Lichen biogeography

Lichen biogeography is the branch of biogeography that studies where lichens occur and how their distributions have changed through time. Lichens are symbiotic organisms: they consist of a fungus living together with a photosynthetic partner, usually a green alga or cyanobacterium. Which photosynthetic partner (the photobiont) a fungus can use, how selective that partnership is, and whether suitable partners are available locally can affect where lichens establish and persist. Research spans scales from microhabitats to comparisons among regions and continents, and from recent range shifts to patterns shaped by deep-time events in Earth history.

A central question is whether wide ranges in lichen-forming fungi mainly reflect long-distance dispersal, or whether they reflect the splitting of populations by new barriers or shifting landmasses (vicariance) and other Earth-history processes. Reported patterns range from narrowly endemic species to taxa that are widespread but still have clear geographic limits, to strongly disjunct distributions such as bipolar ranges at high latitudes in both hemispheres. DNA-based studies show that some species that look uniform by morphology actually contain multiple genetically distinct lineages, each with a different geographic range. Regional species lists are also compared to see which areas share similar sets of lichen-forming fungi. One recurring result is a broad split between Gondwanan and Laurasian elements (a southern-versus-northern pattern at the scale of continents). Across taxa, common themes include local endemism, species that are widespread but regionally bounded, and repeated long-distance disjunctions (especially bipolar and intercontinental patterns); true cosmopolitanism appears uncommon.

Traditionally, lichen biogeography compared regions using morphology-based checklists, but modern work increasingly uses DNA sequencing and phylogeography (the study of how genetic lineages are distributed across geography). Species distribution modelling, which relates occurrence records to climate and habitat variables, is increasingly used to map potential ranges and forecast responses to environmental change. Molecular studies have revealed that many lichens previously thought to be widespread single species are actually complexes of distinct, genetically isolated lineages with narrower ranges. This shift matters for conservation planning, because cryptic diversity (genetically distinct lineages that are hard to tell apart by appearance) can be overlooked. Biogeographic data help assess how lichen communities respond to air pollution, habitat fragmentation, and global climate change.