As ochre sea star ‘baby boomers’ grow up, species showing signs of recovery

(Press-News.org) CORVALLIS, Ore. – The “baby boom” of ochre sea stars that followed a population crash a decade ago is enabling the species to recover on the Oregon Coast, according to new research by scientists at Oregon State University and Cal Poly San Luis Obispo.

The study, published in Ecosphere, does not determine whether the boom was triggered by the wasting disease epidemic that pushed ochre sea stars to the brink of extinction in Oregon, or simply a fortunate coincidence.

But either way, a study of multiple sites along the coast revealed many encouraging signs for ochre sea star populations.

“Wasting disease remains in circulation and populations continue to fluctuate, but there’s also much evidence that ochre sea stars are bouncing back,” said the study’s lead author, Sarah Gravem, an OSU postdoctoral researcher when the project began.

Gravem, now an assistant professor at Cal Poly, and Bruce Menge, a distinguished professor of integrative biology in the OSU College of Science, analyzed ochre sea star populations at eight locations over a 23-year period.

They found that many sea star baby boomers have reached adulthood and that population numbers are now at or exceeding pre-wasting disease abundances.

“After declines in sea star numbers of up to 84% in 2014, we quickly saw an 8,000% increase in young sea stars landing on shore,” Gravem said. “Populations are now large enough that sea stars are on the way to resuming their role as a keystone predator in the intertidal zone; at three-quarters of the study sites, predation on their favorite prey, California mussels, has recovered.”

That means the mussels are unable to form the beds that blanket intertidal rocks and make life difficult for other invertebrates and seaweeds – the sea stars’ predation opens up room for more species to potentially thrive.

“We showed that sea stars have now grown abundant and large enough that they are eating mussels at similar rates to before the epidemic at most sites in Oregon,” said Menge, who has been studying Oregon’s intertidal zone for decades.

However, he added, the average body sizes of the sea stars are still about 25% to 65% smaller than they once were at all but one location, and the sea star populations coast-wide are generally less steady from year to year than before the epidemic, suggesting populations have not returned to their previously stable, adult-dominated state. That’s likely because of continuing pulses of recruits – young sea stars – as well as intermittent resurgences in wasting disease, the researchers say.

“The connection between the sudden decline of ochre sea stars and the baby boom remains elusive,” Gravem said. “It’s certainly possible they are linked, which would mean these sea stars may be resilient to mass mortality events. It is also possible that the baby boom was unrelated to the disease, and simple luck enabled this relatively rapid recovery.”

The cause of sea star wasting disease, a strain of the bacterium Vibrio pectenicida, was only recently uncovered. A collaboration led by scientists at the University of British Columbia, the University of Washington, the U.S. Geological Survey and the Hakai Institute published its findings in Nature Ecology & Evolution.

Wasting disease causes sea stars to develop lesions and twisted arms, taking on the appearance that they’re melting away.

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