Lichens are remarkable organisms and consist of a symbiotic association between two or more organisms namely fungi and one or more photosynthetic partners (green alga and/or cyanobacteria). This relationship has developed on many occasions and a wide range of fungi have independently evolved to become lichens. Only recently has it become recognised that some fungi, once lichenised now have given up the relationship eg. Penicillium.
The photosynthetic partners in lichens provide essential nutrients to the lichen by photosynthesis and the fungus forms the body of the organism known as the thallus and acquires and redistributes minerals whilst also synthesising unique anti-bacterial, fungicidal and UV screening substances to protect the partnership. When cyanobacteria are involved in the partnership they in addition fix atmospheric nitrogen.
There are many forms of lichen, encrusting, trailing, lobed, leaf like etc and they display a variety of colours including yellow, red, orange and grey. In terms of habitat, lichens can be found growing in many environments including woodland, mountains, quarries, sand dunes and urban areas, in fact wherever there are trees, rocks, walls, gravestones and street pavements. Some lichens have even adapted to grow on surfaces so toxic from the concentration of heavy metals no other plants can grow.
Lichens typically grow slowly and some are excellent environmental indicators, often sensitive to changes in air-quality. Air and water pollution continues to threaten lichens and has restricted the range of once common species.
Lichens are also useful to their neighbours, recycling nutrients used by other plants and providing homes for spiders, mites, lice and insects. Though little-studied it is now recognised that a large number of fungi are dependent on lichens-possibly as many as, if not more species, than the lichens on which they live. Humans extract the most incredible range of wool dyes from lichens and eat some of the edible species, while drug companies use lichens to make antibiotics or sunscreen cream. Cyanobacteria-containing lichens fix atmospheric nitrogen much as the bacteria in the root nodules of many members of the pea family do. In woodlands where they are abundant they contribute significantly to the nitrogen cycles of the wood.
According to the Lichen Red Data List for Wales Report (2010), Wales supported approximately 1250 species of lichen representing 68% of the total British lichen flora. Using widely accepted international criteria and data collated by the British Lichen Society a Welsh regional threat status is offered for all Welsh lichens and allied fungi for the first time. The threat status of Welsh species is compared with the already published British Red Data List for Lichens. Of the 1290 species of lichens and fungi traditionally studied by lichenologists in Wales 22 are probably extinct (2% of the total) and 204 more (16%) are threatened with extinction. Of this latter total 28 (2% of the overall total) are Critically Endangered, 24 (2%) are Endangered and 156 (12%) are Vulnerable. A further 131 taxa (10%) are Near Threatened, whilst 152 taxa (12%) lack sufficient information for a threat category to be assigned to them. Of Welsh lichens and fungi studied by lichenologists 38% of the taxa therefore require some action either to better understand their true status or reverse known ongoing declines.