by Whit Gibbons

October 14, 2007

I recently found a life form that keeps my grandson searching through the backyard or woods, no matter where we are. It doesn't bite, sting, or pinch, so I don't worry when he finds some and brings them back. They come in many colors, although green and gray seem to be the most common. Plus, it's kind of cool to hear a four-year-old say "look at the lichens."

What exactly is a lichen? Although appearing to be a single organism, a lichen 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. 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, my grandson and 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, natural 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, including moths, slugs, and mites. Hummingbirds, vireos, and other birds use lichens for nest material. Lichens are even used by humans, such as for dyes, antibiotic salves, and perfumes. Litmus paper, which can determine the acidity of a liquid, is made from a species of lichen. And the alpine reindeer lichen is used to make tiny little trees for model railroad displays.

Most of us pay no attention to lichens, yet they are an important and fascinating part of the living world in your yard, local parks, and woods. 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. That's one of the things I'm trying to teach my grandson.

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