Microfauna are a group of microscopic animals or animal-like organisms that are usually less than 0.1 mm in size and have characteristics of animals, such as a multicellular structure and heterotrophic (i.e., deriving nutrition from complex organic substances) feeding.
In addition to true animals, especially microscopic arthropods, this category also includes some microscopic eukaryotes that are heterotrophic, such as nematodes, rotifers, tardigrades and some protozoa. Other types of microorganisms which are not microfauna include microflora (namely bacteria, archaea, fungi and some algae) and viruses.
Reasons that microfauna are considered animals, in addition to their (1) multicellular structure and (2) heterotrophic feeding, include (3) exhibiting some form of mobility at some stage in their life cycle, (4) embryonic development similar to other animals, and (5) genetic and molecular similarities to other animals.
Microfauna are found in a wide range of environments, including the soil, waterbodies, air, indoors (e.g., dust mites) and inside various other organisms (such as lichens, mosses, insects, fish and even humans). Densities can vary significantly according to the specific environment, influenced by factors such as organic matter, moisture, sediment composition, nutrient availability and pollution levels.
In soil, microfauna densities vary according to the depth, decreasing exponentially due to the constraints of carbon, oxygen and soil compaction. Densities near the surface generally range from about ten thousand to 100 thousand individuals per gram of soil. The number of individual species of microfauna per gram can vary greatly, ranging from roughly 200 to 3,000.
This contrasts with bacteria, whose densities are typically much higher at about ten million to over ten billion cells per gram of soil and the number of species per gram ranging from about 2,000 to millions. Virus densities are generally somewhat correlated with bacterial abundance since many soil viruses infect bacteria, at roughly an order of magnitude or more greater than bacterial densities, often from 100 million to ten billion viral particles per gram of soil.
Microfauna play essential roles in maintaining soil health by driving nutrient cycling through grazing on bacteria and fungi, which accelerates microbial turnover and releases critical nutrients back into the soil, making them available to plants. They also physically fragment and decompose organic matter through burrowing and casting (the excretion of soil particles), thereby improving soil structure by increasing aeration and water infiltration, which in turn promotes root growth.