An unexpected rise in endangered sea otter numbers along Monterey Peninsula has been linked to a dramatic ecological change: the decline in ochre stars due to sea star wasting disease.
Ochre stars are voracious rocky shore predators and previously kept mussel populations in check. Without them, mussels can grow larger and expand into deeper waters, making them more accessible for other coastal predators such as sea otters.
Sea otters on the rise
Despite reintroduction efforts and protective measures, southern sea otters haven’t yet fully recovered from hunting that drove them almost to extinction. Their population today remains only a fraction of what it once was.
Since the late 1980s, however, the number of sea otters around Monterey Peninsula had been relatively stable, leading scientists to think they had reached the maximum number the environment could support.
Long-term monitoring data of ochre stars and mussel beds, observations of sea otter feeding behaviour, and sea otter counts were all used to investigate the impacts of the ochre stars’ disappearance. Scientists were surprised to find that, following the subsequent expansion of mussel beds, and changes in urchin availability, the number of sea otters had increased.
“Mussels had a big benefit for sea otters,” confirms Dr Joshua Smith of the Monterey Bay Aquarium, lead author of the study published in Science Advances. "This shows us how important having multiple predators is for the resilience of ecosystems, and that what's going on in the rocky intertidal also has implications on what's going on in the kelp forests.”
Vast swathes of kelp forest disappeared from Baja to Alaska following a sea star wasting event since 2013, outbreaks of urchins since 2014, and an extended marine heatwave in the northeast Pacific from 2014 to 2016. By eating urchins, which can otherwise overwhelm kelp forests, sea otters play an important role in kelp forest recovery. “Having predators like sea otters to maintain those remnant patches of kelp is really important for the persistence of the forests,” says Smith.
What will happen next?
While the increased abundance of mussels benefitted the sea otters – and therefore the kelp forests – of Monterey Peninsula, this boost is expected to be temporary.
Ochre stars are yet to recover, but what will happen when they do? “We might see the system shift back to what it was before, but we don't know,” explains Smith. “We’re curious to think about what happens next.”
Main image: sea otter in Monterey Bay. Credit: Getty
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