Fungi play an essential role in ecosystems because they are decomposers and participate in the cycling of nutrients by breaking down organic and inorganic materials to simple molecules.
Fungi join bacteria as the primary decomposers in different ecosystems. They feed on the organic remains of dead plants. They release special enzymes that break down lignin, a structurally complex substance in wood.
Because of their ability to break down wood, fungi are considered the major wood decomposers in forests. To absorb organic matter, fungi release acids to melt organic matter, and then they suck the acid back together with the melted organic matter. Decomposing organic matter is considered by ecologists as the major ecological role of fungi.
Fungi often interact with other organisms, forming beneficial or mutualistic associations. For example most terrestrial plants form symbiotic relationships with fungi. The roots of the plant connect with the underground parts of the fungus forming mycorrhizae.
Through mycorrhizae, the fungus and plant exchange nutrients and water, greatly aiding the survival of both species Alternatively, lichens are an association between a fungus and its photosynthetic partner (usually an alga).
For more info about the role of fungi in the ecosystem, see links below:
courses.lumenlearning.com
www.countrysideinfo.co.uk
www.brighthub.com