by Whit Gibbons

April 4, 2004

A fascinating feature of ecology is the endless interaction among plants, animals, and the inanimate world. In fact, so much life swirls around us that we take some life forms for granted and give them little thought. Among the under-appreciated organisms are the lichens.

What exactly is a lichen? Although a lichen appears to be a single organism, a lichen species actually represents a complex relationship between a fungus and algae. The species live together in a permanent symbiotic partnership in which each provides for the other, whereas neither could persist alone.

Both the fungal and algal species contribute to their joint survival in special and essential ways. Like higher green plants, algae convert sunlight into usable energy through photosynthesis, a process a fungus cannot perform. But the fungi are able to absorb vital nutrients from the surface they grow on. Hence, algae offer energy to the association; fungi supply minerals. Also, the fungal structure protects the algae from exposure. Ultimately, both species benefit, resulting in a single living organism.

Few people notice lichens, but they are easy to find. To check this out for myself, I walked outside and found lichens on almost every surface-bricks and concrete walls, tree bark, rocks, and the ground itself. Lichens are everywhere! Next time you see what appears to be a bare cliff face or rock wall, take a closer look. You will find lichens anchored into the rock itself. Many oaks and other trees have patches of greenish or gray lichens that may be spongy or flat and dry. As a rule, lichens cause no harm to the plants they attach themselves to.

Lichens are found in a variety of habitats throughout the world. The lichen known as reindeer moss is eaten by caribou in northern alpine and arctic regions. The lichen carpet in some regions of tundra provides the major source of food and nutrients for the big herbivores, which in turn are a primary food source for wolves. Odd to think that such an awesome predator ultimately depends on a combination of a fungus and algae.

More than 17,000 species of lichens have been described, most belonging to the fungus group that includes the edible morel mushrooms. What might seem to be a fragile life form may actually be one of the toughest organisms around. Lichens not only persist on tundra and mountain cliffs but also inhabit hot deserts and Antarctic sea water. But natural conditions do not have to be harsh for lichens to thrive. They can be found in old growth forests, wetlands, and prairies.

Ironically, the durable lichens are believed to be highly sensitive to some components of modern air pollution. They have been reported to be intolerant of toxic materials such as sulfur dioxide, heavy metals, and fluorine. Lichens have even been proposed for use as indicators of air pollution in areas with high pollution from industrial sources. One report noted a gradual decrease in lichen abundance from the outskirts of an industrial area to its center.

Lichens serve as a food source for many animals besides caribou, such as moths, slugs, and mites. The bright colors on the wings of tiger moths are thought to warn bird predators that, because the moth caterpillars eat lichens, the adult moths are distasteful. Hummingbirds, vireos, and other birds use lichens for nest material. Lichens are even used by humans, such as for dyes (including those used in Harris tweeds), antibiotic salves, and perfumes. Litmus paper, which can determine the acidity of a liquid, is made from a species of lichen. And a lichen species known as the alpine reindeer lichen is used to make tiny little trees for model railroad displays.

Although most of us pay no attention to lichens, they are an important and fascinating part of our natural world. They are on the trees, logs, and rocks in your yard and local parks. Look for the pale greenish or gray coating, sometimes in little patches, on tree trunks, large rock faces, or on the soil itself. It's gratifying to know that two completely different, little noted life forms can live in harmony to make another that is so persistent and pervasive.

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