Winter 2003

Lichens: Symbiotic Mysteries

One of the greatest rewards of a nature hike is finding something you've never seen before. You may think that because you've traversed a favorite path or neighborhood street dozens of times, there is nothing new to discover. But if you are willing to think small, you will discover the world of lichens. Neither plants nor animals — they're part fungi and part algae — lichens fill an important role in Chicago Wilderness. Careful observers are most likely to notice them growing on trees, sidewalks, buildings, and rocks, but lichens can adapt to more unusual surfaces such as sandy soil, old shoes, and even glass.

"Whenever I teach classes, people are just amazed when I show them lichens on the sidewalk right under their feet," says Rich Hyerczyk, a machine designer and botanist who teaches classes for the Morton Arboretum. The lichens found throughout Chicago Wilderness range in color from yellow to olive green and black. Two of the most common species in the region, the battleship-gray Physcia millegrana and the candle flame-yellow Candelaria concolor, adorn many trees with familiar patches and smears.

A lichen is a compound organism created only when a particular fungus and a particular alga get together. Each part has a role: the fungus provides the bulk and shape of the organism, while the alga supplies food through photosynthesis. Science knows relatively little about lichen reproduction, according to Gerould Wilhelm of Conservation Design Forum. "No one can cultivate a lichen," he says. "Scientists don't even know exactly why a certain lichen will appear on a gravestone, for instance. It comes down to an exquisite relationship of minerals, rainfall, wind, sun exposure, and countless other factors that Western science can't measure." An established lichen may spread when natural forces such as wind, rain, or an animal break off a piece and carry it to a spot with such favorable conditions.

Lichens require a stable surface on which to grow. Although they can be the first organisms to colonize bare rock, they often succeed on less solid surfaces only after other organisms, such as mosses or trees, have taken hold. Over time, lichens release acidic chemicals that help to break down these surfaces into soil, creating new habitat for plants and animals. Lichens can proliferate overnight or expand only millimeters over hundreds of years, depending on conditions. If left undisturbed, these organisms can live more than one thousand years, growing outward from their center to form a ring.

Many living things depend on lichens for food, including spiders, caterpillars, moths, snails, and even some mammals. Insects often use lichens for shelter, while local birds use lichens in nest-building. The hummingbird and the blue-gray gnatcatcher line the outside of their nests with the foliose, or flaky and leaf-like, lichen Parmelia sulcata.

Humans have found uses for lichens as well. Native Americans have used lichens to make medicine and dyes, and now ecologists are using lichens as indicators. Because many lichens are sensitive to air pollution, they are effective natural monitors of air quality. Unfortunately, this sensitivity has led to their decline in urban and industrial areas with high levels of air pollution. Additionally, because each species does best in a specific set of conditions, ecologists can follow their population patterns to detect shifts and disturbances in natural communities. Another threat to lichen health is acid rain. According to Hyerczyk, acid rain collects inside a lichen and kills off the algae, causing the fungus to die as well.

Wilhelm estimates that researchers have identified at least 200 species of lichens across Chicago Wilderness. Only a few species survive deep in the forest where the thick tree canopy prevents the penetration of most sunlight — an essential ingredient in lichen photosynthesis. Some lichens respond well to controlled burns in the oak woods. The fires thin the woods enough to allow light, warmth, and rain to reach the surfaces that lichens call home.

The next time you decide to explore the neighborhood or the backyard, Hyerczyk recommends, "Get on your hands and knees and get a hand lens — you'll see a whole world down there!"

— Chris Hardman