Diversity beneath our feet

Some people may think of soil as of an inorganic substance which needs to be fertilized and used for garden plants. In reality, soil a species-rich habitat, but its biological diversity is hidden underground and is therefore poorly known. Soil is an important component of terrestrial ecosystems and contains all groups of micro-organisms, fungi, bacteria, algae and viruses, as well as the microfauna such as protozoa, nematodes and soil mites. The diversity of soil organisms is more extensive than any other environment in the world when all living forms are considered. The total diversity is equal to or greater than any rainforest or coral reef.

 Free-living oribatid mites, known also as ‘beetle’ mites, are the world’s most numerous arthropods living in soil. They can be easily found in the soil of our back gardens. The ‘beetle’ mites can graze on fungi and algae, but their more important role is the breakdown of organic matter. During the decomposition of carrion, dung or plant detritus, organic fluids of various sorts and faecal matter from the resident insects seep into the earth beneath, where it is utilized by legions of different soil-dwelling creatures (soil mites and springtails in particular). Eventually, nothing remains except sweet, rich soil. The abundance and diversity of ‘beetle’ mites in a particular soil serve as good indicators of its ‘health’.

Atopochthonius sp.; © Ritva Penttinen (Turku University, Finland)

Adult ‘beetle’ mites usually have strong exoskeleton, not only hardened by sclerotization but often equipped with numerous projections produced by their skin. In this case they can be called ‘horny’ mites.

Cosmochthonius lanatus; © Ritva Penttinen (Turku University, Finland)

This interesting species of the ‘beetle’ mites (Cosmochthonius lanatus) was originally described from England, though most of its relatives live in the Mediterranean Area. As the previous species its body is highly sculptured and is normally covered with little particles of soil.

Euphthiracarus cribrarius (left) and Phthiracarus longulus (right); © Ritva Penttinen (Turku University, Finland)

Some oribatids, as those on these two photographs, are called ‘box mites’ because of adult body shape, which allows them to fold up in defense. Their hard, calcareous exoskeleton protects them from numerous soil predator-mites.