22 Aug 2019 - Due to soaring temperatures, tropical coral reefs are facing a bleak future. But recent research shows that some of these corals are migrating to cooler subtropical seas, offering a measure of hope that these ecosystems can survive the existential threat of climate change.
From the shores of Florida to the islands of Japan, from the Midway atoll to southern Australia, an unheralded ecological regeneration may be underway. Reef-building coral, which has become ever more threatened in the superheated waters of the tropics, is making a bid for survival by migrating to subtropical climes that meet its temperature requirements.
The coral reefs of the tropics have looked doomed. Bleached by marine heatwaves, suffering mass die-offs, and stuck to the sea bed, they have no obvious escape as the oceans warm. Some experts say they will be gone by mid-century, the first great ecosystem casualty of the climate emergency.
But the news is not entirely grim. It turns out that young corals can be surprisingly mobile, able to move in ocean currents, if their homes become inhospitable, and relocate to more equable waters. “I do believe there is a glimmer of hope for them,” says Nichole Price of the Bigelow Laboratory for Ocean Sciences in Maine, the lead author of the first global study of their sporadic recovery.
Marine ecologists are reporting migration of tropical coral into subtropical regions, part of a wider “tropicalization” of ocean ecosystems as species seek cooler waters away from the Equator. While struggling in their former habitat, they are proliferating between 20 and 35 degrees north and south of the Equator, with young refugee corals creating new reefs hundreds of miles from home.
With coral on the move, the map of the world’s reef-building tropical coral is being redrawn.
Price cautions that the migration does not come close to compensating for tropical decline. The percentage increase in the establishment of new corals in the subtropics, while encouraging, is from a low level. “Many more corals are being lost near the equator than are migrating to the subtropics,” she says. So overall, there has been a global decline in recruitment of 82 percent over the past four decades. But, even so, wipe-out may not loom.
In southern Japan, for example, at 33 degrees north, more than 70 coral species now occupy most of Tatsukushi Bay. In the United States, staghorn and elkhorn corals are extending around the Florida peninsula into the Gulf of Mexico. And in Australia, coral appear to be migrating south from the Great Barrier Reef to the coast of New South Wales around Sydney, at 30 degrees south.
This has been going on for a while, but until now nobody had assembled the complete picture. Price’s six-nation study, published last month, finds that with coral on the move, the map of the world’s reef-building tropical coral is being redrawn.
Her study has two headline findings. First the bad news: The recruitment of new young coral on tropical reefs has declined by a staggering 85 percent since the 1970s. Even reefs that appear healthy are simply not reproducing. But the good news is that there has been a 78 percent increase in recruitment on sites studied outside the tropics. Coral recruitment has been “shifting poleward,” Price concludes.