Sea Otters Maintain Remnants of Healthy Kelp Forest Amid Sea Urchin Barrenshttps://phys.org/news/2021-03-sea-otters-remnants-healthy-kelp.htmlSea otters have long been recognized as a classic example of a keystone species, a dominant predator that maintains the balance of kelp forest ecosystems by controlling populations of sea urchins, which are voracious kelp grazers.
Since 2014, however, California's kelp forests have declined dramatically, and vast areas of the coast where kelp once thrived are now "urchin barrens," the seafloor carpeted with purple sea urchins and little else. This has occurred even in Monterey Bay, which hosts a large population of sea otters.
In 2017, UC Santa Cruz graduate student Joshua Smith set out to understand why. "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," Smith said. "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?"
Smith's findings, published March 8 in Proceedings of the National Academy of Sciences, tell a fascinating story of how the behavioral responses of predators and prey to changing conditions can determine the fate of an entire ecosystem.
... 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.
Yet the urchin barrens remained. A close look at sea otter foraging behavior explained why. Smith's team found that the otters were feeding on urchins in the remaining patches of kelp forest, but not in 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 found that urchins from the kelp beds had much higher nutritional value than those from the urchin barrens, with large, energy-rich gonads. In the barrens, however, the urchins are starved and not worth the effort to a hungry otter.
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. ...
Joshua G. Smith el al.,
"Behavioral responses across a mosaic of ecosystem states restructure a sea otter–urchin trophic cascade," PNAS (2021).
https://www.pnas.org/content/118/11/e2012493118