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In tropical rain forests, vascular plants make up most of the biomass. Animals eat the plants, obtaining food for growth and reproduction, but they feed selectively: They consume leaves, fruits, and seeds, but largely avoid a much more abundant tissue—wood. In fact, most leaves also escape grazing and eventually fall, dead, onto the forest floor. Animals, too, may be eaten by predators, but some die of other causes and contribute their remains to the soil. Given this constant rain of biological materials, we might expect wood, leaves, and animal carcasses to accumulate in great piles, but a walk through the rain forest reveals that the forest floor is remarkably clean. What happened to all the dead plant and animal tissues?

On land, the organisms principally responsible for the decomposition of plant and animal tissues are fungi, one of the most abundant and diverse groups of eukaryotic organisms. Many of us have seen mushrooms and toadstools in a meadow or woodland, but most fungal biomass and most of the metabolic work that fungi do occurs within the soil, out of sight. Fungi play a critical role in cycling carbon because of their ability to locate and break down the complex molecules and bulky tissues in plant and animal bodies. Moreover, because they form intimate relationships with living plants and animals, fungi affect the growth and reproduction of many other organisms. For example, fungi associated with plant roots dramatically increase plant productivity by enhancing the uptake of mineral nutrients from the soil (Chapter 29). Other fungi are major agricultural pests (Chapter 32). Fungi enable us to turn wheat into bread, barley into beer, and milk into cheese, but still other fungi cause athlete’s foot, yeast infections, and, especially in patients with compromised immune systems, overwhelming infections that can lead to death.