Coral species are migrating away from equatorial waters and towards the Earth’s poles, a likely effect of warming ocean temperatures and climate change, according to research published this month in the journal Marine Ecology Progress Series.
Using data collected over four decades, researchers, representing 17 institutions in 6 countries, were able to develop a snapshot of how coral communities have changed over time. A total of 92 studies provided more than 1,200 records of coral populations from 1974 – when such recordkeeping began – to 2012.
What the team found appears to speak to the effects of climate change, explained lead researcher Nichole Price of Bigelow Laboratory for Ocean Sciences.
While the number of young corals on tropical reefs had declined by 85% during the study’s timeframe, in subtropical reefs they had doubled.
“Climate change seems to be redistributing coral reefs, the same way it is shifting many other marine species,” Price said in a Bigelow release.
“The clarity in this trend is stunning, but we don’t yet know whether the new reefs can support the incredible diversity of tropical systems.”
The study explains that coral reefs are vulnerable to a wide-variety of disturbances, whether natural or man-made, but that the most immediate threat to tropical reefs is seawater warming, which can drive coral bleaching and die-off.
Warming in the sub-tropics, however, appears to be encouraging coral fitness and survival. The research team observed coral growth at latitudes as far as 35 degrees north and 35 degrees south – latitudes ranging from Kyoto, Japan, to south of Sydney, Australia.
“We do think that the most parsimonious explanation for the poleward range expansion and equatorial retraction of coral recruits [corals under a year old] is the tropicalisation of high latitude reefs and the heat waves on tropical reefs that are bleaching adult corals, and patterns in our dataset support this theory (i.e., the largest dips in coral recruit abundance at the equator coincide with the strongest annual extended heat waves),” Price said in an email to the Cayman Compass. “But we have not yet formally tested this hypothesis. It is logical though that coral recruits are growing and surviving where temperatures are most optimal.”
Researchers suspect only certain coral species are able to reach these new locations, however. Microscopic coral larvae can only swim or drift as far as their limited fat stores will permit, explained the Bigelow release.
Apparent examples of expanded range include Acropora, small polyp stony corals, along the eastern coast of Florida and several coral species along the eastern coast of Japan.
That does not mean higher latitudes provide ideal environments for coral species, however. The study points out that establishment of young coral could be impeded in such zones due to competition with macroalgae and low winter light intensities.
“The areas exhibiting the most visible establishment of new reefs are parts of the Japanese coastline, and in that case, it is generally Acroporid species that are surviving in new environments. But I wouldn’t consider these new colonies of coral to have a created a ‘coral reef’ as of yet,” Price told the Compass.
“Many coral recruits do not survive through the cold winter periods as reef-building corals cannot tolerate temperatures below 18 degrees Celsius and the low light intensity during shorter winter days can really slow coral growth.”
The drift is also making it more difficult to distinguish between native and invasive species, explained one author of the study, Satoshi Mitarai, an associate professor at Okinawa Institute of Science and Technology Graduate University.
“We are seeing ecosystems transition to new blends of species that have never coexisted, and it’s not yet clear how long it takes for these systems to reach equilibrium,” Mitarai said in the Bigelow release.
“The lines are really starting to blur about what a native species is, and when ecosystems are functioning or falling apart.”
Price added that the distinction between invasive and native species is a matter of perspective, posing a number of questions to keep in mind.
“Have corals always been a part of the local ecosystem, but just present in lower abundances than we may see in the future? Will a higher abundance of corals change ecosystem function or replace other ‘native’ foundational, predominant species (like kelp)? We don’t have these answers, but this is an active area of inquiry for many scientists. Some scientists conjecture that local fish productivity could be increased on tropicalised reefs,” she told the Compass.
“Paleontological evidence (i.e., coring and fossils) suggests that coral reefs similarly shifted distribution to the same latitudes 125,000 years ago during the last interglacial period.”
For now, many questions remain for researchers regarding species suitability and the long-term fate of coral colonies in these new locations.
“The changes we are seeing in coral reef ecosystems are mind-boggling, and we need to work hard to document how these systems work and learn what we can do to save them before it’s too late,” Price said.