Why getting dirty is good for all of us

Darwin extolled the virtues and dedicated his last works to it, few places on Earth are without it, and there is little doubt that human well-being benefits tremendously from it, but what is bioturbation?

 

It is sobering to think that at any time of day or night an army of invertebrates actively modify the soils and sediments beneath our feet, something that they have been doing for millions of years and which most people take for granted. Yet, the emergence and subsequent diversification of these subterranean engineers, about half a billion years or so ago, was one of the most significant events in Earth history. Why? Because the burrowing, feeding and other behaviours of these organisms – processes collectively referred to as bioturbation – had a transformative effect on the rate and magnitude of biogeochemical cycling and the amount of available habitable space.

 

So why do the activities of fauna, such as worms, clams, and shrimps matter? The process of bioturbation can translocate particles and fluids, selectively or indiscriminately, over considerable distances from the surface to depth, from deeper regions of the deposit to the surface, and/or horizontally.

Bioturbation – Worms at Work from Wim van

Bioturbation – Forschen im Watt from nicola wettmarshausen on Vimeo.

As particles are transported in this way, they degrade, aggregate or dissociate, such that the type and extent of microbiological colonisation radically changes. At the same time, the movement of fluid associated with the ventilation of burrows vastly increases exposure to electron acceptors, such as oxygen, that disproportionately fuel the microbial conversion of organic matter to nutrients important to the entire foodweb.

 

Bioturbators are not equal

Given the complex and varied ways in which individuals and species interact with their immediate environment, the major research challenge is to establish a mechanistic understanding of bioturbation that can be applied more generally. One way to achieve this is through the use of models, which are fitted to experimental data that describe the redistribution of tracers in the presence of known species

Using these techniques, seven broad mechanisms of bioturbation can be distinguished:

Epifaunal bioturbators are species whose activities occur predominantly above the surface, such as hermit crabs
Surficial modifiers represent species whose activities are mostly restricted to the uppermost layers (<1–2 cm) of the sediment, such as some sea urchins.
Biodiffusers randomly redistribute particles through every level of the sediment profile over short distances, in a way that is analogous to molecular or eddy diffusion.
Gallery biodiffusers are responsible for extensive burrow networks that connect to the surface, such as many large crustaceans [insert Rachel video], facilitating the redistribution of fluid and sediment particles over both short and long distances.
Upward conveyors are vertically orientated species that feed with the head located at depth, actively transporting sediment from the lower horizons of the sediment profile to the surface
Downward conveyors exhibit a feeding strategy that is the reverse of upward conveyors, located at depth, actively transporting sediment from the lower horizons of the sediment profile to the surface.
Regenerators are excavators that completely transform the entire sediment profile, usually by digging pits or building extensive mounds

 

Hence, the contributions bioturbating species make to ecosystems are not based on their taxonomic identity, rather they are reflect the extent to which they interact with their environment. In other words, different species make different contributions to ecosystems. So you just need to know which of these bioturbator groups species belong to. Well, yes, but its not quite that simple…

 

It ain’t what you do, it’s the way/time/place that you do it

Back in 1982, I didn’t realise how relevant the immortal words to a catchy chart topping pop song by Bananarama– go on, sing it – would become given my chosen career path.  Remember the words to this song and you are most of the way there to summarising where the leading edge of the discipline is currently focussing most effort. Species behaviour varies over time and across relatively short distances, and is further modified by environmental context and historical experience. This means that the assignment of a species to a particular mechanism of bioturbation will not adequately characterise the ecological contribution of a species, because species change what they do, when they do it, and how they do it! Why should we be worried about that? Because knowledge about the response of species to changing conditions and context is fundamental to the accuracy and predictive capabilities of ecosystem models used to estimate the consequences of environmental change.

 

You never realise what you have until it’s gone

And that brings us to a worrying twist to this blog post – the bioturbation revolution initiated half a billion years ago is being reversed by unsustainable human activity. This is worrying because evolutionary patterns in the fossil record, and a wealth of evidence from experiments where investigators have manipulated the type and number of bioturbators, show that a variety of ecosystem properties are dramatically impaired as bioturbator diversity is reduced. In his final book, The Formation of Vegetable Mould, Through the Action of Worms (1881), Darwin concluded, “It may be doubted if there are any other animals which have played such an important part in the history of the world as these lowly organized creatures“. He was right.

 

About the author.

Prof. Martin Solan is a marine benthic ecologist with interests in how sediment-dwelling invertebrates respond to change and effect ecosystem properties. Key components of his research are the use of manipulative laboratory and field experiments to understand the functional role of species and the development of generic concepts of biodiversity-ecosystem functioning within the context of environmental change and anthropogenic forcing.

 

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