Sep 11 2020
According to a new study published in the Science journal, the effects of climate change combined with predator loss are wreaking havoc on living reefs that have defined the Alaskan kelp forests for many years.
Sea otters, the “keystone predator” of Aleutian kelp forests, experienced dramatic population declines in the 1990s. In the wake of this event, herbivorous sea urchins proliferated, de-forested the region, and in recent years—with the help of climate change—triggered the widespread collapse of living reefs that have underpinned Alaskan kelp forests for centuries. Image Credit: J. Tomoleoni.
We discovered that massive limestone reefs built by algae underpin the Aleutian Islands’ kelp forest ecosystem. 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.
Douglas Rasher, Study Lead Author and Senior Research Scientist, Bigelow Laboratory for Ocean Sciences
Built by the red alga Clathromorphum nereostratum, the coral-like reefs are being destroyed by sea urchins. The number of sea urchins increased after the functional extinction of their predator—the Aleutian sea otter—in the 1990s.
Without the presence of the natural predator to control the number of sea urchins, the latter has changed the seascape—first by destroying the thick kelp forests, and now by focusing on the coralline algae forming the reef.
Clathromorphum alga creates a limestone skeleton that safeguards the organism against grazers and, for more than hundreds of years, created a complex reef that supports a rich diversity of marine life.
With kelp forests removed from the menu, sea urchins are presently ravaging the tough protective layer of the alga to consume it—a process that has turned out to be relatively easier because of climate change.
Ocean warming and acidification are making it difficult for calcifying organisms to produce their shells, or in this case, the alga’s protective skeleton. This critical species has now become highly vulnerable to urchin grazing—right as urchin abundance is peaking. It’s a devastating combination.
Douglas Rasher, Study Lead Author and Senior Research Scientist, Bigelow Laboratory for Ocean Sciences
Rasher headed the international research team that included the study’s coauthors Jim Estes from the University of California, Santa Cruz, and Bob Steneck from the University of Maine.
On the basis of their age and size, it is evident that the huge reefs formed by the Clathromorphum algae have historically played a crucial role in the marine ecosystem of Aleutian Islands, including the time when the number of sea urchins increased considerably in the past.
During the maritime fur trade of the 1700s and 1800s, sea otters were poached to near extinction. The reefs held their ground when the populations of sea urchin increased in response to this extinction.
“During the fur trade, Clathromorphum persisted through centuries where urchins presumably abounded,” added Rasher. “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.”
As the Clathromorphum alga adds to its calcified skeleton every year, it produces bands of yearly growth—similar to rings in a tree. Such bands archive regardless of whether the grazing events from sea urchins took place every year. By analyzing polished samples under a microscope, the researchers abruptly discovered a method to look back into the past events of the ecosystem.
This understanding enabled the team to ascertain that urchin grazing events had increased and decreased over time with the previous recovery and recent fall of the populations of sea otters. Most disturbingly, it also showed that grazing rates have increased considerably in the recent past along with the rising temperatures of seawater.
The team also brought live urchins and Clathromorphum algae back to the laboratory and placed them in controlled surroundings that simulated both current and preindustrial seawater conditions, and also those anticipated toward the end of the century.
Following three months, the urchins and algae were combined together to find out how the lethality of the sea urchin grazing events altered as a function of the acidity and temperature of seawater.
The team discovered that lethal grazing events under present conditions were around 35% to 60% higher than those in preindustrial conditions. And under future conditions, the rates increased even more—that is, around an additional 20% to 40%.
The experiment results proved that climate change has lately enabled sea urchins to breach the defenses of algae, forcing this system beyond a crucial 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,” added Rasher.
“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,” Rasher further added.
The latest finding of this interaction between climate change and predators certainly provides some hope—offering numerous ways to tackle the accelerating destruction of reefs. One of the most urgent needs of humanity is to decrease greenhouse gases, but it is a universal effort that needs international coordination and cooperation.
But restoring sea otters is a regional effort that has the potential to ease the erosion of reefs caused by sea urchins, and bring back the ecosystem from its critical tipping point.
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.
Douglas Rasher, Study Lead Author and Senior Research Scientist, Bigelow Laboratory for Ocean Sciences
“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,” Rasher concluded.
The study’s co-authors include senior authors Jim Estes from the University of California, Santa Cruz, and Bob Steneck from the University of Maine and also Kristy Kroeker from the University of California, Santa Cruz, Justin Ries from Northeastern University, and Jochen Halfar from the University of Toronto.
Other authors include Tim Tinker from the US Geological Survey, the University of California, Santa Cruz, Phoebe Chan from the University of Bergen, Jan Fietzke from GEOMAR, Nick Kamenos from the University of Glasgow, Brenda Konar from the University of Alaska, Fairbanks, Jon Lefcheck from Smithsonian Environmental Research Center, Chris Norley from the University of Western Ontario, Ben Weitzman from USGS, NOAA, and Isaac Westfield from Northeastern University.
The study was financially supported by the US National Science Foundation (PLR-1316141 and MGG-1459706) and the National Sciences and Engineering Council of Canada (Discovery Grants).