Wherever there is moisture, moderate temperatures, and a supply of organic food there are fungi. Since they digest their food outside of their bodies, they literally live within their food supplies. When the area around them is depleted, they grow into a new supply. They occur worldwide, although there may be more taxa in the tropics—an assertion that is difficult to support because while there are an estimated 1.5 million species of fungi, less than 10 percent of them have been described. About 500 species are marine; the rest are terrestrial with several thousand described symbionts and plant and animal pathogens.
Fungi usually are the primary decomposers in their natural habitats and are capable of digesting a wide array of organic materials—including, unfortunately, some substances of economic importance to humans. Most are saprobes, but some, like their animal relatives, attack living prey, a notorious example being the fungus that sets hyphal traps, ensnares and then digests nematodes. Many fungi are parasitic and the major pathogens of many crop plants such as corn and wheat.