Disease on pachyseris coral. Photo by Joleah Lamb.

Coral Biodiversity is as Fragile as Glass

By Drew Harvell

The big environmental story this year is that an entire ecosystem around the planet, coral reefs, are endangered by unprecedented warming. A recent report in the journal Science documents coral reefs around the globe have been devastated by the world’s largest bleaching event. In Australia, only 7 percent of all Great Barrier Reefs are unaffected, and in the northern Great Barrier Reef, 80 percent of reefs are severely affected, which means over half the live coral has died. The Great Barrier Reef joins a long list of regions across the Pacific, such as Hawaii, Kiribati, Fiji suffering catastrophic reef mortality between the warmest years on record, between 2014 and 2016. Virtually all 29 coral reef world heritage reefs have been impacted. Scientists found that even remote, pristine reefs that have suffered minimal human degradation, including Papahānamokuākea in Hawaii and the Seychelles have been devastated,” the report states. It’s not over yet. Most recently, the coral reefs of Florida are hit with diseases that are the aftermath of the bleaching.

Drew Harvell, Professor of Ecology and Evolutionary Biology at Cornell University.

Infectious disease is the even uglier twin of coral bleaching. Heat stress causes bleaching by breaking apart the life-giving symbiosis between solar-powered corals and their algal symbionts. The algal symbionts are the energy that drives building the towering calcareous reefs by the tiny coral polyps. But how does the combination of heat stress and the weakened bleached corals fuel new outbreaks of infectious disease? Infectious diseases of humans and wildlife alike are facilitated by a combination of stress to the host and rising temperatures. Our work has shown this is even more so for the extremely temperature sensitive corals, since we and others have repeatedly shown coral disease outbreaks following bleaching events. Diseases are projected to increase with warming.

Bleaching is not always lethal, but the infections that follow it are. As Mark Eakin, coordinator of NOAA’s Coral Reef Watch program said as the 2016 bleaching episode ended: “The good news is we didn’t have any more bleaching. The bad news is we had disease that followed in the aftermath of the bleaching.”
Disease on pachyseris coral. Photo by Joleah Lamb.

While there is now excellent monitoring, often from aerial photographs, to detect and quantify the miles of reef affected by bleaching, the actual extent of infectious disease is harder to document. It is harder to document since 1) infections may occur three to six months after the bleaching event, 2) it requires eyes on the reef to confirm the difference between bleaching and actual dead coral and 3) diagnostic tests are needed to confirm presence of a pathogenic microbe.

There is much that can be done to slow the impacts of climate change; it is the grand challenge of our time, and our reward for success will be an ocean full of nature’s masterpieces. Otherwise, even David Attenborough agrees that in fifty years it may be that all we’ll have left is our sea in glass.


About the author: Drew Harvell is Professor of Ecology and Evolutionary Biology at Cornell University. Her recent book, A Sea of Glass, won the National Outdoor Book Award, honorable mention for Rachel Carson Environmental Award, and Smithsonian top picks for Science and Nature books. Her research on the sustainability of marine ecosystems has taken her from the reefs of Mexico, Indonesia, Hawaii and Australia to the cold waters of the Pacific Northwest. She is a Fellow of the Ecological Society of America and the Atkinson Center for a Sustainable Future, a winner of the Loftus-Hill Award. She has published over 150 articles in journals such as Science, Nature, and Ecology and is coeditor of The Ecology and Evolution of Inducible Defenses.


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