Sea star die-off spurred trophic cascade in B.C. coastal waters

Northern rainbow star afflicted with sea star wasting disease. This species had virtually disappeared from central California kelp forests as of February 2014. Photo: Steve Lonhart / NOAA MBNMS
Northern rainbow star afflicted with sea star wasting disease. This species had virtually disappeared from central California kelp forests as of February 2014. Photo: Steve Lonhart / NOAA MBNMS.

Booming sea urchin population takes bite out of ocean kelp forests

Staff Report

Some sea star populations along the Oregon coast may be recovering from a massive wasting epidemic that all but wiped out some species of the echinoderms, but that’s not the case in Howe Sound, a scenic fjord-like sound on the coast of British Columbia.

There, the die-off had a clear ecological trickle-down effect, called a trophic cascade by biologists. After the sea stars died, populations of their favorite prey, green sea urchins, quadrupled. The urchins quickly gobbled up kelp, reducing by 80 percent. Undersea kelp forests are critical to near-shore ocean ecosystems, providing cover and food for many marine species.

The findings were reported by Simon Fraser University marine ecologists Jessica Schultz, Ryan Cloutier and Isabelle Côté, who studied the after-effects of the die-off that hit the area in 2013, described as one of the largest  wildlife mass mortality events ever recorded.

Student researchers at the University of Washington’s Friday Harbor Laboratories last year discovered that a relatively common ocean virus may be to blame for the die-off, though there’s still no clear answer as to why disease spread so virulently. The virus causes the marine creatures to develops white lesions on its limbs and within days dissolve or  into a gooey mass.

In B.C., the sunflower star was among the most affected. At one meter in diameter, this formerly abundant species is one of the largest sea stars in the world and a voracious predator of invertebrates. Some research has linked ocean warming with the emergence of new diseases, and ocean temperatures are clearly a factor in the global decline of coral reefs.

“Howe Sound lost nearly 90 per cent of its sunflower stars in a matter of weeks,” said Schultz, a SFU master’s student and the Vancouver Aquarium’s Howe Sound research program manager. By repeating underwater surveys done before the mass mortality, the researchers were able to measure changes in marine animal and plant communities around the Sound.

“This is a very clear example of a trophic cascade, which is an ecological domino effect triggered by changes at the end of a food chain, says Côté. “It’s a stark reminder that everything is connected to everything else. In this case, the knock-on consequences were predictable, but sometimes they are not.”

Two summers on, there is still no sign of recovery in sea stars. Until they return, it seems that little will keep urchins in check and their feast on kelp is likely to continue.

The study published in PeerJ, was a joint effort between SFU and the Vancouver Aquarium.

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