lichens as biomonitors
Lichens traditionally have the name of indicating that the environment is clean. This is a simplistic view however. Some lichens will only survive in a clean environment, while others flourish with certain pollutants.
For example, some species of the genus Xanthoria establish and grow abundantly in nitrogen rich areas, such as near farms or chemical factories, while species of the genus Usnea are sensitive to the amount of sulphur in the air and will only grow in areas where the air sulphur content is low.
Lichens, unlike most living organisms, are unable to ‘refuse’ entry to many chemicals into their bodies. This means that chemicals can freely invade them and interfere with their metabolic processes, often killing the lichen, but sometimes increasing their growth rate. Also, lichens are unable to excrete or secrete these chemicals and so they accumulate within the thallus. The lichen is therefore an excellent bioaccumulator. Lichenologists can monitor pollution levels in a habitat by looking at the species present and analysing specific species to see which toxins have accumulated.
An important study into the effect of air pollution on lichens was carried out by Hawksworth and Rose (1970) and Gilbert (1970). These lichenologists divided lichen sensitivity to air borne sulphur dioxide into 10 zones. This 10 zone system is still in use today, although it has been modified and developed since its creation.
Ten Point Hawksworth-Rose Sulphur Dioxide Pollution Scale
The scale depends on the lichen flora on tree bark; two lists were developed, one for acid bark and one for eutrophic bark. Highest levels of pollution are indicated by 0 and lowest levels by 10. With reference only to the acid bark scale the following species are good indicators. Many are profiled on Lichens.ie:
Most Polluted (0) to Least Polluted (10)
0 = No lichens
1 = Desmococcus viridis (not a lichen, but a green alga)
2 = Lecanora conizaeoides
3 = Lepraria incana
4 = Hypogymnia physodes / Parmelia sulcata / Parmelia saxatilis
5 = H. physodes / P. saxatilis / Calicium viride / R. farinacea / E. prunastri / Platismatia glauca
6 = Parmelia caperata / Graphis elegans / Pseudevernia furfuracea
7 = Parmelia caperata / Usnea subfloridana
8 = Parmelia perlata / Normandina pulchella
9 = Lobaria pulmonaria / Dimerella lutea
10= Sticta limbata / Usnea articulata
Thus lichens can be used to give a description of the state of an ecosystem without having to employ the use of expensive (and error prone) scientific equipment.
Common Lichen Air Pollution Indicators
|Lichens of Polluted Areas|
|Desmococcus viridis (algae)|
|Lichens of Moderate Pollution|
|Lichens of Slight Pollution|
|Lichens of Clean Air|