Lichens are composite, symbiotic organisms made up from members of as many as three kingdoms.

The dominant partner is a fungus. Fungi are incapable of making their own food. They usually provide for themselves as parasites or decomposers.

The lichen fungi (kingdom Fungi) cultivate partners that manufacture food by photosynthesis. Sometimes the partners are algae (kingdom Protista), other times cyanobacteria (kingdom Monera), formerly called blue-green algae. Some enterprising fungi exploit both at once.

Lichens are like little sponges that take up everything that comes their way, including air pollution. Most lichens are extremely vulnerable to air pollution. When lichens disappear, they give early warning of harmful conditions.

Lichens are generally regarded as low in protein but high in carbohydrates. Digestibility of lichens is considered by most researchers to be high, although tests done with animals not used to eating them
pointed to a very low digestibility. Lichens consist of roughly 90 percent of a caribou's diet in winter!

Apart from their food value, lichens may be important as a source of free water during periods of cold temperatures. The arboreal lichens in the genus Bryoria are dark-colored and therefore a good absorber of solar radiation. They probably provide liquid for the northern flying squirrel and other animals. Both birds and small mammals who use lichens for nest building undoubtedly benefit from the lichens' insulating properties.

Man has interfered with nature in so, so many ways. One new problem facing animals who eat lichens is the ingestion of radioactive isotopes. Lichens absorb and accumulate radioactive fallout far more than vascular plants and pass them along in the food chain. Studies have shown that  reindeer meat contains 280 times the 137Cs level of beef produced in the same general area. A study in Alaska found that lichens have concentrations of strontium-90 and cesium-137 of from 10 to 100 times that of most other plants from either temperate or northern regions. Caribou and reindeer have concentrations of strontium-90 in meat and bones that are about 25- 30 times that found in meat in the average U.S. diet. Cesium-137 levels are from 3-300 times that found in beef. Strontium-90 in bone in caribou-eating Alaskan Eskimos is being laid down at about four times the rate of that of the average U.S. citizen.