Hidden Biodiversity: Lichens in the city

In urban ecology, a lot of attention has been paid to groups such as birds and trees and how they adapt to life in the human hive, but an often-neglected group are the lichens. These easily overlooked species are developing in their own way in the human dominated urban habitat. All lichens are a symbiotic assembly of a fungal partner and an alga or cyanobacterium (at least). The fungal symbiont extracts nutrients from dust and moisture that deposit on it, while the algal photobiont converts atmospheric carbon dioxide into sugars through photosynthesis. Together they can survive on tree bark or even bare rock, allowing them to colonise new habitats. Because they accumulate resources from dust and air, they grow very slowly into mini-forests (See the image above).

Pollution

Because they draw most of their nutrients from dust, lichens are very sensitive to air pollution. From the industrial revolution onwards, human cities have produced excessive amounts of sulphur dioxide in the air. These emissions were the main cause of acid rain. This combination of air and rain pollution damaged lichens in urban areas throughout the world. Many urban and industrial centres eventually became ‘Lichen Deserts’, as the sulphur poisoning made life close to impossible for these delicate creatures (Warren et al., 2019). In recent decades, industrial regulation and deindustrialisation of cities has led to a reduction of sulphur pollution. Other sources of pollution remain, however: high volume traffic and high intensity agriculture produce high levels of nitroxides and ammonia (Lisowska, 2011; Warren et al., 2019).

In addition to the air pollution, lichens are also affected by drought stresses. Temperatures are often higher in urban environments than surrounding rural or forests areas. This increase in temperature is caused by human life and industry, exacerbated by a lack of shading trees, and is dubbed the ‘Urban Heat Island’ effect (Munzi et al., 2014; Warren et al., 2019). This heat reduces available moisture for lichen growth and survival.

Modern developments

With the reduction of sulphur pollution, some species have been able to eke out a survival in the previous ‘lichen deserts’, despite the nitrogen deposition and heat stress. It is suggested that the effects of nitrogen eutrophication on lichens can be reduced with moisture (Tretiach et al., 2012). In other words, drought resistant species would be more capable of dealing with higher nitrogen levels. The pressures thus serve as a filter, driving the development of a lichen community dominated by drought resistant, nitrogen loving species such as Physcia adscendens (Top picture), and Xanthoria parietina (right) (Lisowska, 2011; Munzi et al., 2014). This dynamic of pollution sensitivity and tolerance has made lichens useful indicators of air quality and the urban heat island effect. This is why many studies into lichen in the city focus on biomonitoring of air quality and temperature effects.

Urban communities

The unique urban environment has driven adaption and community structure in many species, including lichens. Under human made pressures and pollution, a community arises with species that are tolerant or resistant to the ravages of the human hive. This unique community differs from the surrounding rural and natural areas. As such, this may increase regional overall biodiversity, despite the low diversity within the cities (Warren et al., 2019).

Thus, urban pressures extend even to species few would notice: I took the pictures in this post less than 100 metres from my home, while people rushed past baffled by my attention to tree bark. But, when we pay attention, we may discover human influences reflected in communities as minute as lichens. We may discover the effects of our development in oft-ignored species occur right under our noses.

Shameless namedrop: I had the opportunity to work with researchers in this field, working on the project Hidden Biodiversity of the Naturalis biodiversity center in Leiden, the Netherlands, hence the title.

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References

Lisowska, M. (2011). Lichen recolonisation in an urban-industrial area of southern Poland as a result of air quality improvement. Environmental Monitoring and Assessment, 179(1), 177–190. https://doi.org/10.1007/s10661-010-1727-6

Munzi, S., Correia, O., Silva, P., Lopes, N., Freitas, C., Branquinho, C., & Pinho, P. (2014). Lichens as ecological indicators in urban areas: Beyond the effects of pollutants. Journal of Applied Ecology, 51(6), 1750–1757. https://doi.org/10.1111/1365-2664.12304

Tretiach, M., Pavanetto, S., Pittao, E., Sanità di Toppi, L., & Piccotto, M. (2012). Water availability modifies tolerance to photo-oxidative pollutants in transplants of the lichen Flavoparmelia caperata. Oecologia, 168(2), 589–599. https://doi.org/10.1007/s00442-011-2104-z

Warren, R. J., II, Casterline, S., Goodman, M., Kocher, M., Zaluski, R., & Battaglia, J. H. (2019). Long-term lichen trends in a rust belt region. Journal of Urban Ecology, 5(1), juz011. https://doi.org/10.1093/jue/juz011