We are used to seeing lichens in a variety of forms and habitats, ranging from long stringy species living on branches of trees where they may resemble moss, to small crusts on rocks, tree bark, pavement or roofs. Lichens lack a root system, so when they grow on plants, they are not parasites — they use the plant only as a surface on which to grow.
Lichens are a very old and successful group of organisms. They have been around since the time of the dinosaurs and can be found from sea level to the alpine; from the Arctic to the Tropics and even living in rocks (among the grains of rocks) or just blowing around and living where the wind takes them. They may be common, but let’s admit it, they don’t get much attention — not like plants or birds or mammals.
So let’s start with a definition of what a lichen is? It’s a fungus and an alga living together and liken/lichen it. This is a common joke among natural history interpreters when talking about lichens. As a joke, this may leave something to be desired; however, it does encode information. The discovery that lichens were a combination of two organisms goes back to the 1860s, and I suspect the joke is of that vintage as well.
The fungus provides the structure, but fungi can’t produce their own food. The algae or cyanobacteria can photosynthesize, but it needs a home. Living together is, therefore, mutually beneficial to both partners.
The joke and the definition of a lichen, however, now need to be updated. This year saw the discovery of the most major change in the understanding of lichens since that discovery over 150 years ago. Turns out, there is more going on than what meets the eye (or microscope). New research has revealed a third partner — a yeast. The presence of yeast as part of the makeup of the lichen may help explain some of the mysteries in lichenology, such as why lichens that look very different from each other have been found to be genetically identical.
By convention, lichens are given a scientific name, but the two species in the lichen partnership challenges the concept of species — a system of organization based on ancestry. This discovery only complicates that. Lichens are named for the fungus they contain. The alga in the partnership has its own name, but that is not reflected in the name of the lichen. The same fungus may be able to grow with different alga (and maybe yeasts), resulting in growth forms and biochemistry quite different from other partnerships.
Historically, these differences resulted in the lichens being given different names, but modern DNA techniques can reveal that the fungus is the same, and therefore under the present classification system, they should have the same name. My point simply is that the previously clean cut species concept is not so clean cut. While immensely helpful as an organizational tool, it is a human construct. Nature is complicated. Lichens may appear simple, but that simplicity is an illusion.