Decomposition starts right after any living thing dies. Microbes break proteins down, and some of the gases released are chemical cues that signal opportunity to some animals. Scientists still argue about how much the tiniest decomposers matter (as of March 2024). One restoration study counted only animals bigger than 5 mm, because the role of mesoinvertebrates under 5 mm, such as mites and springtails, in decomposition is contentious.
Springtails are among the animals in that argument. They have six legs like insects, but scientists now place them in a separate group. Once counted as an order of insects, they are usually classed today as entognathous hexapods, a sister group of the insects.
Most springtails live in soil and eat fungi, algae, rotting plants and bacteria, though a few hunt tiny soil animals. In moist soil that is rich in rotting plant matter, they can number in the thousands per square foot.
Underground life is harder to study than life above ground. Forest Service scientists say soil biology research has lagged behind work on plants and animals above ground because of method limits and the complexity of below-ground food webs.
Forest Service researchers compared forests using the same kind of leaf. Invertebrate detritivores in tropical forests had a stronger influence on leaf decomposition than they did in cold temperate forests. A drought test across five forest biomes suggests that diverse decomposers may protect forests: where the community of soil animals was more complex, the slowdown from partly blocked rain was softened, and it was fully compensated when large-bodied invertebrates were present.
In a Puerto Rico wet forest, soil fauna accounted for up to 66 percent of the decomposition of tough, lignocellulose-rich Cecropia leaves. Early inventories at El Verde found that about half of the soil animals' weight was in the thin top layer of soil and the litter layer, which is where fallen leaves are broken down.
Millipedes' effect on leaf litter depends on context. One study found they affect decomposition both directly and indirectly, and how much depends on how many millipedes there are and on the quality of the leaves, such as their lignin content. Millipedes are slow and eat rotting plants, unlike centipedes, which are fast hunters of small bugs. Most body parts of a millipede have two pairs of legs, so it has many legs. Millipedes that wander indoors in dry weather usually die, because a house has too little moisture and rotting leaf food for them. Millipedes need damp air, and they hide under leaves, logs and stones.
Earthworms can steer a whole soil food web. A 2026 study of forests from taiga to rainforest found that in mixed broadleaved forests earthworms promote the dead-matter energy paths, overriding climate differences among forests. Tropical and temperate webs differ in other ways: tropical webs have more energy flow and rely more on living roots and predators, and less on bacteria, fungi and dead leaves, a shift from a brown web to a green web.
Invertebrates also speed the decay of fallen wood. A Forest Service review reports that invertebrate activity accelerates decomposition of woody debris, especially in lowland dry forest, and matters early in log decay in middle-elevation wet forests. Decay fungi in rotting wood break down cellulose into smaller pieces before termites bite, so termites eat decayed wood faster than sound wood. Termites eat the soft part of the wood grain first and leave the harder part.
A single termite species can carry up to 12 kinds of protozoan symbionts in its hindgut, along with hundreds of kinds of bacteria, so wood digestion is a team effort between the termite and many microbes. Termites pass gut contents between nest mates, called trophallaxis, and this passes the cellulose-digesting protozoans along to young termites. Termites also help recycle carbon. They thrive on tough materials such as wood and soil, and entomologists say they play important roles in global carbon recycling. In diet tests, about 500 genes in termites and their gut protists changed activity on woody, lignin-rich diets, and the lead scientist believes this may be important for breaking down lignin.
Dung beetles do not work alone. An eastern South Dakota study collected nearly 87,000 arthropods in 13 orders from dung pats. Dung beetles were only about 3 percent early in the season, yet were central to how the whole community used the dung.
A South Dakota test caged some dung pats to keep insects out and left others open. Caged pats broke down much more slowly. Pats that the whole community of insects could use broke down about 30 percent more than pats kept from them. In a lab study, dung with a dung beetle produced 42.2 percent less methane than dung alone, dung with fly larvae produced 77.8 percent less, and nitrous oxide rose 23.4 percent with the beetles, so dung beetles may help or hurt the climate.
A single teaspoon of healthy soil can hold more living things than there are people on Earth. A USDA scientist puts all the soil life in one acre at about two African elephants in weight. A 2026 review says soil animals strongly shape how soil organic matter forms and turns over, yet are absent from nearly all existing frameworks that describe the cycles of elements.
In experiments, University of Connecticut and Bayreuth researchers found that when burying beetles prepared their food, a scent attractant called methyl thiocyanate fell 20-fold, while a deterrent called dimethyl trisulfide rose. The UConn team concluded that burying beetles actively deceive rival carrion feeders by changing the odor of their food. This works by changing the microbes, not by sterilizing, which the team had once assumed.
In a restoration experiment, restorers reintroduced soil and litter invertebrates and fungi (rewilding) to new sites. Decomposition was significantly faster there than in control sites or old remnant sites, largely because of more invertebrate detritivores.
Researchers wrote that despite how much invertebrates and microbes contribute to decomposition, their communities had never been actively reintroduced to speed decomposition (as of March 2024). Picking which species to add is complex, since they depend on each other.
Words to know
- mesoinvertebrates: small spineless animals
- lignin: tough wood stuff
- detritivores: leftover eaters
- symbionts: partner microbes
- rewilding: bringing wildlife back
