels became a major prey surplus for sea otters, revealing a surprising link between the adjacent rocky intertidal and kelp forest ecosystems. The new research into the phenomenon shows how the loss of a keystone predator (Pisaster) in one ecosystem can impart changes to another (sea otters), linking ecosystems.

Mussel bonanza benefits sea otters

Monterey Bay Aquarium researchers have been observing sea otter foraging behavior for decades. Sea otters consume their prey at the surface, which is key to recording what they eat and where they forage. Following the loss of Pisaster, sea otters more than doubled their mussel consumption, increasing from less than seven percent to nearly 18 percent of their overall diet. The local number of sea otters also increased from a decade average (2000-2012) of 373 to 535 during 2014-2024, and the prey surplus supported the increased numbers.

Increases in mussel consumption by sea otters in the rocky intertidal were documented by the Multi-Agency Rocky Intertidal Network (MARINe) – a consortium of organizations that conduct long-term ecosystem monitoring. The MARINe group sampled four locations over several decades in the rocky intertidal along the Monterey Peninsula. For each survey, they recorded information on the number of sea stars, the cover of mussels, and how far the mussel beds extended from the upper intertidal toward the water.

“The long-term monitoring data collected by MARINe were really key to cinching the correlation between the decline in sea stars and the increase in mussels,” Smith said. “The 2013 sea star wasting event was so abrupt. By late 2013 the local population of stars had nearly collapsed.”

“Predators like sea stars and sea otters are essential to maintaining resilient ecosystems. When these predators are removed, everything crumbles,” Smith said. “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.”

From prey surplus to ecological uncertainty

Although sea otters thrived in response to increased availability of mussels, researchers caution this might only be temporary. With mussels now reaching sizes that are too large for sea stars to consume, a return to the previous balance may be slow – even after Pisaster recovers. Researchers hypothesize that sea otters might shift their diets once again, after they deplete the overabundance of large mussels and after Pisaster eventually recovers.

The study also highlights how climate change compounds the effects of ecosystem shifts. Shortly before the mussel population increase, the largest marine heatwave on record occurred in the northeast Pacific Ocean from 2014-2016. In the sweltering sea temperatures, kelp forests declined, kelp-eating sea urchin numbers exploded, and sea otters shifted their diet toward sea urchins. Later, the sea otters pivoted again – this time toward mussels as this prey surplus became available in the rocky intertidal. 

Predator diversity increases resilience

This study highlights the importance of conservation that considers ecosystems and habitats as being fundamentally connected systems.

“Conservation efforts rarely consider how adjacent ecosystems are connected,” said Dr. April Ridlon, co-author of the paper and aquarium director of U.S. and California science. “The findings of this study show that predators are important for enabling ecological resilience, and that when predators decline in one ecosystem, the consequences may be manifest in another.” 

With climate extremes like marine heatwaves on the rise, understanding how changes in one ecosystem impact another is central to developing effective conservation strategies. 

“Predators like sea stars and sea otters are essential to maintaining resilient ecosystems,” said co-author and aquarium Senior Sea Otter Biologist Leilani Konrad. “Our findings of keystone interdependence reveal that conservation of predators in one ecosystem can enhance resilience that transcends ecosystems and protects biodiversity.”

About Monterey Bay Aquarium

With a mission to inspire conservation of the ocean, the Monterey Bay Aquarium is the most admired aquarium in the United States, a leader in science education, and a voice for ocean conservation through comprehensive programs in marine science and public policy. Everything we do works in concert to protect the future of our blue planet. More information at MontereyBayAquarium.org.