Researchers call it the “sea cow effect”: To save the kelp forests, read history

In findings published today in Frontiers in Ecology and Evolution, scientists from the California Academy of Sciences describe how one unassuming undulator, the Stellar’s sea cow, affected the kelp forests ringing North America’s Pacific coast before European visitors hunted it to extinction in the 1760s.

According to the authors, our tendency to assess ecosystems based on current and recent factors, which they call shifting baseline syndrome, may prevent us from seeing the big picture. Ecologically speaking, the sea cows were around until relatively recently, but because “relatively” here means “before scientific method came into its own,” they’ve been left out of most models.

Much like how beavers create openings in the terrestrial canopy by damming streams to create lakes, the sea cow’s mighty megaherbivory cleared away the upper fronds of the kelp forests, allowing shafts of sunlight to pass through the otherwise photon-thirsty upper kelp into the marine understory and smaller plants, most significantly algae. The algae in turn provided food for other organisms.

Over the past few decades, California’s kelp forests have been dying off. Sea urchins have thrived in the area’s warming waters and have devoured the kelp plants, creating desolate stretches called urchin barrens. This may be due to human overhunting of their principal predators—sea otters. But unlike the Stellar’s sea cow, sea otters survived the European onslaught until the advent of protective legislation and are now in recovery. In efforts to save the kelp forests, researchers usually remembered sea otters but forgot the sea cows.

“When kelp forests were evolving millions of years ago, there were large marine herbivores like the Steller’s sea cow, which are now extinct,” says study co-author Dr. Peter Roopnarine. “So when it comes to what’s driving their widespread decline, there might be a major component we’re missing.” Roopnarine went on to compare the sea cows to terrestrial wildfires. Human beings might think they’re helping the forests by putting them out, but they’re actually a necessary and natural part of its life cycle.

Roopnarine and co-authors proposed a new model: the Past-Present-Future approach (PPF). This model incorporates what we think of as modern scientific data, museum samples, and ecological knowledge from traditional Indigenous sources.

The research team built a mathematical model of a kelp forest using data about sea urchins, sea otters, and other extant factors and then added the sea cows. According to study co-author Dr. Roxanne Banker of the University of Nevada, this transformed the forest completely. Their findings suggested that before about 1750, the kelp forests would have been better described as kelp-and-algae forests. With the sea cows clearing the upper layers of kelp away, algae would have dominated the middle levels.

This kelp-algae ecosystem would have been much more resilient to such challenges as drastic changes in temperature or hordes of oncoming urchins. Urchins, Banker points out, like to eat algae, and their voracious appetite for kelp may have been caused by the disappearance of this other food source. The researchers call this the “sea cow effect”: Even though, intuitively, it might seem like removing the large herbivores would help the kelp, it seems to have hurt them and every other organism nearby.

Roopnarine hypothesizes that sending human beings out to trim the kelp fronds might be produce something similar to this older, healthier kelp ecosystem.

Read the full text in Frontiers in Ecology and Evolution.

Roopnarine P, Banker R, Sampson S. Impact of the extinct megaherbivore Steller’s sea cow (Hydrodamalis gigas) on kelp forest resilience. Front. Ecol. Evol. November 28, 2022. https://doi.org/10.3389/fevo.2022.983558