The millipede - Robin Hood of the forest floor

This article is a translated and edited version of a blog first published on the British Council France website. The original can be found here.

Francois-Xavier Joly, the winner of FameLab France 2015 and the public vote at the international final at Cheltenham Science Festival, tells us about the essential role of millipedes in our forests. You can see a video of his winning talk at the end of this page.

The story that I am about to tell is about the amazing interactions that take place in the soil of our forests, invisible to most of us. The organisms involved have an important role for life on Earth. Conservation efforts shouldn’t be restricted to ‘superstar’ species, but should also pay attention to smaller, lesser known species that play an irreplaceable role in the environment. 

The wonderful scene where green leaves slowly turn yellow announces the arrival of autumn. This is the time for retirement for these numerous leaves, which, after months of hard work capturing light, fixing carbon dioxide and producing oxygen, finally fall, blown away by the wind, and end up on the soil.

A vital mission
However, these leaves must accomplish one last, but very important mission. Made of carbon compounds and nutrients, they must decompose, in order to return the carbon to the atmosphere, and the nutrients to the soil. Life in the forest depends upon this mission; without the return of nutrients to the soil, it would quickly become infertile, and without the return of carbon dioxide to the atmosphere, photosynthesis would be just impossible. As plants are at the start of all nearly all food chains, many species would be endangered.

An army of decomposers
Fortunately, many organisms take care of this mission. Soil is far from being as lifeless as we often assume, containing many different species. In particular, some tiny but numerous organisms, namely bacteria and fungi, play an amazing role. They are the decomposers. By releasing enzymes in their environment, they cut the long and complex molecules of the leaves into smaller ones, and ingest them. This in turn releases the nutrients to the soil and the carbon to the atmosphere. 

A world of inequalities
But all leaves are not born equal. Some leaves, like those of the ash-tree, rich in nutrients and readily degradable compounds, manage to attract decomposers, and are decomposed in just a few months. Meanwhile, other leaves such as chestnut leaves, which are poorer, thicker and drier, fail to attract decomposers and need years to be fully decomposed. 

Superheroes with unsuspected powers
However, help is at hand from other soil organisms, like millipedes, which also feed on these leaves. As their digestive system are rather limited, they assimilate only a small part of the leaves, and return most of it to the soil as faeces. And this is where the magic happens. Leaves, once very distinct, become much more similar when turned into faeces, and these inequalities seem to disappear. Rich leaves see their attractiveness slightly reduced, while the attractiveness of poor leaves increases, making them equally attractive to decomposers. This way, this transformation of leaves into millipedes’ faeces gives a new start to leaves in the race for decomposition. Millipedes, by taking from rich leaves, and giving to poor leaves, are maybe the real Robin Hoods of the forest. 

François-Xavier Joly is a PhD student at the Centre of Evolutionary and Functional Ecology in Montpellier where he studies how biodiversity contributes to ecosystem functioning.