Hypogeal animals

An initial concept that must be understood before undertaking a digression on the fauna of the soil is that present day knowledge is not sufficient to list with certainty all the types of animals that inhabit this important natural environment. In fact, the data available nowadays regard animals that have already been studied in most of the ecosystems of the planet. But no one can certify that this data is actually complete with all the hypogeal species, because still today many areas of the planet have not been thoroughly explored as far as the biological component resident in the soil is concerned.
An incredible variety of animals
As we have seen in the previous special report dedicated to soil biology, a frequently used criterion to sort out the fauna that lives in the uppermost layers of soil is that of classifying it according to the dimensions of the various taxonomic groups. This criterion however is considered arbitrary and unsatisfactory because it is unable to offer information that is useful in understanding the relationships existent between one animal species and other cohabiting species. A much more useful way of proceeding would be that of considering not just the body dimensions but also the dietary habits and the role of each individual species in the hypogeal ecological network. A thorough knowledge of these aspects in fact would be very important in order to understand how the soil works where the word soil is intended as a natural system provided with its own specific ecological identity.
Generally speaking, the soil fauna belongs to the taxonomic groups of the Protozoa, Nematodes, Annelids, Molluscs, Arthropods and Vertebrates. Naturally this information reflects only a part of the biological knowledge that  is necessary to be able to explain the ecological complexity of soil. To have a clearer picture other information is necessary, such as that regarding the abundance of a single species, the different distribution (in space and time) and the nature of the  ecological relationships.
In this special report we will try to offer the basic knowledge regarding zoological groups in the soil,  knowing well, however, that among the many species that live just a few centimetres below the surface of the soil, a network of relationships is established of which there is still a lot to discover.

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