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Changing Climate May Favor Smaller Corals

Three things you should know:

1) As global ocean temperatures rise on average, scientists are concerned that some diseases that harm corals will become more prevalent.

2) In response to warming temperaturess over the past few decades, coral populations in the Caribbean have declined and big, slow-growing corals have been replaced by smaller, fast-growing corals. 

3) The new coral communities that are emerging may be more resistant to current diseases, but they do not harbor as rich a community of fish and other ocean life.

The debrief:

The once vibrant coral communities in the Caribbean, such as this one in the U.S. Virgin Islands, are changing as water temperatures rise, but it may not be easy to predict how they will continue to evolve in the future. Credit: NOAA CCMA Biogeography Team.

In the past 40 years, Caribbean corals have been devastated by new disease epidemics and coral bleaching, and overall the coral populations have been cut by about 80 percent. Many researchers claim that the strength with which these coral diseases roar through the Caribbean is linked to the substantially warmer water temperatures in the Atlantic brought on by global climate change, as well as natural climate variability. As the water temperatures rise, the corals are becoming more susceptible to infection. It’s also possible that the diseases themselves could be growing more abundant or virulent in warmer water.

But shrinking numbers aren’t the only changes to Caribbean corals; the types of corals dominating the area have also changed. The large and long-lived reef-building corals synonymous with the Caribbean — like staghorn corals, with their strong colorful branches, and boulder star corals — are being replaced by smaller species that grow and multiply quickly.

“The big corals are like the oak trees of the reef,” says oceanographer Peter Mumby from the University of Queensland in Brisbane, Australia. “But the big corals are disappearing and in their place are these small corals that are more like weeds.” Now, Mumby and his Queensland colleague Laith Yacob say these changing coral populations are a sign that it might not be so easy to predict how climate change is going to affect coral ecosystems as had been previously thought. 

The different types of corals in the Caribbean aren’t all showing the same response to recent disease epidemics, Mumby says, and in a new paper published in the Proceedings of the National Academies of Science, he and Yakob find that the smaller, weedlike corals are more resistant to diseases and may be more likely to survive future epidemics.

“Because these small short-lived coral colonies are growing and dying quickly, it is more difficult for a disease to get started and spread,” says Mumby, though he cautions it is also possible the diseases will adapt in an unexpected way. All coral species may not become more susceptible to disease as the climate continues to change, he says, and we won’t always be able to predict how climate change will influence different ecosystems in the future using the responses we have observed in the past.

But while the response of the coral colonies in the Caribbean to recent diseases hasn’t been as devastating as feared, Mumby says the new community of corals isn’t as beneficial as the ones they are replacing.

“The fact that we’ve now got these reefs dominated by small weedy corals is bad news,” says Mumby. “It’s bad for biodiversity, it’s bad for the people that make their living fishing among these reefs. This is not a good news story.”

Why this science matters:

The coral ecosystems that populated the Caribbean prior to the 1970s had thrived there for thousands of years and currently play an integral role in the region's economy. The reefs provide protection against beach erosion caused by tropical storms that hit the Caribbean each year. Moreover, the distinct corals in the Caribbean play host to a vibrant community of fish and other marine life that attract tourists and sustain local fishing industries. Many Caribbean island nations rely on tourism to bring in as much as half their annual gross domestic product. For the United States commercial fisheries alone, the value of the Caribbean corals is an estimated $100 million annually.

But now it seems that in just a few short decades, the increasingly warm waters in the Caribbean are changing the types of coral colonies that can prosper there, which will have an unavoidable economic impact on the people living in the region.

 

The trend toward smaller and more weedlike corals in the Caribbean is also a red flag to ecologists that are trying to understand how climate change will impact all types of ecosystems around the world. In the past, ecologists thought that higher ocean temperatures would spell further disaster for all types of corals, but Mumby’s new results show that they may in fact favor small and rapidly-growing weedlike coral colonies that are more disease resistant. And this unexpected observation, he says, is just one example of how an old theory on how ecosystems change might not apply in the future as the climate changes.

“As the ecosystems are being altered, we have to be willing to consider that they may behave quite differently than what we’ve seen before,” says Mumby. In order to be accurate with predictions based on future climate change, he says that researchers will need to think more about how the new ecosystems differ from previously observed conditions. 

“The challenge of trying to predict the future just got a bit more difficult.”

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