Scientists find out why hedgehogs are mowing down California’s kelp forests

Purple urchins can emerge en masse to strip parts of the kelp forest. Credit: SBC LTER

Purple sea urchins are chewing their way through California’s kelp forests at a speed and scale that has amazed scientists, fishermen and divers. But kelp forests have long been home to red and purple hedgehogs, so it’s clear that the three species can coexist. Researchers at UC Santa Barbara tried to determine which factors disrupt this harmony.

“Why is it that in some places hedgehogs cause the destruction of a kelp forest, and in other places hedgehogs and kelp can coexist?” asked Associate Professor Adrian Stier. “Our analysis shows what’s going on under the hood. It provides much more insight into when and where you might expect urchins to devour kelp.”

This analysis, led by doctoral students Mae Rennick and Bart DiFiore, appears in the journal Ecology. The authors combined laboratory experiments with 20 years of field data to find out what prompts hedgehogs to start eating from home. The results suggest that the supply of kelp waste, or waste, may be the deciding factor.

Co-author Dan Reed, a research biologist at UCSB’s Institute of Marine Sciences, formulated the hypothesis behind this study decades ago. “In the 1980s, we observed one of our sites on San Nicolas Island switching back and forth between kelp forests and sea urchin barrens with no change in urchin density,” he recalled. “This led us to believe that the availability of kelp detritus changed the urchins’ foraging behavior from passive foraging, when detritus was abundant, to active grazing on live kelp, when detritus was scarce.”

Rennick and DiFiore set out to test this hypothesis. They collected purple and red hedgehogs from the field and brought them back to their laboratory. After acclimating the hedgehogs to the tanks, the researchers withheld food from them for about a week. They then added kelp to different tanks, weighing it before feeding and 48 hours after feeding for purple urchins, and 96 hours for red urchins, to determine how kelp consumption varied with urchin density.

The scientists expected to observe one of three types of density-related feeding behavior. Consumption may decrease with density if hedgehogs compete with each other. Alternatively, urchins may increase their consumption rates, forming a food front like shrimp. Finally, density may not affect consumption rates at all.

Indeed, the experiment revealed that hedgehogs appear to feed independently. “Our analysis showed that there is a linear relationship between hedgehog biomass and consumption rate,” DiFiore said.

Next, the team used time-series data from the Santa Barbara Long-Term Coastal Ecological Research (SBC LTER) site. For 20 years, scientists have routinely tracked all kinds of features in several field areas in the Santa Barbara Channel. The program, part of the National Science Foundation’s LTER network, is designed to support exactly this type of in-depth research.

From their experiment the researchers learned how much urchins need to eat, and from the LTER data they learned how much waste urchins can take, ascertained urchin density and determined the amount of live kelp at a given place and time. Combining the data and lab results allowed them to show how much kelp both purple and red sea urchins may have consumed at each site in the SBC LTER in a given year.

They compared what happened to the stable stock of kelp when the demand for urchin was greater than the supply of detrital. The results were amazing. “We found this 50-fold decrease in the standing stock of kelp biomass when the urchins required more litter than they took in,” DiFiore said. And this does not necessarily mean the end of hedgehogs. Sea urchins are extremely brave and opportunistic. “They can survive on anything,” he added.

A healthy kelp forest is extremely productive and has a lot of biomass turnover as giant algae shed leaves and blades. “It’s shedding its blades over and over again as those individual leaves die,” DiFiore explained. “And so, you have this huge biomass available for the hedgehogs to eat.”

In an ideal world, a hedgehog would run away from the squirrel and grab the scraps of kelp that drifted by with its tube feet. Then he would eat his meal from the safety of his little hole. But if there is no longer enough debris to support this safe lifestyle, the urchin is more likely to go outside and find live kelp. And the less kelp you have, the less waste you produce and the more urchins come out. This creates a feedback loop that can push the ecosystem over a tipping point, causing a sudden change in community composition. Finally, researchers have an understanding of one of the mechanisms that drives hedgehogs to mow down kelp forests.

Scientists have observed similar dynamics in other ecosystems. For example, Littorina snails in salt marshes switch between scavenging debris from marsh grass and consuming the living plant when the supply of debris falls below what the population can consume. And, as in kelp forests, this change can wreak havoc on marsh ecosystems.

There is serious concern about kelp forests on the West Coast. The region has seen large, unexpected declines in kelp north of Point Conception. “The thing is, urchins and kelp can coexist,” Rennick said. “Understanding what’s upsetting that balance is going to be really important before we accept this ‘hedgehog down’ feeling.”

The authors believe their findings can inform how to approach kelp forest management and show where efforts are likely to be effective. “The study suggests that the sites most likely to benefit from restoration are those with a strong supply of detrital algae coming from elsewhere,” Stier said.

“The best place to restore kelp is not in the middle of a urchin barren, where there is no kelp around,” he continued. “It’s actually in places that are closer to other kelp forests. So maybe we should try to extend the leading edge of a particular kelp forest nearby.”

Rennick is currently exploring urchinomics. At present, there is no market for purple hedgehogs, which are the most voracious foragers of the two Southern California species. However, entrepreneurs have proposed harvesting them from the wild, and then growing them to market size in captivity.

Rennick is working to determine how much this might help the kelp forest ecosystem. “‘When and where can urchin removal be beneficial for kelp forest restoration?’ it’s a tremendous question,” said Stier.

Meanwhile, DiFiore is trying to figure out whether non-human predators of hedgehogs can limit populations so they don’t outstrip the litter supply. Sheepshead fish and crayfish are their main predators in SoCal. Of course, lobsters themselves support an important commercial fishery.

“These really big lobsters play a disproportionate role in eating urchins,” Stier explained, “and these are the same individuals that are being targeted in fisheries.” DiFiore is investigating whether marine protected areas can act as a reservoir for predators like these large lobsters, thereby helping to stabilize kelp forests.

Although none of the authors believe that kelp forests will disappear altogether, the ecosystem faces an uncertain future. “The million-dollar question is whether we can do something about it,” Stier said.

Kelp forest destruction and recovery driven by changes in sea urchin behavior

More information:
Mae Rennick et al, Detrital supply suppresses deforestation to maintain healthy kelp forest ecosystems, Ecology (2022). DOI: 10.1002 / ecy.3673

Provided by University of California – Santa Barbara

citation: Scientists discover why urchins are mowing down California’s kelp forests (2022, July 12) retrieved July 12, 2022 from

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