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The impacts of predator loss and climate change are combining to devastate living reefs that have defined Alaskan kelp forests for centuries, according to new research published in Science.

“We discovered that massive limestone reefs built by algae underpin the Aleutian Islands’ kelp forest ecosystem,” said Douglas Rasher, a senior research scientist at Bigelow Laboratory for Ocean Sciences and the lead author of the study. “However, 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.”

The coral-like reefs, built by the red alga Clathromorphum nereostratum, are being ground down by sea urchins. Sea urchins exploded in number after their predator, the Aleutian sea otter, became functionally extinct in the 1990’s.

Without the urchins’ natural predator to keep them in check, urchins have transformed the seascape – first by mowing down the dense kelp forests, and now by turning their attention to the coralline algae that form the reef.

Clathromorphum produces a limestone skeleton that protects the organism from grazers and, over hundreds of years, forms a complex reef that nurtures a rich diversity of sea life. With kelp gone from the menu, urchins are now boring through the alga’s tough protective layer to eat the alga – a process that has become much easier due to climate change.

Researchers from the University of Glasgow’s School of Geographical and Earth Sciences, who have investigated the effects of ocean acidification on marine life around the world, also contributed to the findings.

Dr Nicholas Kamenos, reader in global change at the School, said: “This decline is in part due to global warming and ocean acidification, which makes it hard for these coral-like algae to produce a robust skeleton. That makes them very prone to being broken down by marine grazers like sea urchins.”
 
Based on their size and age, it’s clear that the massive reefs built by Clathromorphum have long played a vital role in the Aleutian Islands’ marine ecosystem, including during past urchin booms. Sea otters were hunted to near extinction during the maritime fur trade of the 1700s and 1800s. When urchin populations spiked in response, the reefs held their ground.
 
Dr Kamenos added:  “This charismatic ecosystem has a defined food web driving its function. Urchins are a key food source for the otters, which helps keep the urchin numbers in check. In turn, with fewer urchins, the reef suffers less grazing. 
 
“Historic otter fur trading has led to previous reductions in otters and thus increases in urchin but the reef was not majorly affected. Things are different now due to the effects of climate change, and with a weakened skeleton, the urchins are having a large impact on the reef as otter numbers have lowered in recent years.”

As the alga adds to its calcified skeleton each year, it creates bands of annual growth – like rings in a tree. These bands archive whether sea urchin grazing events occurred in each year. By examining polished samples under a microscope, the research team found that they suddenly had a way to look back into the ecosystem’s past. This insight allowed them to determine that urchin grazing had waned and waxed over time with the past recovery and recent collapse of sea otter populations. Alarmingly, it also revealed that grazing rates have accelerated in recent time in association with rising seawater temperatures.

The researchers also brought live Clathromorphum and urchins back to the laboratory and put them in controlled environments that replicated preindustrial and current seawater conditions, as well as those expected at the end of the century. After three months, the algae and urchins were paired together to assess how the lethality of urchin grazing changed as a function of seawater temperature and acidity. The scientists found that lethal grazing under current conditions was about 35 to 60 percent greater than in preindustrial conditions. The rates grew even more under future conditions – by about an additional 20 to 40 percent. The results of the experiment confirmed that climate change has recently allowed urchins to breach the alga’s defenses, pushing this system beyond a critical tipping point.

“It’s well documented that humans are changing Earth’s ecosystems by altering the climate and by removing large predators, but scientists rarely study those processes together,” Rasher said. “If we had only studied the effects of climate change on Clathromorphum in the laboratory, we would have arrived at very different conclusions about the vulnerability and future of this species. Our study shows that we must view climate change through an ecological lens, or we’re likely to face many surprises in the coming years.”

The discovery of this interplay between predators and climate change does offer some hope – providing multiple ways to address the accelerating reef destruction. Reducing greenhouse gases is one of humanity’s most urgent needs, but it is a global effort that requires international cooperation and coordination. Restoring sea otters, however, is a regional effort that has the ability to mitigate reef erosion by urchins, and pull the ecosystem back from its tipping point.

Dr Kamenos added: “Our study also provides insight into how we might go about trying to help ecosystems survive in a changing climate - in this case, by helping local otter numbers recover, which will reduce urchin numbers protecting the algal reef. While this is not a silver bullet which could solve the problem entirely, it does provide a buffer until we can reduce carbon emissions and enable the coral-like algae, the basis of this ecosystem, to become robust once more.”

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First published: 11 September 2020