Index ranks which coral reefs are likely to die

Coral reefs "are not rock. Reefs are made of healthy, living animals—individual corals," says Luisa A. Marcelino. "We want to know why corals are bleaching and why they are bleaching differently." (Credit: Oregon State University/Flickr)

Coral reefs are early casualties of climate change, but not every coral reacts the same way to the stress of ocean warming.

Now, a new global index details which of the world’s coral species are most susceptible to coral bleaching and most likely to die.

The world currently is experiencing the longest global coral bleaching event ever recorded, with the Great Barrier Reef and US reefs among those suffering. Bleaching happens when stressed corals expel their life-providing algae, turning coral reefs stark white as their skeletons show through. Some corals rebound, but many do not.

Based on a massive amount of historical data, the index can be used to compare the bleaching responses of corals throughout the world and to predict which corals may be most affected by future bleaching events.

[Are coral reefs headed for another big collapse?]

“Coral bleaching is an inescapable example of the effects of climate change,” says Timothy D. Swain, a postdoctoral fellow at the McCormick School of Engineering at Northwestern University and first author of the study published in Global Change Biology.

“We can see it with our eyes, and we also clearly see the progression of climate change in our data. Our goal is to use data to understand what is driving bleaching and learn how we can protect the world’s coral reefs, so we don’t lose them so quickly.”

“Coral bleaching is an inescapable example of the effects of climate change.”

To produce the index—a standardized measure of vulnerability by species to thermal stress—researchers analyzed publicly available data on nearly half the world’s corals—including actual measurements of bleaching.

It identifies the species most susceptible to bleaching and those most likely to perish as a result of the damage; hardier species also are identified. The index ranks the corals’ susceptibility to thermal stress from 1 to 100, with the most susceptible first in the list.

Rainforests of the sea

The index provides a valuable new tool to conservationists and park managers committed to preserving coral reefs and scientists interested in learning more about the hundreds of reef-building corals.

“Coral reefs are referred to as rain forests of the sea,” says Luisa A. Marcelino, a research assistant professor of civil and environmental engineering. “They are not rock. Reefs are made of healthy, living animals—individual corals. We want to know why corals are bleaching and why they are bleaching differently.”

Highly productive and diverse ecosystems, coral reefs help support approximately 25 percent of all marine fish species, according to the United Nations Environment Programme. As a result, the livelihoods of 500 million people and income worth more than $30 billion are at risk from coral bleaching.

“In our study, we observed a widely variable bleaching and mortality response among corals,” Marcelino says. “Now, with the index, we have a platform we can use to better understand bleaching mechanisms, both intrinsic and environmental. There is value in knowing which species are more resistant and why. With good tools, we can make more informed decisions and better manage coral reefs.”

[Fish need our help to save coral reefs]

The global index represents close to half the world’s corals from 316 sites, emerging from a meta-analysis of all available historical records on coral bleaching from 1982 through 2006—the “sum of human knowledge on species-specific bleaching during this period,” Swain says.

“Using very large data sets, we have teased out valuable information that will help researchers identify global trends and learn about individual corals,” says coauthor Vadim Backman, professor of biomedical engineering. “We want this index to be used to predict how corals might react to future bleaching events.

“To make our analysis possible, we applied financial theory conventionally used to predict changes in stock prices in response to stock market variations to model how individual corals react to a change in the environment.”

The index will be available online, so that data on corals can be added as they become available, making the tool even more robust.

Other researchers from Northwestern and from James Cook University in Australia are coauthors of the study, which was supported by the National Science Foundation and the National Institutes of Health.

Source: Northwestern University