Coral reef spread 'ruled' by volcanoes
Volcanoes rule coral The Earth's ever-shifting geology is affecting the diversity of coral reefs across the Indian and Pacific oceans, a new study shows.
The research, reported in the Proceedings of the Royal Society found reefs and islands created by plate tectonics and volcanic hotspots, generate patterns of coral biodiversity.
The findings explain why the richness of coral varieties tapers off the further one travels from a biodiversity hot spot known as the coral triangle, says paper co-author Professor Sean Connolly of the Australian Research Council Centre of Excellence for Coral Reef Studies.
It also has implications for corals under climate change as it shows they arise from geological processes that take place over millions of years and therefore will be harder to replace if lost due to global warming.
Coral species found in the triangle, an area north of Australia bounded by the Philippines, Indonesia and West Papua, also populate reefs across the Indian and Pacific oceans, from Africa to Hawaii.
However until now, the reasons why coral biodiversity decreases with distance from the triangle, despite similar environments and temperatures, had eluded researchers.
"It's been quite difficult to try to tease apart ... [these] different driving factors," says Connolly.
Previous studies examined how species diversity changed as one moves further away from the coral triangle, so instead, Connolly and colleagues looked at which coral species colonised different reefs.
"We wanted to know exactly where species were dropping out and which species were dropping out," says Connolly.
The authors developed a database of coral species distribution, combining previous surveys of different reef populations with new fieldwork.
They found species further away from the coral triangle, were still subsets of those in the triangle, with only a few new outside species joining in.
More importantly, they discovered that groups of species would fail to colonise a reef, rather than just a single species dropping out on one reef, and another on the next reef.
Connolly and colleagues then discovered the drop-offs occurred on reefs located on tectonic plate boundaries and mantle plume tracks.
These are regions where Earth's crustal plates collide with or slide past each other, and where hot magma from deep mantle plumes push through the crust, building volcanoes that grow and eventually die as the plates they are on, move away from the plume.
The reefs and volcanic island chains generated by these events over millions of years, act as stepping stones for coral colonisation. Species with more specialised habitat requirements are unable to keep going to expand their range and drop-off.
The depth at which corals can survive and the range of water turbidity are suspected to be key factors, according to Connolly.
"We also found corals which tended to belong to older genera or lineages, such as hermaphrodites, tended to be more likely to have broader distributions," says Connolly.
"This is deeper than species in the family tree ... you're talking about millions of years."
The discovery also has serious implications for coral reefs in the face of climate change.
"Climate change is leading to the loss of corals throughout the tropics," says Connolly.
"This study has shown the diversity of corals we see today is the result of geological processes that occur over millions, even tens of millions, of years.
"If we lose these coral-rich environments the recovery of this biodiversity will take a very long time, so our results highlight just how critical it is to conserve the coral reefs that exist today."