An Extinct Sea Cow May Help the Restoration of California’s Dwindling Kelp Forests

Researchers are modeling out what the ocean may have looked like when the seaweed-munching mammal roamed the shores

Steller's sea cow drawing
A drawing of the extinct Steller's sea cow Biodiversity Heritage Library via Wikimedia Commons under CC BY 2.0

For millions of years, gigantic sea cows weighing several tons swam in shallow waters along coasts in the Pacific Ocean. The megaherbivores were three times the length of modern-day manatees and munched on massive amounts of sea kelp to sustain themselves. 

In 1741, German zoologist Georg Wilhelm Steller observed the creatures between Asia and North America, writing in his book that they were “greedy” animals that “eat incessantly,” per the New York Times’ Oliver Whang. He formally described cows, naming them Hydrodamalis gigas, which roughly translates to “giant water cow.”

But just 27 years after Europeans first encountered the animals, they went extinct, possibly because of overhunting. Now, in California, researchers are suggesting the disappearance of these voracious eaters may have left a hole in the ecosystem and contributed to the decline of the state’s kelp forests. Mimicking sea cow herbivory could help keep the kelp forests resilient, they write in a new paper published in Frontiers in Ecology and Evolution

“Kelp forests are highly productive ecosystems. They act as storm buffers, are economically important for fishing, and are home to countless marine organisms, yet they are in steep decline throughout the Pacific,” says Peter Roopnarine, coauthor of the study and a curator of geology at the California Academy of Sciences, in a statement. “When kelp forests were evolving millions of years ago, there were large marine herbivores like the Steller’s sea cow, which are now extinct. So when it comes to what’s driving their widespread decline, there might be a major component we’re missing.”

Scientists attribute the kelp forest decline in California to a combination of factors. In 2014, a mass of warm water called the “blob” raised sea surface temperatures, stressing the kelp and slowing their growth and reproduction. Just a year earlier, a disease killed off sea stars in northern and central California. The population of their prey, purple sea urchins, exploded, and the urchins were left to feed on the kelp uncontrollably.  In just five years, 93 percent of California’s kelp forests had disappeared, replaced instead by “urchin barrens.” 

The research team wondered how historical kelp forests—with Steller’s sea cows and sea otters—would have responded to stresses like ocean warming and disease seen in modern forests, so they created a mathematical model to compare the past to the present. 

They found that grazing from sea cows, which eat on kelp at the surface of the forest, would have allowed sunlight to reach deeper into the water, per the Times. Their models showed this light would have allowed algae to grow and serve as another food source for sea urchins. 

“If you spend time in a kelp forest today, at least a healthy one, one of the impressions we take away is it’s dark, almost cathedral like, and the light is filtering down. The kelp absorbs most of the incoming sunlight right at the surface,” Roopnarine tells Tara Duggan of the San Francisco Chronicle. “If the sea cow was grazing the canopy, there would have been more light penetration.”

Their model suggested that with the stressors of disease and warm water, the ecosystems with otters and sea cows would have been less likely to transition to urchin barrens often seen today. And when they did, they could recover more quickly. 

But Laura Rogers-Bennett, a marine biologist at the University of California, Davis, who was not involved in the research, says that ecosystems are complex, and warns against drawing strong conclusions from simple models, per the Times

“Thinking about what the habitat looked like back then, I would argue that we don’t even know what the habitat looks like now,” she tells the paper. 

Still, Scott Sampson, a coauthor and the executive director of the California Academy of Sciences suggests in the statement that learning more about historical kelp forests can be useful.  

"Today, we are surrounded by severely degraded ecosystems, places that were far healthier a mere century ago, let alone a millennium or more,” he says. “If we are to help guide a given place toward a flourishing future, we must understand not only its current state of health, but past states as well, and then apply these insights toward calculated, regenerative interventions.”