In Graphic Detail: Six Coral Continents

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Some of the world’s most threatened ecosystems — coral reefs — could use a break. Indiscriminate fishing, pollution and coastal development have plagued coral reefs for decades. Add to that climate change, and by 2100, 70 to 99 percent could be damaged or lost. They go hand in hand with food security and livelihoods, biodiversity and beauty.

But researchers behind a new study are finding hope in coral connectivity. Through modeling analysis, they identified six major reef networks around the world that could sustain and regrow corals even after mass bleaching, as long as portions of them fall into climate zones — areas of Earth that are more resilient to environmental stressors like acidification and rising temperatures.

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The largest of these reef complexes — like sunken chains of islands biologically connected by ocean currents — spans nearly 1.8 million square kilometers, nearly the size of Mexico. It includes the Great Barrier Reef and the Coral Triangle, the latter including the reefs of the Philippines and Indonesia. Other large systems are in the Caribbean and Red Seas, along East Africa and around Fiji.

Map of coral reef networks

The largest reef networks (see top map) have been found in the Great Barrier Reef and Coral Triangle (purple); the Caribbean (pink, black); the Red Sea (green); East Africa (red); and near Fiji (dark blue). The warmest colors in the lower map show the reefs with the best sources, or those sending coral larvae to most other reef sites. Map by Greiner et. Al.

To determine which reefs were connected into networks, the study relied on a global dispersal model that estimates how coral larvae travel on ocean currents and resettle in new homes. Researchers then ran three climate scenarios to simulate different degrees of coral bleaching and subsequent recovery.

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In the most likely climate scenario – in which almost a third of the world’s coral persists after a bleaching event because it is protected by refuge zones – the surviving coral can reseed the surrounding reefs and help a large reef network to recover.

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But while these complexes seem to have a knack for dispersing larvae in their own neighborhoods, the study finds that they’re not as good at sending larvae out of their networks, says Ariel Greiner, a graduate student at the University of Toronto. who led the research. Like rebuilding a community after a disaster, these reefs may not be able to help more distant locations.

The other two simulations that wiped out corals in climate refugia showed that global reef networks would collapse, but that surviving corals could still spread larvae after bleaching, just to smaller reef systems. This suggests that in addition to corals in refuge zones, conservationists should also prioritize reefs that are good at dispersing their larvae, says Greiner. If enough of them are identified and preserved, they could serve as stepping stones for coral spread to other areas.

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