Climate change driving marine species poleward
Warming waters have driven thousands of ocean species poleward from the equator, threatening marine ecosystems and the livelihoods of people who depend on them, researchers reported on Monday.
Comparison of data on nearly 50,000 species over three 20-year periods up to 2015 revealed that the exodus from tropical waters was accelerating, they reported in the journal PNAS.
The tropics had long harboured an outsized proportion of marine life, but could see that diversity disappear if climate change was not brought to heel, the authors warned.
“Global warming has been changing life in the ocean for at least 60 years,” senior author Mark Costello, a professor of marine biology at the University of Auckland, said.
“Our findings show a drop of about 1,500 species at the equator,” he added. “This will continue throughout the century, but the pace will depend on how we reduce — or not — greenhouse gas emissions.”
Poleward migration was more pronounced north of the equator, where oceans had warmed more quickly than in the southern hemisphere.
It was also more prevalent among open water fish than socalled benthic species living on the ocean floor.
“Benthic species can only move during their floating life-stage, and thus their shift (poleward) is between generations,” Costello said.
By contrast, species living in the high seas “can move with the water masses in their lifetime”.
Marine life in tropical waters declines when annual average sea temperature rises above 20 to 25°C, depending on the species.
“The ‘missing’ tropical species are likely to be following their thermal habitat as subtropical waters warm,” noted co-author David Schoeman, a professor of ecology at Nelson Mandela University in Port Elizabeth, South Africa.
Fossil records show that the same thing happened 140,000 years ago, the last time global surface temperatures were as hot as they are now.
Based on data in the open-access Ocean Biodiversity Information System, the statistical study does not look at how individual species will adapt to new environments.
In general, open-water species are likely to fare better, earlier research has found.
The impact on commercial fish stocks in the tropics was not addressed either, though it was clear which parts of the world will be hit the hardest.
“Indonesia and other nations near the equator, such as in West Africa, have the most to lose because their stocks can only decrease,” as no new species will replace those leaving, Costello said.