Fungi are heterotrophs and obtain nutrients from dead or living organisms. They provide a critical part of nature's continuous rebirth: fungi recycle dead organic matter into useful nutrients. Fungi include saprophytes (digesting dead organic matter), carnivores (capturing and digesting microscopic animals), parasites (digesting living organisms, to the detriment of those organisms), and symbionts. Many plants are dependent on the help of a fungus to get their own nutrients, living in a symbiotic relationship called a mycorrhizal association. About 80% of all plants grow in such a mutual association with fungi, the fungus providing as much benefit for the plant as the plant does for the fungus and many plants from orchids to pine trees, will not grow without a fungal partner.
Many fungi have great potential as biological controls for insects, weeds, eel-worms, and plant pathogens. They are increasingly used to reduce the use of toxic pesticides that cause environmental pollution. They are also of potential use in cleaning up pollution. In the presence of water, saprotrophs they can consume almost any carbonaceous substrate, including jet fuel ([Amorphotheca resinae]) and wall paint ([Aureobasidium pullulans]).
Fungi are consumers, like animals, though because of their habits often mistakenly grouped with plants. Examples are yeasts and mushrooms. The taxonomy of fungi is variable depending on the author. Phyla and classes are often synonymous in different hierarchical systems.