This little freshwater snail, probably a Helisoma sp., created a record of its movement in the fine sediment layer covering the rocks on the bottom of a shallow pool next to a creek.
Bioturbation is the disturbing and mixing of soils and sediments by organisms that live or feed in them or simply pass thru them. It is a widespread phenomenon that takes place at different scales: the large mass of soil brought up among the exposed roots of a wind-toppled oak tree and the few-millimeter thick sediment layer disturbed along the trail of a tiny aquatic snail are both examples of bioturbation.
A recent review by Meysman et al.1 presents a good introduction to the subject. Charles Darwin was apparently the first naturalist to approach bioturbation from a scientific angle. The result was his book The formation of vegetable mould, through the action of worms, with observations on their habits.
Bioturbation is especially obvious on the ocean floor where many mollusks, worms and crustaceans live buried in the sediment while others plow thru it in search of food. The result is a complex interaction of biological and physical events: "Benthic organisms modify the microtopography of the ocean floor via pellet production, track formation and different types of construction, such as mounds and pits... This biologically induced roughness modifies the hydrodynamics above the sediment layer, which in turn affects erosion and resuspension."
However, Meysman et al. point out that "for most landscapes and seascapes, the importance of the biological imprint compared with purely physical processes remains largely unknown." To return to my original example, how significant is really the reworking of aquatic sediments by snails when a flooding creek after a heavy rain storm can remove or deposit more sediment than can a river full of snails over many, many generations?
When a process is as common as bioturbation, it becomes difficult to rule out what is not bioturbation. Is the imprinting of footprints in mud count as bioturbation? If the formation of footprints had a significant impact in the lives of other organisms, I suppose the process would be bioturbation. What about the weathering of rock outcrops by microorganism and plant roots? Many rock types start out as sediments and weathered rocks eventually turn into soil. But where do we draw the line?
Meysman et al. also assign a "revolutionary" position to bioturbation in the grand scheme of evolution: "Benthic fauna had to adapt to the newly emerging bioturbated sediment conditions, thereby fuelling the 'Cambrian explosion'". But that is pure speculation.
1Filip J.R. Meysman, Jack J. Middelburg and Carlo H.R. Heip. Bioturbation: a fresh look at Darwin’s last idea. Trends in Ecology and Evolution 21:688, 2006. Abstract & Glossary