Sea otters vanish followed by an ecosystem collapse | Polarjournal
In the 1970s, sea otters were found everywhere around the Aleutian Islands. They are the guardians of the kelp forest ecosystem. Photo: Lilian Carswell, US Fish and Wildlife Service

They are fluffy, extremely cute and the most important predator in the kelp forests off the North American west coast from California to Alaska: sea otters. Once they populated the underwater forests in large numbers and kept this enormously important ecosystem healthy. After the otters around the Aleutian Islands in Alaska had recovered from intense hunting in the 18th and 19th century, which almost led to their extinction, more than 90 percent of the otters in the region have disappeared again in the last few decades – with massive effects on the ecosystem, as a recent study now shows.

Kelp forests are among the most biodiverse habitats in the oceans and provide shelter and food for numerous species of fish, invertebrates, marine mammals and birds, including seals, sea lions, whales, sea otters, gulls, terns, herons and cormorants. This complex ecosystem is held together by sea otters (Enhydra lutris), whose favorite food is the sea urchin Strongylocentrotus polyacanthus. Fortunately, because the sea urchin is vegetarian and feeds mainly on algae, including the large brown algae Kelp.

In 1970, the then young biologist Dr. Jim Estes, co-author of the study, travelled to the Aleutian Islands for the first time and was greeted by an ocean full of furry faces. At that time, he could still observe up to 500 sea otters at once. He described to the New York Times how they scrimmaged around the kelp banks, peeling sea urchins and exchanging their characteristic squeaky sounds. “There were so many of them we couldn’t keep track”, Estes says.

Sea otters are native almost everywhere along the North American coast. For the study, the scientists focused on the population in western Aleutian Islands. Map: Google Earth

For about 30 years, however, scientists have observed a massive decrease in the otter population around the Aleutian Islands. According to Estes more than 90 percent of the otters have disappeared. “You can travel down 10 miles of coastline and never see an animal”, he says.

Since the otters are such an essential component in the kelp forests, the entire local food web begins to crumble in the fragile seascape of the Aleutian Islands. And climate change is accelerating and amplifying this process, Dr. Estes and his colleagues report in the paper, published in the journal Science.
The results of the study highlight the importance of sea otters in the Aleutian Islands, where the marine mammals maintain the biological balance through their insatiable appetite. A single sea otter can scarf down almost 1,000 sea urchins per day. “They eat them like popcorn”, says Estes.

“You can travel down 10 miles of coastline and never see an animal.”

Dr. Jim Estes, University of California Santa Cruz

Since the otters disappeared, sea urchins have proliferated and mowed down the seaweed. In the meantime even the reefs, which were created over very long periods of time by crusty red algae (about 0.35 millimeters vertical growth per year) and which the kelp once used as substrate, are in danger. “These long-lived reefs are disappearing before our eyes”, says Doug Rasher, marine ecologist at the Bigelow Laboratory for Ocean Sciences in Maine and lead author of the study.

After kelp has been almost wiped out by sea urchins, they now feed on the red algae. From 2014 through 2017, some reefs shrank by up to 64 percent. Where algae had once coated the Aleutian sea floor like a swath of pink pavement, only patches remained. Photo: J. Tomoleoni, US Geological Survey

The sea urchins gnaw at the algae carpet, which resembles pink bubble gum, and leave holes several millimeters deep, which corresponds to algae growth of up to seven years. However, the sea urchins only succeed thanks to climate change. Due to the increase in the concentration of carbon dioxide in the atmosphere, the seawater has become so acidic that the calcareous skeletons of the algae are less dense. In earlier times, sea urchins had no chance to break through the thick surface of the algae. Therefore, the otter population was able to recover after their hunting had ended. But now, against the background of climate change, the reef’s safety net is gone, as Rasher explains.

In the laboratory, the researchers were also able to show that at higher temperatures – as predicted for the future – the sea urchins gnaw at the algae even faster because their metabolism accelerated.

The vanishing of the otters on the Aleutian Islands is probably caused by orcas, which do not find enough of their actual prey, which are whales. Photo: Lilian Carswell, US Fish and Wildlife Service

A return of the sea otters could become difficult in view of the probable cause for the rapid disappearance of the otters on the Aleutian Islands. Estes suspects that hungry orcas – who may have been deprived of their favorite whales as prey by industrial whaling – have turned in their desperation to the small mammals, which they can devour by the hundreds or thousands every year. This could make it difficult to sustain larger otter populations: Once introduced, they could simply disappear again.

And so it proves once again that everything is related to everything else.

Julia Hager, PolarJournal

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