Most people rarely think about what lives below the ground. If they do, they might think about rabbits and earth worms – or, if they’re in northern Australia, they may think of ants and termites. But these creatures are only a small and unrepresentative proportion of the vast collection of animal life that lives in the soil beneath our feet. It is literally crawling with an incredible array of tiny creatures – many of which look distinctly freaky.
Next time you look at a paddock of sheep, think about the combined weight of all those animals (a sheep probably weighs more than you do). Soil ecologists have estimated that the living organisms below the ground would weigh more than the sheep grazing on the surface. There can be as much as ten tonnes of organisms per hectare of soil, and as many as a five million tiny animals under each square meter of ground surface.
Soil creatures come in a mindboggling array of different shapes and sizes. A lot of them look quite weird, but they are usually recognisable as some type of animal. The ones that make up most of the bulk are tiny – many of them are under one millimeter long – and can barely be seen with the naked eye. Although they play a very important role in the ecosystem, their small size and the fact that they live hidden within the soil means not much is known about their daily habits and how they interact with their surroundings.
Unlike sea creatures, for example, it is not possible for us to get into their habitat to watch what they do and how they live their lives. But we can find out a lot by digging up a chunk of soil and carefully pulling it apart, noting what we find and where. Soil animals mostly don’t make burrows – instead, they crawl through the small pores, cracks, and tunnels which occur naturally in almost all soils. Traps are also often used to capture and study soil fauna.
Soil animals between one tenth of a millimeter and two millimeters long are referred to as soil mesofauna and there is an astonishing range of different categories. Tardigrades are also known as “water bears” because of an apparent resemblance between them and the much larger creatures. Collembola come in a seemingly endless array of different shaped species. Nematodes and rotifers are both tiny wormlike animals. Acari, also known as mites, are related to spiders. Proturans are tiny insects, with no wings, antennas, or eyes. Diplurans are tiny insects with long antennas. Pseudoscorpions look like real scorpions except they have no tails and don’t sting – and, of course, they’re much smaller than their better known cousins. There are other groups too, but mostly they’re just more of the same: tiny and strange.
Tardigrades are found in both soil and water, and are known as “water bears” because their build and the way they walk makes them look a bit like tiny bears – with little claws on each of their eight feet. Tardigrades live all over the planet including dry valleys in Antarctica, deep ocean trenches, and glaciers in the Himalayas.
Tardigrades are particularly noted for their ability to survive extremely low temperatures (down to nearly -200° C) and to survive many years almost totally dehydrated. They are also very tolerant to radiation. In 2007, dehydrated tardigrades were sent into space on two separate missions, to find out how well they tolerated the hostile conditions of space. They survived the vacuum and some of them survived the extreme solar radiation too.
About 1200 species of tardigrades have been named so far. The soil dwelling species mostly eat plants or bacteria, but some prey on other soil creatures like rotifers and nematodes, and sometimes, also, other species of tardigrade. When environmental conditions get too tough, they can go into a form of suspended animation and wait until things improve.
Approximately eight thousand species of collembola have been named so far, some of them dating back as far as 400 million years in the fossil record. There are probably many more yet to be discovered. The species which live above the soil surface are often brightly coloured, but those which live entirely in the soil usually have no pigmentation and no eyes. They are often found in large quantities – up to a hundred thousand creatures in a square meter of soil. They are also known as “springtails” because the surface dwelling members of the group have a tail which they can use for jumping. The species which live permanently in the soil don’t jump – because there’s nowhere to jump to down there!
Collembolans are thought to mostly eat fungi growing on decomposing vegetable matter although some species eat live plants. One species of collembola (Proisotoma minuta) has been shown to aid cotton production by browsing on a fungus which damages the roots of the cotton plant. But another species, known as the “lucerne flea” (Sminthurus viridis), is considered a pest in Australia because of its predilection for some agricultural crops, notably grains and lucerne.
Nematodes are also known as “roundworms” or “eelworms”, however they are not related to earthworms. They are mostly less than a millimeter long and move with snake-like undulations. Some nematodes are plant or animal parasites. Mostly, though, they live on bacteria or the spores of fungi.
While nematodes eat fungi, fungi also eat nematodes. Predatory fungi grow organs to capture their prey. Some grow sticky nets rather like a spider’s web, others grow rings, rather like a lasso, which tighten on any nematode unlucky enough to try and wriggle through it. The fungus then penetrates its victim’s body and and rapidly consumes its insides. As well as the predatory types of fungus, some of the spores which the nematode eats can grow inside its body and consume it from the inside out.
Acari, or mites, are found in large numbers in the soils of all ecosystems and there may be hundreds of mites, of many different species, in a single handful of soil. They have eight legs and no head and many species have a rather spiky look about them. Some look a bit crab-like, while others look more like tiny spiders, but there are a wide range of body types and shapes. Some types of mite produce silk threads, which they use for various purposes – including making platforms for eggs and wrapping themselves for protection while they moult.
Some mites predate on other mesofauna, including nematodes and collembolans. One species of mite engages in a particularly bizarre form of predation – springtails are attracted to the rear end of these mites and tap them with their antennas. When this happens, the mite secretes a fluid which the springtail consumes. The fluid makes the creature run in a zigzag and then fall down paralysed – at which point, the mite can eat it.
Soil is a weird and wonderful world, inhabited by an amazing variety of freaky looking creatures – all going about their day to day business unseen by human eyes. The little we do know about what happens down there is quite spectacular – in a microscopic sort of way. New tools and techniques may one day allow us to watch the behaviour of these tiny animals more closely.