Historically, lichen fungi have through most of the nineteenth and twentieth centuries been arranged in its own class, Lichenes, based on their symbiotic life form as expressed by their composite thalli. By convenience, the name of the lichen fungus is usually but inaccurately applied to the feature referred to as a lichen, as if that was an organism. Actually, lichens are small ecosystems (Section 1.6), comprising associations with two or more components, an algal producer and a fungal consumer. The components are individual organisms; lichens are not. Consequently, lichens per se cannot be classified into natural systems because they have no phylogeny. Today lichen fungi are classified together with other chitinous fungi and incorporated into a common fungal system.

Systematics

Systematics – the science of studying the diversity and hierarchy of nature – is not only the oldest natural science, but is also a science where the modern development is progressing at a dramatic pace. Systematics is built up by four major parts: taxonomy (the delimitation and description of taxa), nomenclature (the formal naming of taxa), phylogeny (the natural relationships among taxa), and classification (the organization of taxa into a hierarchical system). In the present treatment, we focus on the phylogeny and classification of lichen fungi. Taxonomic methods and nomenclatural rules and principles are beyond the scope of the present book.