Lichens are one of those groups of organisms that defies categorisation. A lichen is a symbiosis between a fungus and either a green alga or cyanobacteria. As such, they are neither strictly fungus, nor plant, nor bacteria.
Nor are all lichens the same. The relationships between fungi and their photosynthesising partners did not all evolve from a common ancestor, so each relationship is unique. In all cases, the fungus has the upper hand—capturing and enveloping its algal partner, and drawing carbohydrates from it. In some relationships, the fungus is only mildly parasitic, and in others it better resembles a disease on the alga.
The algae, however, do reap some benefit from the relationship. The fungal hyphae protect the alga from excess sunlight and keep it moist, allowing it to thrive in places it would otherwise be unable to survive.
Then there are the lichens that are parasitic on other lichens, stealing their algae, and those that begin as a lichen, but then become independently living fungi—there seems no upper limit on the complexity of lichen relationships.
Separate a lichen into its component organisms, and each will take on a form quite different from their joint lichen form–the lichen is more than the sum of its parts.
Together, lichens can colonise some of the most inhospitable places on the planet—bare rock, polar regions, mountain peaks, intertidal zones, and even the backs of living insects.
They certainly like to colonise my yard—benches, tree trunks, my office deck, even the roofs of house and shed are home to lichens. Many of the lichens in our yard are leaflike or shrubby in form, which is a good indication that our air quality is high. Shrubby and leaflike lichens don’t like air pollution, and tend to vanish in more industrial areas.
Biology and ecology aside, lichens are simply beautiful, with their intricate shapes and sometimes vivid colours. There’s a lot to like about lichens.