symbiosis

A lichen is a symbiotic association between an alga and a fungus - in other words, each lichen is not one organism but two. Lichens grow very slowly, usually as encrustations on rocks, walls, or wood. They are found throughout the world but are unable to survive where the atmosphere is polluted, so they are good indicators of clean air.

The tree and fungus both benefit from their symbiotic relationship.

Fig tree in Arusha National Part, Tanzania. Two olive trees, one either side of the path, have been enveloped by a ‘strangler’ fig (genus ficus), in a symbiotic relationship that will eventually kill the host trees.

Any close relationship between two organisms of different species, and one where both partners benefit from the association. A well-known example is the pollination relationship between insects and flowers, where the insects feed on nectar and carry pollen from one flower to another. This is sometimes known as mutualism.

Strictly speaking, symbiosis refers to continuous, intimate contact between mutually benefiting species, such as the fungus and alga in lichen.