Are our Cities Polluted? Just ask Lichens

 Urban environments are everywhere. It is estimated that almost three percent of the earth’s land is urbanized, and that number is predicted to rise significantly in the near future. Gao et al., 2020 predict that by 2100, the total area that is urbanized will rise by a factor of 1.9-5.9. This is incredibly significant for the Earth’s species, because urban environments are different from natural environments [citation needed]. In a paper by Coffee & Fahrig (2012), the authors attempt to separate the impact of three of these changes brought on by urbanization—loss of habitat (which they measure using tree cover), vehicle pollution (measured by the number of roads in the area and bus traffic [Fig. 1]), and moisture levels in the outer bark of trees—on Lichens in the city of Ottawa. Lichens have been proposed as an indicator of air pollution in cities because they receive their nutrients from the air, so harmful pollutants are easily taken up by the lichen and quickly reach toxic levels. However, Coffee & Fahrig wanted to understand whether the effect of air pollution was all it took to cause poor lichen abundance and diversity, or whether these other effects of urbanization were also connected. 

Fig. 1: Mapping air pollution in the city of London. Notice how pollution maps really strongly with major roads, showing the importance of vehicle pollution. From https://gsttcharity-uk.shorthandstories.com/thecausesofairpollutionininnercities/index.html

To separate the impact of each of the three effects of urbanization is challenging, as it really isn’t possible to experimentally “block” the effects of two effects in order to isolate one effect of the urban landscape. Instead, the authors used a “mensurative” experiment, where data are collected in situ and they separate the effects using statistical techniques when analyzing the data. By choosing sites carefully, they can tease apart the impact of each predictor on their response variables, species richness and cover (how much of each tree had lichen).

Their experiment showed that air pollution did have a significant effect on the cover of lichens, but not on species richness (Fig. 2). They found that species richness was positively correlated with colonization sources, especially when looking at large spatial scales. Thus, they conclude that lichen cover could serve a purpose as an indicator of air pollution in cities. However, they point out that if managers in urban spaces are attempting to increase species richness of lichens, they would do better to increase tree cover than simply attempting to lower vehicle emissions.

Fig. 2: Correlations between species richness and cover at different spatial scales with (a) vehicle pollution (b) colonization sources. Adapted from Coffee & Fahrig (2012)

This experiment is very relevant to my group’s urban ecology project in Grinnell. In our project, we want to measure how different factors (distance to roads/parking lots and distance to the nearest tree), affect several measures of lichen populations growing on lampposts (lichen abundance [measured using presence/absence and percent cover], species richness, and the largest lichen in each of five sections [a potential correlate of population longevity/health]). To do this, we have sampled lichen on each lamppost contained within South campus. One thing that will be interesting to see is whether the effects that these authors have shown will be present on a much finer scale within South campus. Additionally, we hope to explore colonization sources more closely by analyzing the direction of the nearest tree and determining whether it correlates with areas of higher lichen density on lampposts. If that were the case, we could have a much better understanding of how lichen colonize the lampposts, which could help with lichen management.

References:

Coffey, H. M., & Fahrig, L. (2012). Relative effects of vehicle pollution, moisture and colonization sources on urban lichens. Journal of Applied Ecology, 49(6), 1467-1474.

Gao, J., & O’Neill, B. C. (2020). Mapping global urban land for the 21st century with data-driven simulations and Shared Socioeconomic Pathways. Nature communications, 11(1), 1-12.