Sea otters nurturing remnants of vital kelp forests after explosion in voracious urchins, study reveals

‘Keystone species’ targeting most nutritious prey is helping to save one of the world’s most important and dynamic ecosystems

A southern sea otter with a purple sea urchin in Monterey Bay, California. (Morgan Rector)

The mighty kelp forests off California’s coast are highly valued as biodiverse ecosystems essential for sequestering carbon, but they have been hit hard by a range of different foes, including the ravages of heat waves, destruction by fishing and armies of voracious sea urchins.

In the past century kelp forests have declined by up to 75 per cent in some areas, but new research from the University of California at Santa Cruz reveals otters may be guarding what remains of these vital habitats by upping their consumption of sea urchins.

Otters are recognised as a keystone species - a dominant predator which maintains the balance of an ecosystem by controlling populations of prey - in this case urchins.

Though pollution and industrial fishing beginning in the 20th century have been blamed for the long term decline of the kelp forests, since 2014, there has been a dramatic decline in California’s kelp forests, which has been compared to the Amazon losing its trees.

Vast areas of the seafloor where kelp once flourished as one of the most productive and dynamic ecosystems on Earth are now deserted “urchin barrens” - subaquatic wastelands patrolled by plagues of the spiny animals.

But while some areas are completely empty of kelp and full of urchins, they are in many cases next to thriving areas of kelp, mystifying scientists.

University of California graduate student Joshua Smith, who is studying the complex relationships between species which have contributed to the demise of the kelp said: "Here in Monterey Bay, we now have a patchy mosaic, with urchin barrens devoid of kelp directly adjacent to patches of kelp forest that seem pretty healthy.

"We wanted to know how did this sea urchin outbreak happen where there are so many otters, how did the otters respond, and what does that mean for the fate of kelp forests here on the Central Coast?"

Together with a team of sea otter researchers at UCSC, the US Geological Survey, and the Monterey Bay Aquarium, Mr Smith conducted intensive underwater surveys along the Monterey Peninsula over a span of three years.

The study built on decades of long-term monitoring of sea otter populations and kelp forest ecosystems along the California coast.

It reveals how the behavioural responses of predators and prey to changing conditions can determine the fate of an entire ecosystem, and its potential recovery.

A major factor in the outbreak of the urchin plague was the 2013 emergence of a disease called sea star wasting syndrome, which has decimated starfish populations around the world, hitting the West Coast of the US particularly hard.

Among the worst affected species was the sunflower sea star which is one of the sea urchin’s main  predators.

Meanwhile, factors such as the 2014 marine heatwave known as “the blob”, along with warm water from an El Nino event also contributed to a decline in kelp productivity. It is believed this triggered a change in urchin behaviour.

The scientists explained that in kelp forests, sea urchins usually occupy crevices in rocky reefs on the seafloor where they are protected from predators.

Pieces of kelp then periodically drift down onto the reef like falling leaves in a forest, delivering food directly to the urchins in their shelters.

Kelp thrives where cold, nutrient-rich water wells up along the coast from the ocean depths, and giant kelp (the dominant species on the Central Coast) can grow more than a foot per day in good conditions.

In the warmer water during the heatwave, kelp growth rates dropped dramatically. This meant less kelp detritus drifting into the crevices of the reefs, and as a result sea urchins began to emerge in search of food.

With no starfish around to attack them, the urchins mowed down the living kelp fronds, turning kelp forests into urchin barrens.

"It happened so fast, before we knew it we had lost over 80 percent of the historic kelp forest cover in Northern California," Mr Smith said.

"We also had an urchin outbreak on the Central Coast, but not to the same extent as in the areas north of San Francisco."

But Mr Smith and his colleagues found that sea otters on the Central Coast responded to the urchin outbreak by increasing their urchin consumption dramatically, eating about three times as many sea urchins as they had prior to 2014.

Thanks to an abundance of prey (including an increase in mussels as well as urchins), the sea otter population increased substantially after 2014, from about 270 to about 432 sea otters in the Monterey region at the southern end of Monterey Bay.

But despite a boom in otter numbers and huge numbers of urchins visible on the seabed, the urchin barrens remained.

A closer examination at the sea otters’ foraging behaviour explained why.

Mr Smith’s team found the otters were only feeding on urchins in the remaining patches of kelp forest, and not those on the urchin barrens.

“It’s easy to see from shore where they are diving repeatedly and coming up with sea urchins,” he said.

The dive team surveyed those places, as well as areas not being targeted by otters, and collected urchins to examine in the lab.

The researchers discovered the urchins from the kelp beds had much higher nutritional value than those from the urchin barrens. In the barrens, however, the urchins are starved and not worth the effort to a hungry otter.

"Some people call them zombie urchins,” Mr Smith said. “You open them up, and they’re empty. So the otters are ignoring the urchin barrens and going after the nutritionally profitable urchins in the kelp forest.”

By doing this, the sea otters are helping to maintain those patches of healthy kelp forest, which are now crucially important for the persistence of giant kelp along the coast. Spores produced from those remaining patches could eventually reseed the barren areas and restore the kelp beds.

But the scientists warned that sea otters alone won’t disrupt the urchin barrens.

Mr Smith said another predator could help knock down the urchin population, or a disease, or even a major storm bringing large, bottom-scouring waves. Some groups are even exploring human interventions, sending teams of volunteer divers out to remove sea urchins in an effort to restore the kelp forests.

Mark Carr, professor and chair of ecology and evolutionary biology at UC Santa Cruz, said the differences between kelp forests on the southern, central, and northern coasts of California are striking. Professor Carr, who is Smith’s adviser but not a coauthor of the research, said Southern California’s kelp beds had not declined to the extent seen on the central and northern coasts.

“The difference in Southern California is that even though they lost the sea stars, they have other predators like the spiny lobster and California sheephead that are able to control urchin populations and allow the kelp forests to persist,” he said.

“It is possible that the presence of a healthy sea otter population in the north might have made those kelp forests more resilient, but it’s hard to speculate,” Professor Carr said.

“The role of a predator can be very different depending on where you are.”

"This study not only fine tunes our understanding of the role of sea otters in kelp forests, it also emphasises the importance of animal behaviour," Mr Smith said. "So much of this is driven by behaviour - the urchins shifting their behaviour to active foraging, and the otters choosing to prey on healthy urchins in the kelp forest - and these behavioural interactions have implications for the overall fate of the ecosystem.”

The study is published in the Proceedings of the National Academy of Sciences.