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Microfauna (Ancient Greek mikros "small" + New Latin fauna "animal") refers to microscopic organisms that exhibit animal-like qualities. Microfauna are represented in the animal kingdom (e.g., nematodes, small arthropods) and the protist kingdom (i.e., protozoans). This is in contrast to microflora which, together with microfauna, make up the microzoa.
Microfauna are present in every habitat on Earth. They fill essential roles as decomposers and food sources for lower trophic levels, and are necessary to drive processes within larger organisms.
One particular example of the role of microfauna can be seen in soil, where they are important in the cycling of nutrients in ecosystems. Soil microfauna are capable of digesting just about any organic substance, and some inorganic substances (such as TNT and synthetic rubber). These organisms are often essential links in the food chain between primary producers and larger species. For example, zooplankton are widespread microscopic animals and protists which feed on algae and detritus in the ocean. They include foraminifora and krill, which are the primary food source for even animals such as whales. Microfauna also aid in digestion and other processes in larger organisms.
The microfauna are the least understood of soil life, due to their small size and great diversity. Many microfauna are members of the so-called cryptozoa, animals that remain undescribed by science. Out of the estimated 10-20 million animal species in the world, only 1.8 million have been given scientific names, and many of the remaining millions are likely microfauna, much of it from the tropics.