Sea Otters Shape Changing Aleutian Ecosystems

The marine habitats surrounding Alaska’s Aleutian Islands are besieged by two powerful forces: the loss of sea otters and global climate change. The ecosystem is changing rapidly, but Senior Research Scientist Doug Rasher has discovered a critical strategy that would help stabilize the region in the near future.

“We need to manage the return of sea otter populations if we’re going to save this ecosystem,” Rasher said.

The influence of sea otters in the Aleutian Islands is well-known. At the heart of the food web is a relationship between otters, sea urchins, kelp, and reef-building algae. Without otters, urchin populations flourish and consume the local kelp forests. Rasher discovered that urchins then graze on their next-preferred food: Clathromorphum, the calcifying alga that builds the reef.

Rasher found that when otters disappear – which has happened historically from both over-hunting by humans and predation by orcas – their loss reverberates down the food web and drastically changes the entire ecosystem.

One significant result of this change is the rapid erosion of the reef, which serves as a substrate for kelp and habitat for many other marine organisms. Ocean warming and acidification are weakening the limestone skeleton produced by Clathromorphum. In areas without otters, urchins graze on the weakened algae, which degrades more easily. Currently, the reefs of the Aleutian Islands are being eroded faster than they are growing.

“Fostering the return of sea otters to this system would effectively mitigate the whole urchin problem,” Rasher said.

Rasher found that the effects of otter loss and climate change are intertwined and compound one another. This also means that mitigating one problem would help the other.

He believes the benefits of otter recovery would cascade through the ecosystem. By eating large urchins and removing them from the reef, otters would curb the problem of rapid erosion. This would also foster the return of kelp, which in turn may reduce CO₂, potentially creating a buffer against ocean acidification. These healthier ocean conditions would “buy time” for the alga in the face of climate change.

As researchers around the world study climate change impacts across a range of species, it is essential to place these studies in the context of the food web, Rasher says. Running laboratory experiments revealed only small changes to the alga’s skeleton caused by ocean warming and acidification, but placing them in an ecosystem context revealed how grave these changes are.

“Climate change may cause only subtle physiological changes for some species, but when you put them into a food web context, they can scale up to alter entire ecosystems,” Rasher said. “This isn’t an end of century problem. This is now.”