Lichens are good indicators of pollution because they absorb substances present in the air and are sensitive to the quality of the environment. Their presence or absence, as well as their appearance, allow scientists to assess the level of air pollution.
Lichens are highly sensitive to pollutants due to their very simple structure: they lack roots, protective cuticles, and any complex tissues like those of typical plants. This makes them particularly vulnerable, as they absorb directly through their entire surface whatever is in the air or rain: nutrients, but also toxic substances. Furthermore, their very slow growth rate forces them to accumulate these pollutants over long periods. Therefore, when there is a lot of air pollution, it disrupts their ability to carry out photosynthesis, limits their growth, or even outright kills them. They then become very clear indicators for measuring the quality of an environment.
Lichens are simple; they have neither roots nor protective cuticles, so they absorb directly from the air whatever falls on them. Their surface acts a bit like a sponge, easily capturing atmospheric pollutants, including heavy metals (such as lead, mercury, or cadmium) as well as gases (like sulfur dioxide). This direct absorption makes them particularly sensitive to atmospheric changes. Thus, when the air becomes too polluted, lichens quickly accumulate these toxic substances, directly harming their growth or even causing their disappearance in certain areas. By analyzing lichens, we can see precisely which pollutants are present in the air and in what quantities: this quickly gives a good idea of the quality of the air we breathe.
Lichens do not all have the same tolerance to pollution: some are very sensitive to pollutants, while others are more resilient. Generally, three main categories are observed. First, crustose lichens, which are flat and embedded in surfaces (rocks, walls), are relatively resistant to pollution. Next are foliose lichens, which resemble small flat leaves: they are less tolerant to pollution and disappear as soon as the air quality declines slightly. Finally, fruticose lichens, which appear as small tufts or shrub-like forms, are extremely fragile against pollutants and clearly indicate clean air when present. By simply taking a look at the lichens present in an area, one can quickly get an idea of air quality.
To use lichens as bioindicators, mainly observe their abundance, diversity, and types present at a site. If you notice that fruticose or foliose lichens (those resembling small leaves or tufts) become rare or nonexistent in favor of crustose lichens (flat, encrusted on rocks or trees), it is often not a good sign: it generally indicates an increase in air pollution. We can also directly measure the amount of certain pollutants accumulated in lichens in the laboratory; it's more precise but obviously more complicated. Typically, we compare the state of lichens in potentially polluted areas with those in protected or natural places to have a clear reference on the state of the surrounding air quality.
In several industrial regions of Europe (such as northern France or the Ruhr in Germany), sensitive lichens like those of the genus Usnea have virtually disappeared due to high levels of sulfur dioxide (SO₂). In contrast, studies in Scandinavia and the mountains of Canada show that these lichens are gradually returning once pollutants are better controlled. In the United Kingdom, the Hawksworth and Rose scale has been used since the 1970s to quickly characterize air pollution based on the presence or absence of certain indicator species. More recently in the United States, research conducted in national parks clearly shows that sensitive lichens are disappearing near major cities, while more resistant species (such as Xanthoria) continue to thrive. These cases unambiguously illustrate how the health and type of lichens present in an area can reflect the actual air quality we breathe.
Lichens do not have roots. They absorb almost all of their nutrients and pollutants directly from the atmosphere, making them very accurate indicators of ambient air quality.
Some lichens can absorb radioactive pollutants like cesium, which has allowed for monitoring the environmental impact after the Chernobyl disaster in 1986.
The oldest lichen ever discovered is estimated to be around 8,600 years old! Its exceptional longevity allows it to serve as a witness to the evolution of the environment over long periods.
Due to their extreme sensitivity, some lichens completely disappear from urban areas, making their presence or absence a valuable tool for quickly assessing urban pollution.
Lichens mainly serve as qualitative indicators that reflect the overall state of air quality in a given environment. They do not replace precise chemical analyses but are an excellent tool for initial environmental diagnostics.
Lichens are very sensitive to atmospheric pollutants such as sulfur dioxide and nitrogen oxides, which are emitted abundantly in urban environments. This explains why there is often a significant decline in their diversity and abundance in cities compared to rural areas.
A healthy environment is typically characterized by a high diversity of lichens, including sensitive species such as fruticose or foliose lichens. In contrast, a polluted environment usually only shows resistant crustose lichens, or even no visible lichens at all.
Lichens primarily detect pollutants related to air quality, including sulfur dioxide (SO₂), nitrogen oxides (NOₓ), ozone (O₃), as well as certain heavy metals such as lead, mercury, and cadmium.
No, lichens are generally not toxic or dangerous to humans. On the contrary, they indicate the quality of the surrounding air, thus indirectly helping to protect human health by signaling polluted areas.
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