Lichens must surely be one of the most unusual organisms on the planet for they are not really a true species but a community of two or three completely unrelated organisms living in the same skin. The organisms living together are symbiotic with one another – that is, none can live without the other.
The dominant partner is a fungus. Because fungi cannot make their own food, they usually live as decomposers or parasites. But in lichens, fungi maintain symbiotic relationships with other species that can make food. The symbiont partners make food by photosynthesis; they are usually an algae, a cyanobacteria, or both. Up to 18,000 species of fungi have been “lichenized” and about 40 genera of algae and cyanobacteria are found in lichen partnerships.
Lichen primarily reproduce vegetatively, so most lichens start life complete with all their parts. Any lichenized fungus born without its symbionts will die unless it can acquire them in some other way. Not just any symbiont will do – a lichenized fungus can live with only one species of algae or bacteria; the reverse is also true.
Fungal partners tend their symbionts by providing them with a protected environment, organizing them into patterns that permit optimum photosynthesis, and exchanging metabolites with them to improve their nutrition. They harvest their crops by secreting chemicals that cause the algae to “leak” nutrients to the surrounding tissue or grow minute tendrils that pierce the symbionts’ walls to extract food. The harvest is pretty good. Up to 80% of the photosynthesized carbohydrates go to the fungus. If cyanobacteria are present, the fungus extracts nitrogen and other biologically useful compounds it could obtain in no other way.
Lichens have been around for at least 600 million years. Their hardiness is legendary. They can live in the Arctic tundra and in the intense heat of Death Valley. They can dry out completely without being harmed, and are practically immune to ultraviolet radiation. In 2005 the European Space Agency carried lichen into outer space and left them outside the spacecraft for over two weeks. When returned to earth and watered, the lichen resumed photosynthesis as if nothing had ever happened.
Lichen colonies can be quite large and can quite literally change the appearance of the landscape. In this photo, lichen have almost completely masked the red color of the rocks with their own bluish-grey tint.
Both wildlife and people eat lichen. Mule deer have a taste for them; their foraging often creates a visible browse line on tree trunks. The Japanese use lichen in many traditional foods, and hikers sometimes eat them as survival food. Many lichen turn up in commercial products such as ointments, deodorants, tonics, and expectorants. One chemical which is derived from lichen, is the active ingredient in litmus paper. It changes color according to pH. Home swimming pool owners around the world use litmus paper, made with lichen extracts, to monitor the condition of their pool water.
Over 50 species of birds, including many hummingbirds, use lichen in their nest building. The female Anna’s Hummingbird covers the outside of her nest with lichen to camouflage its presence. A few insects also use lichen for camouflage. And, of course, moths and butterflies have evolved to mimic the colors and patterns of lichen to camouflage themselves. The military has noticed the patterns and colors and realized that the appearance of lichen changes very little through the seasons. A new, patented, camouflage pattern is now being used. It is claimed that this camouflage is exceptionally difficult to detect.
Lichen even play a significant role in geological processes. They produce acids which dissolve rock and help break it down. Some lichen attach themselves to rock by sinking root-like tendrils into the rock. These tendrils regularly grow as much as 8 inches beneath the surface. As they swell and contract, they create fissures in the stone. Water flows into these fissures, hastening the erosional process.
Lichens grow in roughly four shapes –
Folios lichens are flat, leaf-like organisms. You are most likely to notice them growing on tree limbs or trunks after a rain because the edges tend to curl up then and reveal the “leafy” structure.
Crustose lichens grow like crusts on the surface or between the crystals of volcanic rocks, or buried in tree bark. The brilliant red, yellow, and orange lichen you see growing in the Davis Mountains are mostly crustose lichens.
Fruticose lichens look like miniature shrubs. They often resemble masses of string hanging from trees or other plants.
Squamulose lichens are scaly and are usually comprised of numerous small rounded circles. Our “ground lichens” are of this type. On loose soil they sometimes appear to be growing on tiny mesas or pedestals. These elevated patches result when rain washes soil away everywhere except under the lichen.
Some lichen have such pronounced antibiotic properties that they are commercially valuable. Lichens from the genus Usnea are used in ointments and other products sold as aids to healing wounds. Lichens may be found in deodorants, laxatives, and expectorants. Ongoing research is beginning to indicate that some lichens may be useful against certain cancers and viral infections.
Lichens are one of the many “little things” that make exploring the Chihuahuan Desert interesting and rewarding.