Photograph by Gerry Ellis, Minden/National Geographic

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The mangrove-fringed bays of Hurricane Hole on the island of St. John are a haven for 33 coral species.

Photograph by Gerry Ellis, Minden/National Geographic

To Save Coral Reefs, First Save the Mangroves

Coastal forests are a refuge for corals at risk of bleaching in a warming climate.

With coral reefs in decline and NOAA calling for a larger protected area for reefs in the Gulf of Mexico, U.S. Geological Survey scientists are pointing out another strategy to save reefs: First save the mangroves.

Mangrove trees' thickets of stilt-like roots protect coastal land from erosion and help mitigate the damage of tsunamis and hurricanes.They may also serve as a haven for corals, according to a recent report in Biogeosciences. (Read more about how mangroves support animal life.)

Warming waters have not been kind to coral reefs. Heat causes corals to release the photosynthetic algae that live within and help feed the reefbuilding creatures—a phenomenon called bleaching, which is often fatal. In the Caribbean, where bleaching is widespread, more than 50 percent of the area that was covered by reefs in the 1970s is no longer.

In the mangroves of Hurricane Hole on St. John, U.S. Virgin Islands, however, Caroline Rogers of the USGS made a startling discovery. Growing among the tree roots were more than 30 coral species, including four threatened species. Some of the older corals must have survived bleaching that devastated the nearby reefs, suggesting the mangroves protected them.

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Reefs worldwide are facing destruction at every turn, as can be seen in this map of the Mesoamerican Reef, at the other end of the Caribbean from St. John.

Why It Matters

The corals in Hurricane Hole mangroves appear to be thriving, especially where the shade of the trees protects them from extra heating, Rogers says. She and colleagues report that the corals recovered rather well from a bleaching episode in 2010.

The corals in mangroves may also have evolved to be more resistant to bleaching than their brethren on reefs, the researchers speculate. Previous studies suggest that corals that are accustomed to environmental fluctuation can better weather extreme heat.

That raises hope that these more bleach-resistant corals may be able to recolonize dead reefs.

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Threatened species, including star corals, take shelter in the shade of mangrove trees.

The Big Picture

By 2030, more than 90 percent of the world's coral reefs will be endangered by bleaching, acidity from rising atmospheric carbon dioxide, tourism, and other threats.

Researchers are looking for refuges for corals to hide out in, including deeper reefs with cooler water. These places may become top conservation priorities. (Read more about the search for refuges.)

What's Next

Preserving mangroves may be one in a portfolio of strategies to help corals survive the effects of climate change. Hurricane Hole is part of the protected Virgin Islands Coral Reef National Monument, but mangroves worldwide are being destroyed at a prodigious rate.

The variety of coral species makes prediction difficult, says Rogers. But it also ups the chances that at least some species will survive the coming changes.

"That's to be optimistic," she says, "about a fairly dire circumstance."

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