Posted by: David Green | March 24, 2010

Indicator Species

Lichens are mutualistic associations of a fungus and an alga or cyanobacterium. They are sensitive to sulphur dioxide in the air which rapidly accumulates in their tissues. Lichens are commonly used as air quality indicators since some species are more pollution tolerant than others. The most sensitive lichens are shrubby and leafy whereas the most tolerant lichens are crustose. In city centres, lichens may be entirely absent. If the air is clean, shrubby, hairy and leafy lichens colonise every available surface.

Moss and lichen colonising a piece of ten year old Shap Granite in a Warrington garden. Behind and to the right is a piece of luxullianite (the rock used for the Duke of Wellington's tomb) which was also collected ten years ago, but is relatively untouched.

City smogs in the mid-twentieth century resulted in legislation that gradually improved air quality in Britain. As a result, lichens have recolonised towns and cities… which brings me to the subject of this post. Ten years ago the museum was redeveloping its galleries and there were many different ideas for new displays. Specimens needed to be collected and some were suitable whereas others were not. A few of the least suitable lumps were consigned to the garden rockery. One such was a specimen of ‘type three’ granite from Shap in Cumbria, collected for an igneous petrology display that never happened. Type three granite is late stage and contains lots of tiny cavities. Over the years, its surface weathered and the cavities accumulated scraps of detritus. Eventually plants started to colonise. Ten years on, very little bare rock is visible. It is encusted in moss, and there are patches of the attractive shrubby lichen Cladonia. Air quality is clearly getting better in towns and cities…

Close view of newly grown Cladonia cups, the most exciting thing I have seen in the garden this year!

There is another sense in which lichens are an indicator species. They are used in the manufacture of litmus, a water-soluble mixture of different dyes which is absorbed onto filter paper to produce a pH test. It seems ironic that lichens, which are so sensitive to sulphur dioxide, produce one of the oldest tests for acidity: sulphur dioxide, after all, is the major contributor to acid rain.

David Green

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