The ocean otter has lengthy been acknowledged as a basic instance of a keystone species, a dominant predator that maintains the stability of forest kelp ecosystems by controlling populations of sea urchins, that are voracious grazers of kelp.
Since 2014, nevertheless, California’s kelp forests have declined dramatically, and enormous areas of the coast the place kelp as soon as thrived at the moment are “sea urchin moorlands,” the seabed carpeted with purple sea urchins and nothing else. This has occurred even in Monterey Bay, which is residence to a big inhabitants of sea otters.
In 2017, UC Santa Cruz graduate scholar Joshua Smith tried to determine why. “Right here in Monterey Bay, we now have an uneven mosaic, with sea urchin moorlands devoid of kelp straight adjoining to patches of kelp forest that look fairly wholesome,” Smith stated. “We wished to understand how this sea urchin epidemic occurred the place there are such a lot of otters, how did the otters react and what does this imply for the plight of the kelp forests right here on the coast? central? “
In collaboration with a workforce of sea otter researchers from UCSC, the US Geological Survey and the Monterey Bay Aquarium, Smith carried out intensive underwater research alongside the Monterey Peninsula over a interval of three years. The examine builds on many years of long-term monitoring of sea otter populations and forest kelp ecosystems alongside the California coast.
Smith’s findings, revealed on March 8 in Proceedings of the Nationwide Academy of Sciences, inform a compelling story about how the behavioral responses of predators and prey to altering situations can decide the destiny of a complete ecosystem.
It began in 2013 with the onset of a mysterious illness referred to as starfish losing syndrome, which decimated starfish alongside the west coast. Among the many most affected species is the sunflower starfish (Pycnopodia helianthoides), a serious predator of sea urchins. However in keeping with Smith, this was solely one of many components main to an enormous sea urchin outbreak.
“We imagine that a number of components triggered the ocean urchin epidemic,” he stated. “The lack of a serious sea urchin predator was quickly adopted by a decline in kelp productiveness attributable to climatic stressors.”
In kelp forests, sea urchins primarily inhabit the crevices of rocky reefs on the seabed the place they’re protected against predators. Items of kelp drift throughout the reef like leaves falling in a forest, delivering meals on to the ocean urchins of their shelters.
Kelp thrives the place chilly, nutrient-rich water gushes alongside the coast from the depths of the ocean, and big kelp (the dominant species on the central coast) can develop greater than a foot a day. in good situations. In 2014, nevertheless, an unprecedented sea warmth wave hit the northeastern Pacific. Often known as the “blob”, it has unfold from the west coast of Alaska to central California. Across the identical time, a serious El Niño occasion introduced sizzling water to the coast from the south.
With all this heat water bathing the coast, kelp progress charges have dropped dramatically. This meant much less kelp detritus drifting into the reef crevices, and sea urchins started to emerge searching for meals. With no starfish to assault them, the ocean urchins mowed down the dwelling kelp fronds, turning the kelp forests into sea urchin moorlands.
“It occurred so quick, earlier than we knew it, we had misplaced over 80 p.c of the historic kelp forest cowl in northern California,” Smith stated. “We additionally had an outbreak of sea urchins on the central coast, however to not the identical extent as in areas north of San Francisco.”
Smith and his colleagues discovered that central coast sea otters responded to the ocean urchin outbreak by dramatically rising their consumption of sea urchins, consuming about thrice as many sea urchins as earlier than 2014. Because of a abundance of prey (together with a rise in mussels in addition to sea urchins), the ocean otter inhabitants elevated considerably after 2014 from about 270 to about 432 sea otters within the Monterey space, on the tip south of Monterey Bay.
But the sterile sea urchin remained. Cautious examination of the ocean otter’s foraging conduct has defined why. Smith’s workforce found that otters preyed on sea urchins within the remaining patches of the kelp forest, however not within the sea urchin barrens.
“It is simple to see from the shore the place they repeatedly dive and arrive with sea urchins,” he stated.
The dive workforce investigated these places, in addition to areas not focused by otters, and picked up sea urchins for laboratory examination. Researchers discovered that sea urchins from kelp beds had a a lot greater dietary worth than these from sea urchin moorlands, with massive gonads that had been excessive in power. Within the moors, nevertheless, sea urchins are ravenous and never definitely worth the effort for a hungry otter.
“Some folks name them zombie sea urchins,” Smith stated. “You open them, and so they’re empty. So the otters ignore the ocean urchin moorlands and prey on the nutritionally worthwhile sea urchins within the kelp forest.”
By doing this, sea otters assist preserve these patches of wholesome kelp forest, which at the moment are critically necessary to the persistence of big kelp alongside the coast. The spores produced from these remaining patches might finally reseed the sterile areas and restore the kelp beds.
Nevertheless, sea otters alone is not going to disturb sea urchin barrens. One other issue is required to take away sufficient sea urchins from the heathlands to permit new kelp vegetation to develop there. Smith stated one other predator might assist carry down the ocean urchin inhabitants, or illness, or perhaps a main storm inflicting massive waves to scrub out the underside. Some teams are even exploring human interventions, sending groups of volunteer divers to take away sea urchins in an effort to revive kelp forests.
Mark Carr, professor and director of ecology and evolutionary biology at UC Santa Cruz, stated the variations between kelp forests on the southern, central and northern coasts of California are hanging. Carr, who’s Smith’s advisor however not a PNAS co-author, has been finding out kelp forests all alongside the West Coast for years. He stated southern California kelp beds had not declined as a lot as on the central and northern coasts.
“The distinction in Southern California is that despite the fact that they’ve misplaced the starfish, they produce other predators just like the spiny lobster and the California shepherd which might be in a position to management sea urchin populations and permit for kelp forests to persist, ”Carr stated.
The kelp beds of northern California, then again, had been a lot worse. The absence of sea otters north of San Francisco could also be an element, but it surely’s onerous to say as a result of the dominant species of canopy-forming kelp is totally different, with bull kelp (Nereocystis leutkeana) dominant within the northern and big kelp (Macrocystis pyrifera) dominant on the central and southern coasts.
“It is attainable that the presence of a wholesome sea otter inhabitants within the north made these kelp forests extra resilient, but it surely’s onerous to invest,” Carr stated. “The position of a predator may be very totally different relying on the place you might be.”
Pioneering research of sea otters and kelp forests carried out within the Aleutian Islands by James Estes, now professor emeritus of ecology and evolutionary biology at UCSC, have proven that whereas the ocean otter inhabitants of the Aleutians had been recovering from close to extinction, otters had been reworking sea urchin moors into kelp forests in the course of the recolonization of the islands. However these interactions play out in another way on the central coast and will have one other consequence in Northern California.
“This examine not solely refines our understanding of the ocean otter’s position in kelp forests, it additionally emphasizes the significance of animal conduct,” Smith stated. “A lot of that is motivated by conduct – sea urchins undertake lively feeding and otters select to prey on wholesome sea urchins within the kelp forest – and these behavioral interactions have implications for the general destiny of the ocean urchins. the ecosystem.
Along with Smith, the article’s co-authors embrace Joseph Tomoleoni and Sophia Lyon from the USGS Western Ecological Analysis Middle, Michelle Staedler and Jessica Fujii on the Monterey Bay Aquarium, and Tim Tinker at UC Santa Cruz. This analysis was supported by the Nationwide Science Basis.