Soil animals

Soil harbours a huge number of animal species (30% of arthropods live in soil), whether over their entire life or at least during larval stages. Soil offers protection against environmental hazards, such as excess temperature and moisture fluctuations, in particular in arid and cold environments, as well as against predation. Soil provisions food over the year, especially since omnivory seems the rule rather than the exception, and allows reproduction and egg deposition in a safe environment, even for those animals not currently living belowground. Many soil invertebrates, and also some soil vertebrates, are tightly adapted to a subterranean concealed environment, being smaller, blind, depigmented, legfree or with reduced legs, and reproducing asexually, with negative consequences on their colonization rate when the environment is changing at landscape scale. It has been argued that soil could have been a crucible for the evolution of invertebrate terrestrial faunas, as an intermediary step in the transition from aquatic to aerial life.

Soil fauna have been classified, according to increasing body size, in soil microfauna (20 μm to 200 μm), mesofauna (200 μm to 2 mm), macrofauna (2 mm to 2 cm) and megafauna (more than 2 cm). The size of soil animals determines their place along soil trophic networks (soil foodwebs), bigger species eating smaller species (predator-prey interactions) or modifying their environment (nested ecological niches). Among bigger species, soil engineers (e.g. earthworms, ants, termites, moles, gophers) play a prominent role in soil formation and vegetation development, giving them the rank of ecosystem engineers.

From a functional point of view soil animals are tightly interconnected with soil microorganisms (bacteria, archaea, fungi, algae). Soil microorganisms provide food to saprophagous and microbivorous species, and play a significant role in the digestion of recalcitrant compounds by saprophagous animals. In turn, soil animals, even the tiniest ones, create environments, e.g. digestive tracts, feces, cavities, favourable to soil microorganisms, allow their dispersal for those unable to move by their own means (e.g. non-motile bacteria), and regulate their populations.

The identification of soil animals, needing to be extracted (e.g. microarthropods, potworms, nematodes), expelled (earthworms), trapped (e.g. carabids) or searched by hand (e.g. termites, ants, millipedes, woodlice) before being observed under a dissecting, light microscope or electron microscope, has slowed down the development of soil zoology compared to the aboveground. To a few exceptions (e.g. vertebrates) the identification of soil animals was only done by specialists, using various published or unpublished keys and their own collections. From a few decades on molecular tools such as DNA barcoding helped field ecologists to achieve complete lists of species or OTUs. Such automated tools have been implemented in the study of nematodes, protozoa, and are still in development for other soil invertebrates such as earthworms and collembolans. They will be most useful for giving us reliable estimates of soil biodiversity, taking into account the huge amount of cryptic species which cannot be identified by morphological criteria.