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‘Hopping hotspots’ reveal how the planet drives life

01
Aug 2008

An international team of marine scientists has proposed that huge explosions of life may be due in part to continental ‘traffic accidents’ – the Earth’s tectonic plates slamming into one another.

The researchers have tracked the global heartbeat of tropical marine biodiversity halfway round the world, from a focal point over western Europe/Africa around 50 million years ago, to its present-day location around South East Asia and Australia, identifying three major hotspots along the way.

The explanation for these “hopping hotspots” with the greatest array of marine species appears to be plate tectonics as well as climate, says the team led by Dr Willem Renema, Professor John Pandolfi and Professor David Bellwood of the Australian Research Council Centre of Excellence for Coral Reef Studies. Their review paper appears in the latest issue of the journal Science.

“By examining fossil and molecular evidence, we could clearly see that the global centre of marine mega-diversity was over western Europe and Africa in the Eocene, around 50 million years ago – a time when there was a shallow ocean called Tethys between the two continents,” says Professor Pandolfi.

“By the start of the Miocene, around 25 million years ago, the hotspot of sea life had moved to the region of the Arabian Peninsula, Pakistan and west India – an area that is still quite rich today and was starting to appear in the East Asia region.

“Then, in more recent times, the global centre of the action hops to the Indo-Asian-Australian region, which is the greatest hotspot of marine biodiversity today, leaving an ageing Arabian fauna to senesce.

“In each of these cases we think that the emergence of many species and their gradual dwindling can be largely explained by plate tectonics, as well as climate change.”

Each of the three marine biodiversity hotspots coincides with the site of a major collision between continental tectonic plates.

The gradual approach of the African plate to the European plate created shallow, warm seas with many islands and bays, ideal for new species to evolve in, while the currents of Tethys which was open at both ends, swept through. These conditions would promote the emergence of new species and their spread, Prof. Pandolfi says.

Similarly, the gradual closure of Tethys to form today’s Mediterranean, the constriction of major currents and a plainer, cooler, more stable environment would reduce the diversity of species – whereas further east, where Arabia was just starting to collide with Asia the conditions were ideal for a new hotspot of sea creatures.

As Arabia joined the Asian landmass, conditions became less favourable. However, further east again, the progressive collision of the Australian plate with SE Asia, and the eruption of island chains forming modern Indonesia, the Philippines, and Papua New Guinea created conditions ideal for sea life to flourish in all its diversity, the team says.

“The evidence seems to be saying that when habitats are disrupted, increasing in area, and highly variable, you get more species.  When they are reduced in size and number, and become more consistent, the range of species declines,” says Prof. Pandolfi.

“As continents merge tectonically, seas close up, nutrients increase, currents subside – and the range of habitats and species also declines.  This period of senescence is of particular interest, as it is important to understand whether species richness in a particular area is declining naturally due to tectonic changes, or due to man-made changes,” says Dr. Renema.

The scientists were able to track the migration of highly diverse sea life around the planet by studying the range of fossil corals, mangrove pollen, shellfish, plankton and other species in each region over time.  For example, nine of the 20 varieties of the well-known modern coral Acropora flourished in the region of Paris, France, 50 million years ago, when the West Tethys hotspot was thriving.

They also used molecular techniques to ‘clock’ the time at which new species emerged. “This shows, for example, that most cowrie shell species and coral reef fishes appeared around 20 million years or more ago, and very few have evolved in the recent past. This contradicts a popular assumption that many coral reef species appeared in the last few million years,” says Prof. Bellwood.

“Because they occur over such huge time-scales, there has been a tendency to discount the role of tectonic changes in the development of life, but we are suggesting here that it is in fact one of the major driving forces,” Prof. Pandolfi says. “We can see from the fossil record how many tropical organisms are hitch-hiking round the planet, as conditions change.”

The team says its work provides a new way to view biological hotpots and the factors that cause them to arise and decline. This will help researchers to better understand the factors propelling species decline in particular regions.

More information:
Dr. Willem Renema, CoECRS and Nationaal Natuurhistorisch Museum, The Netherlands, +31715687576 (office), +31642396039
Professor John Pandolfi, CoECRS and UQ, +61 7 3365 3050 or +61 0400 982 301
Professor David Bellwood, CoECRS and JCU, +61 7 4781 4447 or +61 0419 422 815
Jenny Lappin, CoECRS, +61 7 4781 4222
Jim O’Brien, James Cook University Media Office, +61 7 4781 4822

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Coral Reef Studies

ARC Centre of Excellence for Coral Reef Studies
James Cook University Townsville
Queensland 4811 Australia

Phone: 61 7 4781 4000
Email: info@coralcoe.org.au