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 Monday.
Comparison of data on nearly 50,000 species over three 20-year periods up to
2015 revealed that the exodus from tropical waters is accelerating, they
reported in the journal PNAS.
The tropics have long harboured an outsized proportion of marine life, but
could see that diversity disappear if climate change is 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, told AFP.
"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
have warmed more quickly than in the southern hemisphere.
It was also more prevalent among open water fish than so-called 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 explained.
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 degrees Celsius, depending on the species, the study
found.
40 percent drop by mid-century
"The 'missing' tropical species are likely 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 is not addressed either,
though it is 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.
Worldwide, about 1.3 billion people live in coastal tropical areas, many of
which rely on fisheries for food.
A recent review article in Nature estimated that the maximum catch potential
of tropical fish stocks in so-called exclusive economic zones—200 nautical
miles (370 kilometres) from the coast—would decline 40 percent by
mid-century if global warming continues unabated.
In most Pacific island nations, combined catch of skipjack and yellowfin
tuna—the two most exported fish—would drop up to 40 percent under the same
scenario, while coral reef fish consumed locally could decline even more.
The study in PNAS began with the University of Auckland doctoral
dissertation of Chhaya Chaudhary.
Reference:
Chhaya Chaudhary el al., "Global warming is causing a more pronounced dip in
marine species richness around the equator," PNAS (2021).
www.pnas.org/cgi/doi/10.1073/pnas.2015094118
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Planet and Environment