Mushrooms are the visible fruiting bodies of fungi, and fungi are among the most essential organisms in any terrestrial ecosystem. They decompose dead material, feed nutrients to living plants, build soil structure, regulate animal and insect populations, and move staggering amounts of carbon underground. Without fungi, forests would be buried in dead wood, most plants would starve for nutrients, and entire food webs would collapse.
The mushroom you see above ground is only a small part of the story. Beneath the surface, a vast network of thread-like filaments called mycelium does the real work. These networks can stretch for miles through soil, connecting trees, breaking down rocks, and channeling nutrients between organisms. Here’s how that plays out across every major ecosystem function.
Breaking Down Dead Material
Fungi are the planet’s primary decomposers. When a tree falls, an animal dies, or leaves blanket a forest floor, fungi are usually the first organisms capable of dismantling that material at a molecular level. This matters because most of the tough structural compounds in wood and plant tissue, particularly cellulose and lignin, are nearly impossible for bacteria or animals to break down on their own.
Fungi handle this by releasing specialized enzymes directly into dead material. One group of enzymes attacks cellulose, snipping long sugar chains into smaller fragments that can be absorbed. Another group, collectively called ligninases, tackles lignin, the rigid compound that makes wood hard. These ligninases work through a chain reaction involving reactive molecules called free radicals, which essentially rip lignin apart at the chemical level. White-rot fungi are especially effective at this, producing multiple types of these enzymes simultaneously to break down the full complexity of wood.
The end products of all this enzymatic activity are simple sugars, minerals, and carbon compounds that wash back into the soil. This is nutrient cycling in action: the locked-up carbon, nitrogen, and phosphorus inside a dead tree get returned to a form that living plants can absorb through their roots. Without fungal decomposition, nutrients would stay trapped in dead tissue indefinitely, and ecosystems would slowly run out of the raw materials needed to support new growth.
Feeding Plants Through Underground Networks
More than 80% of all terrestrial plants form partnerships with mycorrhizal fungi. In these relationships, fungal threads weave into or around plant roots and extend far into the surrounding soil, acting as an extension of the plant’s root system. The fungi absorb phosphorus and nitrogen from soil particles too fine or too distant for roots to reach, then deliver those nutrients directly to the plant. In return, the plant feeds the fungus sugars produced through photosynthesis.
This exchange is not trivial. A 2023 study published in Current Biology estimated that terrestrial plants allocate roughly 13.12 gigatons of CO2 equivalent per year to mycorrhizal fungi, about 36% of current annual fossil fuel emissions. That carbon flows from plant roots into fungal mycelium underground, where some of it is stored in soil for months, years, or decades. Mycorrhizal networks are, in effect, one of the planet’s largest carbon sinks.
These fungal networks also connect multiple plants to one another. A mature tree linked to a mycorrhizal network can share nutrients with a struggling seedling nearby, channeling sugars and minerals through the fungal threads that connect their roots. This interconnection helps maintain plant diversity by giving smaller or shaded plants a better chance at survival.
Helping Plants Survive Stress
Some fungi live entirely inside plant tissues as endophytes, providing benefits that go well beyond nutrient delivery. These internal fungi help plants tolerate drought, extreme heat, cold, and salty soils through several mechanisms.
During drought, fungal hyphae penetrate deep into soil to access water that plant roots alone cannot reach. The fungi also influence plant hormones that regulate water loss, helping the plant close its pores more efficiently and retain moisture. In heat stress, endophytic fungi boost the plant’s production of protective compounds like antioxidants and stress-response proteins. Research on soybeans and sunflowers found that plants colonized by certain fungi showed significantly higher levels of these protective molecules at 40°C compared to uncolonized plants. In cold conditions, some fungi trigger the plant’s own cold-hardening genes, reducing cellular damage from freezing temperatures. In bananas exposed to 4°C, fungal colonization reduced harmful oxidative molecules in root tissue while increasing the plant’s stores of protective sugars.
For salt-stressed plants, endophytic fungi improve nutrient uptake, adjust the plant’s internal chemistry to balance out excess sodium, and neutralize damaging reactive oxygen species inside cells. The net result is that fungal-colonized plants consistently produce more biomass and yield more in harsh conditions than plants growing alone.
Building and Protecting Soil
Fungi don’t just live in soil. They actively construct it. Mycorrhizal fungi produce a glycoprotein called glomalin, a sticky, glue-like substance that coats soil particles and binds them together into stable clumps called aggregates. These aggregates are the structural foundation of healthy soil. They create pore spaces that allow water to infiltrate, air to circulate, and roots to grow. Soil with strong aggregate structure resists erosion, holds more moisture, and drains more efficiently.
Glomalin also plays a direct role in carbon storage. By binding organic matter and clay particles into aggregates, it physically protects carbon molecules from being broken down by microbes and released as CO2. This means fungal activity in soil simultaneously improves soil quality for plants and locks away carbon that would otherwise enter the atmosphere.
Feeding Wildlife
Mushrooms are a critical food source for a wide range of animals. Deer, squirrels, chipmunks, voles, and many marsupials eat fungi regularly, and for some species, mushrooms make up a major portion of their diet. Eurasian red squirrels can meet up to half their daily energy needs from fungi alone. Deer in several species rely heavily on mushrooms as a seasonal food source. The eastern bettong, a small Australian marsupial, shows measurably better body condition when fungi make up a larger share of its diet.
Nutritionally, mushrooms offer something unusual compared to other forest foods. Some species accumulate high levels of selenium, a trace mineral that many animals struggle to get enough of from plant material alone. Mushrooms also provide protein, carbohydrates, and water in a compact, accessible form. Animals that eat fungi and then travel through the forest spread fungal spores in their droppings, helping fungi colonize new areas and maintaining the underground networks that plants depend on. This creates a feedback loop: fungi feed animals, animals spread fungi, and fungi feed plants that sustain the broader ecosystem.
Controlling Insect Populations
Not all fungal relationships are cooperative. Parasitic fungi, particularly a group known as entomopathogenic fungi, infect and kill insects. These fungi are found in most terrestrial ecosystems, where they act as natural population regulators for arthropods. When an insect population grows too large, fungal infections spread more easily through dense groups, knocking numbers back down before any single species can dominate and destabilize the ecosystem.
These insect-killing fungi operate both above and below ground, targeting pests in soil as larvae and on plant surfaces as adults. Beyond direct pest control, many of these fungi also promote plant growth and help plants defend against other threats. Their role in keeping insect populations in check is one of the less visible but most important services fungi provide, preventing outbreaks that could strip forests of leaves or destroy plant communities.
Cleaning Up Pollution
The same powerful enzymes that let fungi break down lignin also enable them to degrade synthetic pollutants. This ability, called mycoremediation, allows fungi to break down heavy metals, polycyclic aromatic hydrocarbons (toxic compounds from burning fossil fuels), pesticides, herbicides, and even pharmaceutical waste. White-rot fungi and other lignin-degrading species are particularly effective because their enzymes are nonspecific: they attack chemical bonds found in many different toxic molecules, not just those in wood.
This means fungi can detoxify contaminated soil and water in places where chemical spills, agricultural runoff, or industrial waste have accumulated. In polluted sites, fungal networks gradually convert hazardous compounds into less harmful or inert forms, restoring soil health over time.
Driving Ecological Succession
When land is disturbed, whether by fire, volcanic eruption, or agricultural abandonment, fungi play a central role in rebuilding the ecosystem. In the earliest stages of recovery, fast-growing fungal species dominate, breaking down residual organic matter and beginning to cycle nutrients. Over time, the fungal community shifts. Research tracking abandoned farmland found that in recently abandoned fields, bacteria captured most of the carbon flowing from plant roots. But in long-term abandoned fields, fungi had taken over as the dominant processors of root-derived carbon.
As succession progresses, the fungal community transitions from fast-growing, sometimes pathogenic species to slower-growing, mutualistic species like mycorrhizal fungi. This shift has cascading effects: it changes how carbon moves through the soil food web, alters nutrient availability, and ultimately determines which plant species can establish and thrive. Fungi, in this sense, don’t just respond to ecological change. They drive it, shaping which plants succeed and how quickly a landscape recovers.