The decline of kelp forests doesn't get the same attention as the loss of coral reefs or rainforests, but could be almost as important. Otters serve as the guardians of these vital ecosystems, saving them from being wiped out by sea urchin plagues, but marine biologists have been puzzled why these defenses sometimes fail. Now they think otters' preference for prey with big genitalia is decisive.

Sea urchins usually live in harmony with kelp forests, consuming pieces of kelp that fall into their safe havens in rocks. Sometimes, however, they go on the rampage, wiping out the living kelp and leaving marine deserts known as “urchin barrens”. Kelp forests not only provide great biodiversity, they're also exceptionally effective at removing carbon at 20 times forests' rate per unit area.

This makes what is happening along the California coastline a tragedy of global significance. Immense areas of the West Coast kelp forests have vanished in recent years. We understand some, but not all, of the reasons. University of California Santa Cruz doctoral student Joshua Smith has found otter feeding preferences fill some gaps.

Kelp forests get destroyed when urchins become so numerous and ravenous they eat them to destruction. In healthy north and central California ecosystems, predation by sea stars and sea otters ensures it never gets this bad.

The door to urchin-apocalypse was opened in the 19th century when otters were hunted to near extinction, and the crisis really took hold in 2013 when a mysterious disease caused mass sea star die-offs, leading to an outbreak of urchins.

Nevertheless, Smith noted in a statement, things are complex. "Here in Monterey Bay, we now have a patchy mosaic, with urchin barrens devoid of kelp directly adjacent to patches of kelp forest that seem pretty healthy." The flourishing kelp forests have stayed that way because many otters turned from other prey to feasting on sea urchins, which raises the question of why the same thing hasn't happened next door.

The problem, Smith found, was that a period of unusually hot water affected kelp production in 2014. Urchins, no longer getting their regular source of kelp detritus came out of their homes and began eating living kelp instead. Without sea stars to keep their numbers under control, forests were quickly denuded.

Sea otters stepped up their consumption of the urchins, and in some places this was sufficient. In others, urchin barrens appeared before the otter army could mount a fightback. The puzzling aspect of all this is why the otters were not extending their range into the barrens, consuming enough urchins to allow kelp to retake lost territory.

Smith found that urchins in surviving forests have large gonads, which otters consider the tasty parts of the echinoderms. With less kelp to feed on in a barren, the urchins' genital organs are less developed. Otters don't see the point in diving all the way to the bottom to collect under-endowed urchins, so once an area becomes barren the urchins are left alone to keep it that way.

"Some people call them zombie urchins," Smith said. "You open them up, and they're empty. So the otters are ignoring the urchin barrens and going after the nutritionally profitable urchins in the kelp forest."

The findings, published in Proceedings of the National Academy of Sciences, show how a small shift can flip an ecosystem into a different state where it gets stuck. Unless something decides even poorly-hung urchins are worth consuming, restoring the barrens to kelp forests will be a monumental task. In southern California predators like the spiny lobster have kept urchins from destroying the forests, so an expansion of their range may be the best hope.