Sulfur dioxide and acid deposition are a major cause of Lichen diversity loss. Even in non-industrial areas of the UK acid tolerant lichens are becoming more dominant than those which are not.
Lichens are good indicators of air quality. They rely on dissolved solids in rainwater for their nutrients, so any increase in acidity has a direct effect.
A lichen is a symbiotic partnership between a fungus, usually an ascomycote, and a chlorophyte or a cyanobacterium. Technically lichens are called a 'form phylum'.
Lichens are nature's pioneers, colonising some of the most barren and inhospitable parts of the world. From there they slowly begin the process of creating a foundation for succession by other species. Lichens are among the most fascinating organisms on this planet. Their very structure is unique: a symbiosis of two organisms, a fungus and algae, so complete that they behave and look like an entirely new being. A lichen can literally eat stones, survive severe cold, and remain dormant for long periods without harm.
Lichens can be divided into three basic forms: crustose, or crust- like; foliose or leaf-like; and fruticoseor stalked. Crustose lichens are flaky or crust-like. They can be found covering rocks, soil, bark, etc. often forming brilliantly coloured streaks. Foliose (leaf-like) lichens can be papery thin or, in more advanced forms, netted branch-like. Branched foliose lichens have a distinct top and bottom surface, thus differentiating them from most fruticose lichens. Fruticose lichens are the most highly developed lichens. Their branches are much closer in form to "true" branches although, unlike most plants, the lichen branch has no specialized vascular system for transporting fluids.