Warming oceans and an infectious wasting disease have combined to devastate what was once an abundant type of sea stars along the West Coast, scientists say in a newly published study.
The study, published today by the open-access journal Science Advances, provides fresh evidence for the climate-related decline of multiple species of sea stars, a class of marine invertebrates popularly known as starfish.
For the latest report, researchers tracked populations of sunflower sea stars, one of the larger varieties.
“At one time plentiful in nearshore waters, the sunflower sea stars right now cannot be found off the California coast and are rare into Alaska,” Cornell University biologist Drew Harvell, one of the study’s lead authors, said in a news release. “Numbers of the sea stars have stayed so low in the past three years, we consider them endangered in the southern part of their range, and we don’t have data for northern Alaska.”
Sea star wasting disease, which is linked to a type of virus, is implicated in the sunflower stars’ decline, as it has been for about 20 other species. But climate change is also a major factor, Harvell said.
“The heat wave in the oceans — a product of increasing atmospheric temperatures — is exacerbating the sea star wasting disease,” he said. “It’s a lethal disease, and when you add a higher temperature to that, it kills faster, causing a bigger impact.”
Past studies have pointed to the same combination as the killer in a mass die-off of ochre stars in the coastal waters of Washington state’s Puget Sound, the San Juan Islands and Olympic Peninsula.
Unlike the ochre stars, sunflower sea stars aren’t recovering from the decline, saiid co-lead author Diego Montecino-Latorre, a wildlife epidemiologist with the One Health Institute at the University of California at Davis.
“This is likely because this disease has many hosts, and other species that tolerate the pathogen better may spread it to the sunflower star,” Montecino-Latorre said.
The sunflower sea star plays a significant role in marine ecosystems, in part because of its size (which can get as big as a manhole cover) and its appetite (which is voracious).
“In California, Washington and parts of British Columbia, sunflower sea stars keep urchins under control,” said Joseph Gaydos, the study’s senior author and director of UC-Davis’ SeaDoc Society program. “Without sunflower stars, urchin populations expand and threaten kelp forests and biodiversity. This cascading effect has a really big impact.”
The decline was documented through several chains of evidence.
Scientists and trained divers affiliated with the Florida-based Reef Environmental Education Foundation, or REEF, conducted nearly 11,000 shallow survey dives in West Coast waters stretching from California to Alaska from 2006 to 2017. Before 2013, the divers reported an abundance of sunflower sea stars, but saw the population collapse in the years since then.
Researchers recorded a rise of as much as 7 degrees Fahrenheit (4 degrees Celsius) in ocean temperatures at the REEF survey locations, beginning in 2014.
Nearly 9,000 deep-ocean trawl surveys, conducted between 2004 and 2016 under the supervision of the National Oceanic and Atmospheric Administration, found a similar pattern. Sunflower sea star populations declined by 80 to 100 percent, and the worst collapse came during the last four years of the survey, when sea surface temperatures were anomalously high.
Yet another survey, conducted by researchers from Simon Fraser University and the Hakai Institute, confirmed a dramatic die-off in the 2014-2015 time frame off the shores of British Columbia’s Calvert Island — as well as a correlation with rising sea urchin populations and declining kelp forests.
In addition to Harvell, Montecino-Torre and Gaydos, the authors of the Science Advances study, titled “Disease Epidemic and a Marine Heat Wave Are Associated With the Continental-Scale Collapse of a Pivotal Predator (Pycnopodia Helianthoides),” include J.M. Caldwell, J.M. Burt, K. Bosley, A. Keller, S.F. Heron, A.K. Salomon, L. Lee, O. Pontier and C. Pattengill-Semmens.