In the soil, the main biological role, in purely quantitative terms, is played by micro-organisms and in fact, researches have been concentrated mainly on fungi and bacteria. However, if it is true that micro-organisms represent the living creatures that are most ‘present’ in the hypogeal environment, it is also true that they alone cannot explain all the ecological phenomena that take place in the soil. For example, knowledge regarding the relations that concern the higher levels of the soil food chains is still rather poor. The consequence of this delay is a still incomplete and inadequate knowledge of biopedological dynamics. Many phenomena that are, for example, connected to the nutrient cycle, in fact concern organisms that occupy very different positions in the ecological networks of the soil and are still not well known to date. In spite of the difficulties, some studies have clarified the identity of single species that normally live in the soil and this has lead to the possibility of formulating some hypotheses on their ecological functions.
Today, the role of particular biological groups can be studied in different ways and allow the elaboration of a sort of organizational chart of hypogeal ecology. A principle often used when classifying these biological groups takes their dimensions into account. As a result, five fundamental categories of hypogeal organisms have been identified:
- microflora: bacteria and fungi;
- microfauna: protozoa and nematodes;
- mesofauna: springtails, mites, enchytraeids (potworms) and others;
- macrofauna: isopods, molluscs, miriapods, earthworms and others;
- megafauna: amphibians, reptiles and mammals.
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