From the coast of South Africa to southern Alaska, sea urchins and algae coexist peacefully—until they don’t. In recent decades, more and more kelp forests have been destroyed by hungry urchins. In these places, “barren urchins” have replaced once lush and diverse ecosystems.
“At worst, it’s hedgehogs as far as the eye can see. They are literally stacked on top of each other, jostling for space,” says May Renick, Ph.D. candidate in marine ecology and evolutionary biology at the University of California, Santa Barbara. “These changes can happen very quickly.”
So what ultimately tips the scales? This summer, Renick and her colleagues published a report in the diary Ecology which gives an idea: In healthy kelp forests, urchins feed mainly on detritus, or algal debris, that is shed naturally by the giant kelp. However, when detritus is scarce, urchins begin to feed on live algae.
A long-standing theory
Biologist Dan Reid has been thinking about urchins and algae for more than 40 years. In the 1980s, he noticed something he wanted an explanation for: one of the scientist’s sites on San Nicola Island off the coast of Southern California had it went back and forth between kelp forests and sea urchin deserts — all with no change in hedgehog density.
That mere observation was a hint. The barrenness of hedgehogs cannot be explained by overpopulation alone. Something else was driving the process.
In 1985, Reed and colleagues published a hypothesis: If detritus was abundant enough to feed a population of sea urchins, the creatures would consume the leftover algae from the safety of cracks and crevices on the ocean floor—away from predators. However, if the supply of detritus is not sufficient, the urchins will venture out of safety and feed on algae.
Testing the theory
Thirty-seven years later, Reid co-authored the paper that Renick and Bart DiFore, Ph.D. candidate in ecology, evolutionary and marine biology at UC Santa Barbara, published with their colleagues. Using a combination of approaches, the team tested Reed’s hypothesis for decades.
The first step in the process was deceptively simple. Researchers collect urchins in tanks and feed them algae. Some tanks were full of spiny herbivorous invertebrates, while others were scarce. The researchers wanted to find out if hedgehogs’ feeding behavior would change based on their population density – no. Regardless of how many neighbors were piled on top of them, the hedgehogs ate at a constant rate.
The scientists then applied their new model to historical data. Lots of historical data. For 22 years, students and researchers led by Reed have monitored the abundance of giant kelp, red and purple urchins on nine locations along the Southern California coast. “A lab experiment can only tell you so much,” Difore says. “But being able to pair it with a long-term data set like this is really powerful.”
Researchers have drawn solid support for Reid’s hypothesis from this wealth of data. A model that takes detritus delivery into account fits the data much better than one based only on hedgehog distribution. And for the first time, they were able to give rough numbers to the phenomenon. “When there was less detritus than the urchins could eat, there was an average 50-fold reduction in algal biomass,” explains Difiore. “It’s a big change.”
An edible solution
Where kelp forests are healthy, they are diverse and productive ecosystems. They provide sustenance and habitat for marine plants and animals and benefit humans as well. Like terrestrial forests, algae forests sequester and store the most abundant greenhouse gas in the atmosphere: carbon dioxide. They also provide fish for local communities, the fishing industry and outdoor enthusiasts.
But in places where urchins have stripped entire kelp beds, these benefits disappear. And once a hedgehog is infertile, it tends to persist. “These hedgehogs can survive without eating for a very long time,” Renick says. “Once it flips over to a barren urchin, there’s really no chance the algae will come back.”
Renick describes the process as “feedback loop after feedback loop.” As urchins eat standing algae, the supply of detritus decreases, which in turn causes the urchins to consume more standing algae. As the forest disappears, there is less habitat for hedgehog predators such as lobsters, allowing hedgehog populations to grow even more. “Hedgehogs stay there until they reach a density threshold that allows disease or some other control to come in,” she says.
As the relationship between urchins and algae becomes clearer, more research is focusing on recovery. How can humans intervene to promote healthier kelp forests?
A company with an appropriate name Urchinomics, has made it his mission to create a commercially viable seafood industry around harmful urchins like the purple urchins that infest the Southern California coast. For his next project, Renick will work with Urchinomics to develop a strategic plan that will balance ecosystem restoration with profitability. “How can we catch these purple urchins to maximize algal recovery,” she asks. “What would happen if this budding purple hedgehog industry became as big as the red hedgehog industry?”
While the root causes of barren urchins are coming into focus, it’s still unclear what might be driving them back into kelp forests. But scientists like Renick hope the answer is somewhere out there. “We know that urchins and algae can coexist in a sustainable way,” she says. “Hypothetically, there is a way to do this effectively.”