Bioturbation and how this contributes to the well-being of the planet

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By definition, bioturbation is the stirring of the soil or sediment by living organisms. This includes burrowing, ingestion and defecation of soil or sediment grains. Bioturbating activities by plants and animals have a profound positive impact on the environment. If the stirring of sediment did not occur, plants would be unable to grow in dense soil deprived of oxygen and this would have dire consequences on the environment. Fortunately, nature has taken care of bioturbation.

Dung beetle activities are a good example of bioturbation

Ancient worm burrows have been found in caves in South Africa. These burrows had a diameter of several centimetres. These burrows filled with sediment and have been preserved in the rock as a tube-shaped rock. The number of cylinders present in the deposit testifies to the bountiful food available to those organisms who can adapt to a life in mud. Living in mud is not easy since there are very little oxygen and the ever-present danger of being inundated by the next deposit.

Moles are considered a nuisance to our neatly manicured lawns but without their digging, the soil would be too hard for the grassroots to penetrate. Water and air can penetrate down the tunnels. In addition to the moles, there are all those earthworms, crickets, lizards and dung beetles. Fly larvae dig into the ground to pupate.  Antlion larvae only dig a small conical pit to trap terrestrial insects but if you consider that over millions of years, they alone, have moved mountains worth of soil.

The most famous insects that turn the soil are undoubtedly the termites. During the construction of their mounds the termites bring soil from deep below the surface and dump it in the form of a mound. The fine particles that have filtered into the soil are then stirred to the surface. The clay particles comprising of some salts which leech down every time it rains filtering these particles into the soil. When the salts are in the lower layers the mammals cannot get to it. The termites make it available when they discard the clay in the form of their mounds.

Termite mount (c) Ben Coley

Antelope and elephants have been known to eat the soil of a termite mound when there is no other source of salt. Some neighbouring mounds will have tunnels that pass each other. If a connection is made the workers will seal off the breach and dig in another direction. Ant nests can also be found very close to termite mounds. The tunnels of these two species intertwine but do not join.

Many wasp species use mud for the construction of their brood chambers. Mud is collected and moved to a suitable site. When the chambers are later abandoned the mud falls off at the new site. Pompilid wasps commonly known as ‘spider hunters’, dig tunnels in the soil where they keep immobilised spiders. They lay an egg on the food source, in this case, the spiders and then cover the hole. When the young hatch, they feed on the spider food source and dig a hole out which is left open. Insects as individuals do not move much soil but when we take the untold numbers of single insects as a collective into account, the resulting amount of soil that is moved is enormous.

Baboon spiders and solifuges dig holes in the ground for shelter. When the solifuge is foraging they do a lot of digging. Every little bit helps to loosen the soil or to make hollows where water can collect instead of running off down the slope.

Baboon spider coming out its hole in the ground (c) Rebecca Evans

Birds also do their bit to turn the soil. Swallows collect mud to build their nests and the very act of removing the mud makes hollows in the ground. When the nests fall to the ground, then it is in a new position. The hammerkop, a unique species of wading bird found predominantly in Africa lines its nest with mud, the flamboyant flamingos gather the mud off the floor of the lake where they are nesting and form it into turrets which are small tower-like structures. Kingfishers that nest in tunnels in earth-banks remove the soil and weaken the embankments.

Mammals like the elusive aardvark dig much larger holes than the insects using their short but powerful sharp shovel-shaped claws found on their fore and hind feet. The foraging actions of bushpig and warthog turn the top layer of soil over large areas. These pigs are using their snouts to root out tubers (enlarged structures in some plant species used to store nutrients). Many mammals practice geophagia which is the eating of soil to supplement the minerals that are deficient in their diets. By digging in sites where the soil contains traces of salts, the animals remove the soil in their guts and then defecate it out some distance away.

Warthog diggings

Tracks and pathways of herds of hard-hoofed animals like the buffalo and wildebeest wear the soil and grind the sand into a fine dust or powder. Much of the soil is moved by the movement of mammals but is also carried by the wind. The roots of trees and shrubs that are bulldozed over or pulled out of the ground by elephants break the soil when they stretch and snap.

Bioturbation has to happen. The turning of the sediments increases the productivity of those sediments by allowing the introduction of oxygen and water. The movement of the soil prevents the particles from becoming stuck together to form sedimentary rock.  If bioturbation did not occur, plant growth would be severely reduced, thus negatively impacting the overall productivity of the planet. We owe a great deal to all these industrious animals for the preservation of our planet.


About the Author:
Mark Gunn

Mark Gunn

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