There’s a saying that when one door closes, a window opens. In the natural world, you could say that the collapse of one marine predator can stand to benefit another. At least, that’s what new research led by the Monterey Bay Aquarium would suggest – particularly when it comes to ochre sea stars and sea otters.

Published today in the journal Science Advances, this new study indicates that a mass die-off event of these ochre sea stars (Pisaster ochraceus) due to a sea star wasting event perpetuated a mussel boom – which is today, 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 the study’s 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.”

It was back in 2013 that 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. This species of sea star – orange and purple in colour – happen to have a rather large appetite for the mussels dwelling in the rocky intertidal.

Without these voracious sea stars around, mussel populations exploded, expanding in cover from around 5% to more than 18% within three years. In the wake of the sea star die-off, many more mussels were available to sea otters to the point of becoming “a major prey surplus” for the species, revealing what researchers have called a “surprising link between the adjacent rocky intertidal and kelp forest ecosystems.”

It’s an example – says the paper – of how the loss of one keystone predator (Pisaster) in one ecosystem can impart changes to another (sea otters) in a linking ecosystem.

Monterey Bay Aquarium researchers have been observing sea otter foraging behaviour for decades. Sea otters consume their prey at the surface, which allows researchers to record what they eat and where they forage. 

Following the loss of Pisaster, sea otters more than doubled their mussel consumption, increasing from less than 7% to nearly 18% of their overall diet. The local number of sea otters also increased in this time, from a decade average of 373 between the year 2000 and 2012 to some 535 in the ten years between 2014 and 2024. 

Rather handily, the prey surplus supported these increased numbers.

The increase in mussel consumption was, at the time, recorded by the Multi-Agency Rocky Intertidal Network (MARINe) – a consortium of organisations that conduct long-term ecosystem monitoring. The group sampled four locations over several decades in the rocky intertidal along the Monterey Peninsula.

“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,” said Dr Smith. “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. And 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.”

Researchers have cautioned, however, the benefit currently being enjoyed by thriving sea otters, may only be temporary. With mussels now reaching sizes that are too large for sea stars to consume, any return to the previous equilibrium may be slow – even once Pisaster recovers. 

It’s been hypothesised, therefore, that sea otters may end up shifting their diets once again, after they deplete the overabundance of large mussels and after Pisaster eventually recovers.

It’s also worth due consideration of the fact that shortly before the mussel population increase, the largest marine heatwave on record surged through the northeast Pacific Ocean between 2014 and 2016. In these sweltering sea temperatures, kelp forests declined, kelp-eating sea urchin numbers exploded, and sea otters were forced to shift their diet towards them.

Later, the sea otters pivoted again, this time toward mussels as the prey surplus became available in the rocky intertidal.

“Conservation efforts rarely consider how adjacent ecosystems are connected,” said Dr April Ridlon, co-author of the paper and aquarium director of US and California science. “The findings of this study show that predators play a critical role in ecological resilience, and that when predators decline in one ecosystem, the consequences may be transmitted to another.”

With climate extremes like marine heatwaves on the rise, understanding how changes like this impact upon neighbouring ecosystems will be crucial in developing effective conservation strategies in the future.

“Predators like sea stars and sea otters are essential to maintaining resilient ecosystems,” said Leilani Konrad, co-author on the study and the aquarium’s senior otter biologist. “Our findings of keystone interdependence reveal that conservation of predators in one ecosystem can enhance resilience that transcends ecosystems and protects biodiversity.”

The paper – Keystone interdependence: Sea otter responses to a prey surplus following the collapse of a rocky intertidal predator – has been published in Science Advances.

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