A surprising ecological twist reveals how the collapse of one marine predator—ochre sea stars—has led to a feeding opportunity for another—Southern sea otters.
Overview: How the Loss of Ochre Sea Stars Boosted Mussel Populations
New research from the Monterey Bay Aquarium, recently published in Science Advances, uncovers a remarkable case of interconnected marine ecosystems. A mass die-off of ochre sea stars (Pisaster ochraceus), triggered by sea star wasting disease, led to a dramatic increase in mussel populations along the California coast. This “mussel boom” created a rich prey source for Southern sea otters (Enhydra lutris nereis), reshaping their diet and boosting their population numbers.
Sea Star Wasting Syndrome and Its Ecosystem Impact
Beginning in 2013, populations of ochre sea stars along the West Coast of North America were devastated by a rapidly spreading wasting disease. These sea stars, commonly found in rocky intertidal zones, are keystone predators known for controlling mussel populations.
With their primary predator gone, mussels expanded significantly, growing from around 5% coverage to over 18% in just three years along the Monterey Peninsula. This sudden abundance of mussels spilled into nearby ecosystems, notably the kelp forest habitat, where sea otters forage.
Sea Otters Adapt: A New Prey Opportunity
The team at Monterey Bay Aquarium observed a significant increase in sea otters’ mussel consumption, rising from under 7% to nearly 18% of their diet. Between 2014 and 2024, the local sea otter population also rose from an average of 373 (2000–2012) to 535 individuals.
This dietary shift was documented by the Multi-Agency Rocky Intertidal Network (MARINe), which has monitored California’s intertidal ecosystems for decades. Their long-term data confirmed a strong link between the decline of sea stars and the expansion of mussels.
A Concept Called Keystone Interdependence
Dr. Joshua Smith, the lead researcher, describes this dynamic as “keystone interdependence”—the idea that the loss of a predator in one habitat can benefit predators in another. It illustrates how ecosystems are deeply connected and how predator presence or absence influences food web structures far beyond a single zone.
“Our findings reveal how keystone predators like sea stars and sea otters are essential to ecosystem resilience,” said Dr. Smith.
Climate Change Complicates Recovery
The study also notes that climate change is amplifying these ecological shifts. From 2014 to 2016, the Northeast Pacific Ocean experienced the largest marine heatwave on record. Rising sea temperatures weakened kelp forests, increased sea urchin populations, and forced sea otters to shift their diet multiple times—from sea urchins to mussels, depending on what was available.
As mussels continue to grow in size—potentially becoming too large for recovering sea stars to control—scientists warn the balance may not easily return. Sea otters may eventually have to adjust their diet again.
Conservation Lessons: Connect the Dots Across Ecosystems
The key takeaway from this study is the need for holistic marine conservation strategies that recognize the interconnectedness of adjacent ecosystems. Protecting predators like sea stars and sea otters not only maintains biodiversity but also helps ecosystems better withstand environmental stress.
“Conservation often overlooks ecosystem connections,” said Dr. April Ridlon of the Monterey Bay Aquarium. “This study shows that resilience depends on predator diversity across ecosystems.”
With climate-driven changes accelerating, understanding these connections is vital for developing adaptive, forward-thinking conservation efforts.
Key Takeaways for Marine Ecology and Conservation:
- Ochre sea star collapse led to a mussel population explosion.
- Southern sea otters adapted by increasing mussel consumption.
- Keystone interdependence shows ecosystems are interconnected.
- Long-term monitoring is critical to understanding ecological shifts.
- Climate change further complicates recovery and food web stability.
- Integrated conservation efforts are necessary for ecosystem resilience.
Read the full study:
Joshua Smith et al., “Keystone interdependence: sea otter responses to a prey surplus following the collapse of a rocky intertidal predator,” Science Advances (2025). DOI: 10.1126/sciadv.adu1028
Credit: Monterey Bay Aquarium