Traditionally, speleolobiologists have not studied the soil fauna, but the soil is aphotic, and the fauna shows a number of shared morphological features. In addition, several shallow subterranean habitats are embedded in the soil (milieu souterrain superficiel, epikarst, and the hypotelminorheic). Organic carbon and nutrients decline with depth and abundance of animals declines as well. In the context of soils, milieu souterrain superficiel sites are macropores. In general, pore size imposes a constraint on the size and shape of soil animals. In the upper layers of the soil, organic carbon content is 10 – 100 times that of other shallow subterranean habitats, but organic content sharply declines with depth, approximately 50 percent per meter in one study. Both in terms of abundance and species numbers, soil faunas are very rich. Tens to hundreds of thousands of arthropods can be found in a cubic meter of soil, and tens to hundreds of species. The number of species and their abundance fall off with depth, but some species are specialized for deeper soils. Both nematodes and mites play a diverse and key role in soil food webs, and are herbivores, bacteriophages, omnivores, and predators. Overall, the global pattern of soil nematode diversity shows a maximum at mid-latitudes, a pattern also found in the terrestrial cave fauna. The typical morphology of soil organisms is miniaturization, body thinning (except for some mites), eye and pigment loss, and appendage shortening. They share eye and pigment loss with cave animals, and miniaturization with the interstitial aquatic fauna.
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