The unique symbiotic partnership found in lichens, between fungi and algae/cyanobacteria, has allowed them to colonise many seemingly hostile environments where higher plants struggle to survive.
Bare rock surfaces often are entirely covered by lichens. On mountain tops [Figure 1: Fuscidea cyathoides] and rocky seashores [Figure 2: Seashore lichens] they are often abundant and diverse. Stone walls and graveyards are important lichen habitats too.
Many lichens grow on the bark of trees [Figure 3: epiphytic Parmelia]. Some are sensitive to pollution, particularly from atmospheric sulphur dioxide and ammonia [Figure 4: Oceanic lichens]. They have long been used as environmental indicators.
Other specialist lichen communities are found on peat bogs, sand dunes [Figure 5: Cladonia heath] , the periodically submerged margins of lakes and rivers [Figure 6: Fluvial zonation], and even on soil or rock contaminated with heavy metals.