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Darwin’s last idea is more important than ever thought: reworking the sediment mixed/s up the earth

05.01.2007
We all know Charles Darwin as the founder of the theory of evolution. Less known is that 125 years ago, he was the first to show the importance of burrowing earthworms.

He himself thought of this as a nice, but not very important subject. Researchers of the Netherlands Institute of Ecology (NIOO-KNAW) now provide proof of the contrary. Burrowing organisms are not only real “ecosystem engineers”, but the digging has also played a crucial role in the evolution of the modern animal forms. In the December issue of the scientific magazine TREE they put it this way: ‘Darwin would have been amazed!’

In his last book ‘of small importance’, The formation of vegetable mould, through the action of worms, with observations on their habits from 1881, Charles Darwin describes the role of earthworms in soil formation and erosion, and their impact on the landscape. For a large part, the book is based on experiments in Darwin’s own garden, carried out with the help of family members. Biogeologist Filip Meysman of NIOO: ‘Darwin’s book taught the wide audience the importance of soil organisms. Beforehand, earthworms & friends were mostly looked upon as pests that should be exterminated. Nowadays, we call the influence of burrowing animals and rooting plants on the soil “bioturbation”. What we now know of the impact of bioturbation on biodiversity and evolution, would have probably amazed Darwin enormously.’ Together with his colleagues Jack Middelburg and Carlo Heip, Meysman shows why.

Not only Darwin’s garden, but almost the complete earth surface is being reworked – including the ocean-floor. Meysman: ‘Burrowing soil animals show the important ecological principle of ecosystem engineers. Just like beavers, moles, sea urchins, and lugworms, earthworms strongly shape their environment. Their digging determines how the environment looks like and which other organisms can live there.’

For the current biodiversity research these burrowing creatures are very important: if a ecosystem engineer would be lost, by climate change for example, the whole ecosystem is affected. The engineer effect does not stop at the surface. The soil or sediment stores plant seeds and “resting stages” of algae and small planktonic animals. By uncovering it or reversely, digging it into the soil, bioturbation partly determines the vegetation aboveground and plays a role in the bloom of harmful algae.

The burrowing biota proves to have an unforeseen link to Darwin’s most well-known work: On the Origin of Species, the start of the theory of evolution. The evolutionary importance of burrowing surfaces approximately 540 million years ago, during the Cambrian explosion. In that era many new animal forms came to life within a short period of time, including all the modern forms. The first multicellular animals were actually nothing more than “softies”, filtering algae from the water or browsing the microbial mats on the ocean-floor. But soon afterwards, they discovered that it is also possible to eat each other. The first predators evolved, starting off an escalating “arms race”. The prey began to shield itselve with skeletons and by hiding in the sediment. Consequently, the predators developed weapons of offence and pursued their prey into the sea-floor. Meysman concludes: ‘On that moment, the ocean-floor changed drastically: it was a true burrowing revolution. The stable microbial mats from the Precambrium were replaced by the reworked sea-floor we still see. The organisms of the ocean had to adapt to this new world. Bioturbation was the driving factor of this rapid evolution, and this is where Darwin’s two books meet.’

The Netherlands Institute of Ecology (NIOO-KNAW) studies the ecology of land, freshwater and brackish and seawater. The Centre for Estuarine and Marine Ecology in Yerseke studies life in the sea and in estuaries. The NIOO employs about 250 people and is the largest research institute of the Royal Netherlands Academy of Arts and Science (KNAW).

Froukje Rienks | alfa
Further information:
http://www.nioo.knaw.nl

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