From the coast of Southern Africa to Southern Alaska, sea urchins and kelp coexist peacefully — until they don’t. In recent decades, more and more kelp forests have been razed by the hungry urchins. In these places, “urchin barrens” have replaced once lush and diverse ecosystems.
“At its worst, it’s urchins as far as the eye can see. They’re literally stacked on top of each other vying for space,” says Mae Rennik, a Ph.D. candidate in ecology and evolutionary marine biology at the University of California, Santa Barbara. “These shifts can happen very quickly.”
So what ultimately tips the scales? This summer, Rennick and her colleagues published a paper in the journal Ecology that provides a clue: In healthy kelp forests, urchins feed mainly on detritus, or scraps of kelp that shed naturally from the giant algae. However, when detritus is scarce, urchins begin to feed on living kelp.
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