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Why are urchins sometimes feeding on living kelp?

Purple sea urchins are currently munching their way through California’s kelp forests at an unprecedented speed and scale that has amazed scientists. However, since kelp forests have long been home to purple and red urchins, it is clear that the three species can get along. A research team led by the University of California, Santa Barbara (UCSB) has set out to investigate what prompts urchins to munch their way out of a home. They discovered that the availability of kelp scraps, or detritus, might be the deciding factor. 

“Why is it that in some places urchins cause the demise of a kelp forest, and in other places urchins and kelp can coexist?” asked study senior author Adrian Stier, an associate professor of Ecology, Evolution, and Marine Biology at UCSB. “Our analysis shows what’s going on under the hood. It offers a lot more resolution in explaining when and where you might expect urchins to devour kelp.”

By combining laboratory experiments with two decades of field data, Professor Stier and his team discovered that the availability of kelp detritus alters the foraging behavior of urchins from passive feeding, when detritus is abundant, to active grazing of living kelp, when detritus is scarce. The analyses revealed that when urchin demand was greater than detrital supply, there was a stunning 50-fold decrease in the standing stock of kelp biomass. 

Thus, the less kelp available to supply enough detritus, the more are urchins likely to venture out and forage on living kelp. This phenomenon creates a feedback loop that can push an ecosystem over a tipping point, causing a sudden shift in the composition of kelp-urchin communities.

These findings can inform how to effectively approach kelp forest management. “The study suggests that the places most likely to benefit from restoration are those with a hardy supply of detrital kelp coming from elsewhere,” Professor Stier explained. “The best place to restore kelp isn’t in the middle of an urchin barren, where there’s no kelp around. It’s actually in places that are closer to other kelp forests. So maybe we ought to try extending the leading edge of a given kelp forest nearby,” he concluded.

The study is published in the journal Ecology.

By Andrei Ionescu, Staff Writer  

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