Sunflower sea stars could help save kelp forests

In July 2013, sea stars of several different species began to die along the northwest Pacific coast, all the way from Washington to Baja California. Populations of the sunflower sea star (Pycnopodia helianthoides), among other species, were decimated by what became known as sea star wasting disease (SSWD) and numbers declined by more than 97 percent. Since then, there have been few signs of recovery, and the species is now considered critically endangered. The ecological consequences have been massive.

The sunflower sea star is one of the largest in the world, reaching more than 1m in diameter. It is a highly mobile and voracious predator that hunts down a range of invertebrate prey, including bivalves, gastropods, echinoderms, crabs and carrion of any origin. In particular, they readily prey on purple sea urchins (Strongylocentrotus purpuratus), that are important herbivores in several different marine and coastal ecosystems. 

The loss of sunflower sea stars in parts of British Columbia and California has been associated with drastic changes in community structure in the forests of bull kelp (Nereocystis luetkeana) in the nearshore waters off these regions. With fewer predators, sea urchin populations exploded and the herbivores began to graze the kelp to the ground. 

The occurrence of a simultaneous marine heatwave in the same area compounded the problems as warm temperatures are also a major stressor of kelp forests. The action of the sea urchins has led kelp forests to collapse along the California coast, where they have been replaced by so called “barrens” – areas where sea urchins have settled in abundance and have grazed away the seaweeds and left very little primary production upon which to feed.

There is now growing interest in understanding how to control sea urchin numbers and encourage the recovery of kelp forests. With this in mind, a research team including scientists from Oregon State University, the University of Oregon, the University of Washington, and The Nature Conservancy has undertaken a study into the feeding habits of sunflower sea stars, focusing particularly on whether they will consume the starving sea urchins that are typical of barren ecosystems and, if so, how fast can they eat them. 

The findings, published in the Proceedings of the Royal Society B, indicate that these sea stars play a much more important role in kelp forest health than was previously thought. In fact, the researchers suggest that sunflower sea stars may be key in controlling urchin density and therefore kelp forest growth.

“What we saw suggests a clear link between the crash of sea stars, the explosion in sea urchin populations and the decline in kelp,” said Sarah Gravem, a research associate in the Oregon State College of Science. “It also points to sea star recovery as a potential key tool for kelp forest recovery.”

The researchers collected 24 sunflower sea stars and kept them in laboratory tanks for four months while conducting feeding experiments. They found that the sea stars consumed urchins at an average rate of 0.68 per day, and that they even ate the starving individuals that are associated with urchin barrens. In fact, they consumed the starving urchins 21 percent more rapidly than they did the well-fed urchins that are typically found in healthy kelp forests. The authors report that this rate of consumption is sufficient to maintain and even restore the health of kelp forests. 

“Eating less than one urchin per day may not sound like a lot, but we think there used to be over 5 billion sunflower sea stars,” said Gravem. “We used a model to show that the pre-disease densities of sea stars on the U.S. West Coast were usually more than enough to keep sea urchin numbers down and prevent barrens.”

Bull kelp is an important commercial commodity that is harvested for use in medicines and fertilizers, and as feed in aquaculture. Kelp forests thrive in cold waters and form a crucial habitat for a large number of other species, both invertebrates and vertebrates. Their sensitivity to certain growing conditions means climate change and a warming ocean are particularly problematic for them.

Researchers have long suspected that the sea star decline upset the balance between predators and herbivores in kelp forest ecosystems, allowing the herbivore populations to go unchecked. The overabundance of urchins was thought to place additional pressure on kelp forests that are already stressed by marine heatwaves. However, prior to the current study, the relationship between sea stars, urchins and kelp had not been quantified.

“This study addresses that gap, and the findings are significant and somewhat surprising,” said principal investigator Aaron Galloway of the University of Oregon Institute of Marine Biology. “We found that these stars are eager consumers of purple urchins and, most importantly, they even eat the nutritionally poor, starving ‘zombie’ urchins.”

Other important predators of purple sea urchins, such as sea otters, are generally known to avoid eating starving urchins from barrens, where the dense underwater carpets of urchins have devoured all available food and can live for years in an emaciated state, waiting for the kelp to grow back.  

The researchers emphasize the need for sunflower sea stars in the remaining kelp forests in order to prevent overgrazing by urchins. Since sea star recovery has been negligible since the SSWD event, they also support the reintroduction of sea stars that have been bred in captivity. These individuals may be able to aid in the recovery of kelp forests that have become barrens because they eat the emaciated urchins that reside there. 

The authors conclude that sunflower sea star recovery may be an effective and long-lasting method to control sea urchin populations and restore kelp forest ecosystems.

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By Alison Bosman, Earth.com Staff Writer

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