The Ecology of Fungi

How do fungi affect homeostasis in other organisms and the environment?

Fungi play an essential role in maintaining equilibrium in nearly every ecosystem. But there are some species that cause disease in plants and animals.

Decomposition Many fungi feed by releasing digestive enzymes that break down leaves, fruit, and other organic material into simple molecules. These molecules then diffuse into the fungus. The mycelia of fungi produce digestive enzymes that speed the breakdown of wastes and dead organisms. Many organisms, especially plants, remove important trace elements and nutrients from the soil. If these materials were not returned, the soil would quickly be depleted, and Earth would become barren and lifeless. Fungi are champions of decomposition. Many species help ecosystems maintain homeostasis by breaking down dead organisms and recycling essential elements and nutrients.

FIGURE 21–20 Parasitic Fungi Corn smut infests the kernels of a corn plant, reducing the farmer's crop yield (left). A moth falls victim to the Cordyceps fungus (right).

Parasitism As useful as many fungi are, others can infect plants and animals. Parasitic fungi can cause serious diseases in plants and animals by disrupting homeostasis.

▸ Plant Diseases A number of parasitic fungi cause diseases that threaten food crops. Corn smut, for example, destroys corn kernels and wheat rust affects one of the most important crops grown in North America. Some mildews, which infect a wide variety of plants, are also fungi. Fungal diseases are responsible for the loss of approximately 15 percent of the crops grown in temperate regions of the world and even more of the crops grown in tropical areas.

FIGURE 21–19 Champions of Decomposition The mycelia of these mushrooms have released enzymes that are breaking down the wood tissues of the decaying tree trunk.

▸ Animal Diseases Fungal diseases also affect insects, frogs, and mammals. One deadly example is caused by a fungus in the genus Cordyceps. This fungus infects grasshoppers in rain forests in Costa Rica. Microscopic spores become lodged in the grasshopper, where they germinate and produce enzymes that slowly penetrate the insect's tough external skeleton. The spores multiply in the insect's body, digesting all its cells and tissues until the insect dies. To complete the process of digestion, hyphae develop, cloaking the decaying exoskeleton in a web of fungal material. Reproductive structures, which will produce more spores and spread the infection, then emerge from the grasshopper's remains.

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