In the summer of 2013, the sea stars that once carpeted North America’s west coast began dying by the millions, prompting an SFU research team to investigate.
“Evidence for a trophic cascade on rocky reefs following sea star mass mortality in British Columbia” is a study recently published by SFU’s Jessica Schultz, Ryan Cloutier, and Dr. Isabelle Côté. The paper documents the findings of the group as they dove in the waters of Howe Sound in the wake of one of the largest sea animal mortality events ever recorded.
The mortality of the sea stars is attributed to wasting disease, as researchers from Cornell University reported in 2014. The virus has been present in sea stars for decades, but became lethal for undetermined reasons. The marine biologist team at SFU examined the effects the sea star mortality had on the surrounding ecosystem.
“[Our team was] in a good position to do this [research], because a couple years before, we had done surveys around Howe Sound for a separate study where [. . .] we had collected information of parts of the ecosystem which are now affected,” Dr. Côté explained to The Peak. To examine the differences between the ecosystem before and after, the team surveyed a total of 20 sites.
Their study found that the population of the sea star prey, green sea urchins, had quadrupled while the population of sea urchin prey, kelp, went down by about 80 percent. Furthermore, the abundance of small shrimp and crabs in the water decreased as well, possibly owing to the dramatic decrease in their primary food and shelter source, the seaweed. This domino effect in the marine ecosystem due to the disappearance of the top predators (sea stars) is a prime example of what biologists call a trophic cascade.
When asked what the most remarkable finding from the dives were, Dr. Côté responded with an observation the team made about the sea urchin population: “Given the size that the urchins were, they were probably already around before the sea stars died,” she said. The phenomenon is known as a release of fear effect, where prey may have been previously hiding in undersea cracks and crevices, only to come out of hiding when the sea stars disappeared.
Dr. Côté and her team have yet to determine whether the sea star disappearances constitute a one-off event, or are part of a larger global warming effect. “When you look at the sea star and urchin populations, they are known to go through cycles of boom and bust for various causes,” stated Dr. Côté.
If anything, she maintained that her alarm regarding the situation was due to the geographical extent of the sea star disappearance, from California to Alaska. The taxonomic extent was also concerning, as 20 different species of sea stars have been affected.
“I’d like to think, and I’m pretty certain,” opined Dr. Côté, “that the sea stars will re-establish themselves.”