Fungi play vital roles in the biosphere. They are essential to the recycling of nutrients in all terrestrial habitats because they are the dominant decomposers of the complex components of plant debris, such as cellulose and lignin. As opportunistic heterotrophs, they have evolved hyphae to penetrate solid substrates, and spores for long-range dispersal. They cause many diseases of plants and animals, but they also have established mutualistic symbioses with a wide range of organisms: cyanobacteria and green algae (in lichens), bryophytes, pteridophytes, gymnosperms and angiosperms (in mycorrhizae), and coleopteran, dipteran, homopteran, hymenopteran and isopteran insects. As parasites or pathogens they are well equipped to penetrate host organisms and to liberate spores that will effectively transmit them from one host to the next, and many species produce toxic compounds (mycotoxins). Some fungi affect human health in various ways.

Key concepts:

• Many fungi are opportunistic heterotrophs, disposers of, or recyclers of, organic substrates, especially those of plant origin.
  
• Parasitic fungi attack almost all known taxa of plants and animals.
  
• Fungi have established mutualistic symbioses with cyanobacteria and chlorophycota (green algae) to form lichens.
  
• Fungi often form symbiotic associations with the roots of plants (bryophytes, pteridophytes, gymnosperms and angiosperms) in mycorrhizae.
  
• Some fungi live as symbionts with coleoptera, diptera, homoptera and hymenoptera.
  
• Some fungi cause diseases (mycoses) in humans or excrete toxic compounds (mycotoxins).
  
• Fungal spores can cause severe allergies in humans.