Balance Lost: Disappearance of Sea Otters Speeding Up Climate Damage to Alaska Reefs

(CN) — Coral reefs off the Alaskan coast have steadily declined in recent years, hampered by a combination of climate change and an explosion in sea urchin populations which have flourished in the absence of sea otters.

Coral reefs are built slowly by a species of algae over the course of centuries to millennia and host a wide array of marine life, as well as providing the foundation for vital kelp forests. In their absence, the balance of predator to prey shifts dramatically, foretelling cascade failure for the local ecosystem.

Among the greatest contributing factors to this reduction in coral reef mass are declining otter populations. These keystone predators feed on the sea urchins which themselves feed on the coral reefs and kelp. In an untouched ecosystem they create a nice balance which keeps both predator and prey in check. Tip the scales too far in one direction, however, and you have trouble.

Researchers detailed the complex interplay between these creatures and their changing ecosystem in a study published Thursday in the journal Science. They found that the overhunting of otters, coupled with ocean warming and acidification brought on by climate change are the primary culprits behind the decline in Alaska’s coral reefs.

"Ocean warming and acidification are making it difficult for calcifying organisms to produce their shells, or in this case, the alga's protective skeleton," said Douglas Rasher, who led the international team of researchers behind the study, in a statement. "This critical species has now become highly vulnerable to urchin grazing - right as urchin abundance is peaking. It's a devasting combination."

Alaska’s coral reefs have been effectively ground to dust by sea urchins since the Aleutian sea otter became functionally extinct in the 1990s, allowing the urchins to graze unchecked in the time since — first by mowing down the kelp fields, then turning their attention to the underlying reefs themselves.

“These long-lived reefs are now disappearing before our eyes, and we're looking at a collapse likely on the order of decades rather than centuries,” Rasher added.

The authors found that lethal coral grazing has increased 34% to 60% over preindustrial times, and when combined with ocean acidification and warmer waters brought on by climate change, the one-two punch may be more than some marine ecosystems can handle.

Future grazing is projected to increase by another 20% to 40%. A single sea urchin bite can remove up to seven years of algal growth, and in some areas the urchins have eaten away entire centuries of growth.

Aleutian kelp forests are built on top of a vast foundation of coral reefs, constructed over the course of hundreds or thousands of years by an especially long-lived form of red algae. Coral is particularly resilient to herbivore grazing because of the algae’s calcified exterior shell, but warm, acidic waters have begun to chip away at its armor in recent years.

Much like a tree, the algae grow in annual rings, allowing scientists to determine which years they’ve been hardest hit — or more accurately, bit.

"During the fur trade, Clathromorphum [red algae] persisted through centuries where urchins presumably abounded," Rasher said. "However, the situation has drastically changed this time around. Our research shows that sea urchin grazing has become much more lethal in recent years due to the emergent effects of climate change."

Otters traditionally kept the urchins in check, but over the past two centuries their populations have been decimated both by the fur trade and by overzealous fisherman. In more recent years, orcas have turned their attention to the remaining otters even as they fall prey to whaling ships. That’s given local urchin populations the run of things.

Reducing greenhouse gas emissions and restoring sea otter populations are two potential remedies. While global warming is a planet-scale problem requiring concessions from numerous governments, sea otter repopulation could be achieved on a local level and prove more attainable before the breaking point is reached.

"This is exciting because it suggests that resource managers have opportunities to manage large predators in ways that can help slow the rate with which climate change is deteriorating our natural ecosystems," Rasher said. "In the case of Aleutian kelp forests, restoring sea otter populations would bring many ecological benefits, and would also buy us time to get our act together on curbing carbon emissions, before this foundational reef builder is lost."

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