This past weekend my daughter and I hung out on some rocks lining the bank of the West River in Brattleboro, Vermont. While she launched little twig boats into the swirling water, I took photographs of the local ecology. There were fuzzy fiddleheads, budding witch hazel, springtails, moss, and lichen.

One spot of lichen and moss on a rock caught my attention. It was beautiful in its own way: organic and rough. In the middle was a tuft of seductive entodon moss, Entodon seductrix, taking advantage of the minerals released by the lichen.

So often I have passed by lichen without much pause, perhaps because it is not as yielding to identification as plants. Being slow-growing, it is also not as dynamic.

I recently decided to learn more about lichen, so I purchased Lichens of North America. This massive tome––a model of scholarly excellence––has detailed information on the identification of North American lichen, but I am still in the early stages of learning to use it. Lichen biology is rather different from that of trees and plants.

What are lichen?

Lichen are fascinating. They are not singular organisms, but communities. They are symbiotic composites of a fungus and a green alga or cyanobacterium (blue-green alga). In other words, lichen fungi are able to team up with simple organisms that can photosynthesize food, something that the fungi cannot do on their own.

The fungus of a lichen provides structure, distributing minerals and water to its algae or cyanobacteria––the photosynthetic symbiont, or “photobiont” of the lichen. The photobiont produces food energy from sunlight that is shared with the fungus.

There is some debate over whether this is a mutualistic arrangement, since the algae and cyanobacteria can thrive independently of the fungi that harness their photosynthetic powers. The fungi, on the other hand, need their photobionts to survive. In the words of one lichenologist, Trevor Goward, lichen are “fungi that have discovered agriculture.”

There are some 14,000 species of lichen. One can spot them on rocks at all latitudes and altitudes, and on tree branches and soil. They are ubiquitous. Or at least they were. Some lichen are proving sensitive to pollution and are disappearing from polluted areas. Their sensitivity to pollution means that scientists can use them to efficiently measure air quality over a broad area.

Lichen die from pollution not just because these composite organisms are in a delicate balance, but because they are good at absorbing things from the environment. They produce acids that gradually erode rocks, freeing up minerals for use by living cells. Many obtain their minerals from rain, fog, and dust.

Lichen are like little prospectors living at the interface of the inorganic and the organic, the mineral and the living. By bioaccumulating inorganic compounds, they help to gradually build up soil. They are small but mighty levelers of mountains.


Comparing my photograph with those of Lichens of North America, I wondered if my lichen might be a shingled rock shield, Xanthoparmelia somloënsis. Internet image searches seem to support this, though online images are not always accurate, especially at the species level. 

The online image search did, however, turn up a surprise: A Xanthoparmelia species is marketed as a male aphrodisiac, a performance enhancer for treatment for erectile disfunction (ED). Companies in China and North America––and worldwide?––are marketing Xanthoparmelia scabrosa, jokingly referred to as “sexy footpath lichen” and “sexy pavement lichen,” because it contains a chemical that acts like Viagra. 

Large commerce sites such as Amazon and Alibaba are selling powdered lichen so that people can fulfill sexual desires. Is this a problem? 

There are several reasons why it is. I am not opposed to the use of natural supplements for helping with ED, but there are plenty of plant-based alternatives that are more sustainable. Lichen grow very slowly, just millimeters each year. They are a resource like old-growth trees that we should utilize only cautiously, especially if collection is coming from wild populations. 

Even if X. scabrosa is abundant, there is also the problem of bioaccumulation of toxic compounds. The superpower of lichen is to efficiently accumulate minerals. This can be helpful or harmful, depending on the context. 

Lichen can concentrate human-made pollutants and toxic heavy metals, poisoning the photobiont and killing the lichen. This is why they are so sensitive to pollution. Even if they do not poison themselves, animals like us humans can be harmed by ingesting these bioaccumulated toxins. (The authors of Lichens of North America spell this out clearly, pp. 39-40). 

As the common name “sexy pavement lichen” suggests, X. scabrosa grows well on old asphalt roads. The tar in these roads contains toxic compounds that can be picked up and accumulated by the lichen, poisoning those who ingest it.

Who can say that the companies harvesting the lichen are doing so in pristine environments and using sustainable methods? Plants such as horny goat weed (Epimedium grandiflorum) provide safer alternatives, until someone learns to farm X. scabrosa on toxin-free substrates.

Rock-Hard Relationships

Just as lichen operate at the interface of the mineral and the living, they seem to be powerful intermediaries between hard and soft processes. They are decomposers of rock, slowly liquifying hard minerals that can then be used by living organisms. And they are little pharmacies of chemicals, some of which can influence vasodilation in human bodies. 

What was once just a lichen on rock, growing almost imperceptibly in the quiet margins of species evolution, has become a fluid object of human desires. We scrape it, powder it, imbibe it, and temporarily experience some hardness, but to what end? Only to want more, until there is none left to scrape? Will some of our lichen species go the way of the dodo and heath hen? 

Human desires are a powerful force in the commodification of natural products. Whether human reason is an equal force I do not know, but lichen deserve more of our reasoned attention.

Like trees, the slow growth of lichen and their sensitivity to environmental change should remind us of relationships that exist on large time-scales. Lichen eat rocks so that we do not have to. They warn us that we are degrading our environment with pollution, so that we can clean up our act. 

What we need to be made firm, enduring, even rock-hard, are the long-term relationships that our lichen species have with the rest of the organic world. 


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