Why does coral reef diversity cluster in southeast Asia?

Plate tectonics might have driven the movement of coral reef biodiversity over time, according to new research.

Courtesy of Simon Gingins
Coral reefs such as the Great Barrier Reef (shown here) are extremely species-rich habitats.

Coral reefs in southeast Asia are made up of the most diverse species of corals and fish globally. Boasting hundreds of species of corals and thousands of different reef fish, the region has been called The Coral Triangle. 

But how did this biodiversity come together? Plate tectonics may have something to do with it, say scientists. 

The way in which the Earth's plates have moved over millions of years may have stimulated diversification among coral reef species, according to a new study. 

"Shallow reefs occur on the fringe of continental plates," study author Loïc Pellissier, a landscape ecologist at ETH Zurich and the Swiss Federal Institute for Forest, Snow and Landscape Research, told The Christian Science Monitor in an e-mail. So "plate tectonics should have largely reshaped the position of this habitat and in turn influenced the evolution of their associated species."

Dr. Pellissier and a team of researchers modeled the historical diversity of coral reef populations along with the way the plates have moved in the past 140 million years and they found a correlation between the two. Their findings were published Friday in the journal Nature Communications. 

So what happened in southeast Asia?

Some 140 million years ago, the landmasses that are today South America, Africa, India, and Australia were smushed together, forming a supercontinent called Gondwana. A body of water called Tethys stretched between Gondwana and Laurasia, the supercontinent's northern counterpart. 

As Gondwana began to break up in a process that ultimately led to today's continents, the movement of the plates fragmented and consolidated the tropical reefs across Tethys.

"Because of the plate tectonic processes, new habitats emerged in different locations over the course of millions of years, while others merged or disappeared," Pellissier said in a press release.

The structure of the seafloor shifted and changed as the plates moved. This split and merged reef habitats. As a single species was separated in two different locations, the two groups would evolve separately to adapt to the new ecological niche, thus becoming distinct species over time.

"This is a process also referred to as geographic speciation," Pellisier tells the Monitor. 

The waters of southeast Asia hold the major coral reef biodiversity today, but that hasn't always been the case. That hotspot has shifted over the past 50 million years, starting in the western Tethys. 

"Now, for the first time, our models provide an explanation for this movement," Pellissier said in the press release. "These dynamic structures encouraged the relocation of the focal point of species diversity."

In addition to the influx of western Tethys species into southeast Asian marine environments, the Australian continental plate also moved north some 15 million years ago, effectively merging Tethys and Australian marine biodiversity. These two processes together fed into the spectacular coral reef biodiversity in the region today.

New species don't just emerge when one group is isolated from another. Sometimes new species evolve within their ancestral population. Although both processes feed into global biodiversity, the researchers found that, in their models, geographic speciation often best explained fossil biodiversity. This fits with their assertion that plate tectonics helped stimulate diversification.

"Today's reef ecosystems have a very long history," Pellissier said. These processes of diversification happen over long timescales. But, in light of current global warming trends, he warns that coral reefs are particularly sensitive to temperature changes.

"It took millions of years to shape [today's] coral reef diversity and it might take less than 100 years to destroy them as a consequence of climate changes," Pellissier tells the Monitor.

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