Lichens have been used as air quality bio-indicators because of their sensitivity towards pollutants including ammonia, heavy metals, and in particular, sulfur dioxide. This is due to their efficient absorption systems (they take in nutrients from water and the surrounding air) which results in the accumulation of sulfur dioxide when exposed to high levels of sulfur dioxide pollution. The use of lichens as an air quality indicator was first discovered when many lichens disappeared from trees in England after the Industrial Revolution, which also resulted in an increased occurrence of acid rain. Since then, many research has been done on the impacts of air pollution on lichens, and it has been established that urban lichen distributions are related to patterns of urban air pollution.
One such research by Coffery & Fahrig (2012) attempts to identify the relative importance of vehicle pollution (based on road density and bus traffic at each measurement site), moisture, and colonisation sources in determining lichen distribution within the city (Ottawa, Canada). Both lichen richness and extent of lichen cover was monitored. Results of the study (84 lichen sites were surveyed) indicated that vehicle pollution is most strongly linked to lichen cover (on trees) within 300m with insignificant variation of lichen richness in relation to vehicle pollution. This supported the use of the Lichen Diversity Index (LDI) as an indicator of urban vehicle pollution patterns. From an ecologist point of view, increasing the availability of lichen colonisation sources such as trees will be more efficient in promoting lichen diversity than controlling vehicle pollution.
Many of these study of lichens, however, are from Europe and North America. Related studies are lacking in other regions around the world. There is a study by Ng et al (2006) on lichens as bioindicators of heavy metal pollution in Singapore. The species Dirinaria picta (see below for picture!) is monitored because they can be found on the trunks of commonly found trees in Singapore. Results of these study have shown that there is potential for D. picta to be used extensively as a bioindicator for the monitoring of atmospheric metal contamination in the Southeast Asian region.
Picture of Dirinaria picta. Image retrieved 3 February 2015, from http://www.sharnoffphotos.com/lichensB/lic_img2/dirinaria_picta_2.jpg
With so many trees along Singapore’s streets, it will be interesting to see how monitoring of lichens can be used to compare different levels of pollution in Singapore. This will make a rather interesting and doable fieldwork project for us students to learn more about air pollution too.
Air Pollution Information System (2011) Lichen Diversity Value (LDV) (European Method). [Online] Retrieved from http://www.apis.ac.uk/node/1210 [Last accessed: 3 February 2015]
Coffey, H. M.P. and Fahrig, L. (2012). ‘Relative effects of vehicle pollution, moisture and colonization sources on urban lichens’. Journal of Applied Ecology. 49(6) pp. 1467-1474.
Ng, O. H, Tan, B.C. and Obbard, J. P (2006). ‘Lichens as Bioindicators of Heavy Metal Pollution in Singapore’. Environmental Monitoring and Assessment. 123(1-3), pp. 63-74.
Unwin, H. (2009) Using Lichens as Indicators of Air Quality. [Online] Retrieved from http://www.theecologist.org/campaigning/cleaner_air_water_land/363298/using_lichen_as_indicators_of_air_quality.html [Last accessed: 3 February 2015]