Complex soils behind Cape plant diversity
12 April 2011, by Tamera Jones
The Cape region of southern Africa is one of the world's top hotspots for plant diversity, because it's blessed with a huge range of soils and habitats, and its climate has been relatively stable for millions of years.
Protea flower, South Africa.
That's the conclusion of a study to find out why the area boasts over 9000 different types of plants – more than England, Wales, Scotland and Northern Ireland put together. Around 70 per cent of Cape plants aren't found anywhere else in the world.
Scientists found that a range of different soils, a stable climate, and regular fires would've encouraged new plants to evolve and would have led to lower extinction rates.
'Despite climate changes throughout the northern hemisphere over the last few million years, southern Africa is one of the few regions to have been buffered against these changes,' explains Dr Jan Schnitzler from Imperial College London, who led the study.
As the Earth's climate changed over thousands of years, the northern hemisphere went through multiple ice ages. But southern Africa was left largely unaffected.
Biologists have long wondered why plants have done so well here, and have come up with a number of suggestions.
'Our study shows that the remarkable plant diversity in the Cape of southern Africa is not the result of a recent and rapid radiation triggered by climate changes, but that diversification took place over an extended period of time.'
Dr Jan Schnitzler, Imperial College London
One idea says that changes in the climate around five million years ago, which led to a cooler and more arid climate, would have triggered an 'orgy of speciation'. Another idea suggests the plants' relationships with their pollinators would have driven the evolution of new plant species.
'We set out to test some of these ideas, because recent studies have suggested that many of these plants started evolving long before the change in the climate five million years ago,' says Schnitzler.
Together with UK, South African and American colleagues, Schnitzler analysed and built up family trees for 470 plant species, including Protea plants, a 'tribe' of legumes, and two subfamilies of the iris family. The plants the researchers chose to study 'represented three of the seven largest plant families in this biodiversity hotspot'.
As well as drawing up detailed family trees, the scientists collected information about the habitat, geography, soil and pollinators in different parts of the Cape.
They found that for Protea and the iris subfamilies lots of different types of soils were responsible for a large amount of their diversification. But fire-coping strategies would have been important for driving the evolution of the legume species they studied.
They also found that all 470 species started diversifying long ago: throughout the Oligocene and Miocene – from around 35 million years ago – and that diversification rates have stayed pretty much constant in that time.
'Our study shows that the remarkable plant diversity in the Cape of southern Africa is not the result of a recent and rapid radiation triggered by climate changes, but that diversification took place over an extended period of time,' write the authors in their study, published in the journal Systematic Biology.
'We didn't find that pollinators could explain the high plant diversity here either,' says Schnitzler. 'But we didn't expect soil to be so important.'
Schnitzler says the next thing would be to compare the diversity of the Cape region with other Mediterranean-type biodiversity hotspots like southwest Australia, California and central Chile.
Jan Schnitzler, Timothy G. Barraclough, James S. Boatwright, Peter Goldblatt, John C. Manning, Martyn P. Powell, Tony Rebelo and Vincent Savolainen, Causes of Plant Diversification in the Cape Biodiversity Hotspot of South Africa, Systematic Biology, First published online: February 28, 2011, doi: 10.1093/sysbio/syr006
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