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 ) 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.
Definition and scope
Lichens are symbioses in which a fungus (the ) partners with a photosynthetic organism (the ), usually a green alga and/or a cyanobacterium. A single lichen-forming fungus can associate with more than one photobiont type, and some lichens contain both a green alga and a cyanobacterium (). Beyond the mycobiont and photobiont, the lichen body (thallus) often contains additional fungi (including endolichenic and lichenicolous forms) and commonly harbours bacteria. Lichenization (forming lichens) is common among fungi. One 2017 estimate put lichen-forming fungi at about 27% of known fungi (more than 19,409 species). Work in the 2010s moved beyond the simple "one fungus and one photobiont" picture of lichen symbiosis. Instead, some authors describe the thallus as a dynamic metacommunity: a linked, interacting community of multiple organisms living together. This broader view has been linked to the discovery of specialist basidiomycete yeasts living in the (outer layer) of some lichens whose main fungus is an ascomycete. However, cystobasidiomycete yeasts appear rare rather than ubiquitous: a broad DNA-based metagenomic survey of 413 thalli detected them in only nine species (2.7%).
Biogeographic research describes where organisms and lineages occur, and it examines the processes that shape those patterns. Lichen biogeography examines distribution patterns across multiple spatial and temporal scales, considering both historical events and contemporary ecological factors. DNA-based family-tree studies (molecular phylogenetics) can reveal genetically distinct lineages that are hard to distinguish by appearance within widely recognized morphospecies (species defined mainly by morphology). As a result, morphospecies-based range maps can obscure lineages with more restricted distributions.
Background and deep-time context
Biogeography examines where organisms occur now and where they occurred in the past, then seeks explanations by testing ideas against new observations at different scales. The fossil record for lichen-forming fungi is sparse. Lichen fossils have been recovered from the Early Devonian Rhynie chert of Scotland (about 400 million years old). Older Ediacaran material from marine phosphorites of the Doushantuo Formation at Weng'an (c. 635–551 million years ago) has also been described as lichen-like associations. Later reviews treat that Precambrian (Ediacaran) interpretation as uncertain.
Perspectives and major ideas
Biogeographers often organize work around two complementary kinds of explanation: historical processes (such as tectonics, vicariance, and dispersal) and ecological constraints that limit present-day ranges. The two perspectives are complementary rather than competing, but they remain useful for organizing work. Ecological and historical approaches often overlap, and ecological biogeography can still include reconstructions of past demography, interactions, and environmental constraints.
A recurring question is whether wide ranges mainly reflect frequent long-distance dispersal (sometimes summarized as "everything is everywhere") or vicariance and other Earth-history processes. Evidence supports both explanations, and the balance differs among taxa: similar present-day ranges can arise through different histories. For example, a historical biogeographic analysis of Chroodiscus linked its present-day distribution to vicariance and dispersal between nearby or connected continental shelves, rather than to recent trans-oceanic long-distance dispersal. Analyses in lichen biogeography draw on several traditions. Some emphasize dispersal, some emphasize vicariance using cladistic relationships and area histories, some use panbiogeographic "tracks" (repeated map patterns), and some use phylogeography to infer past population movements from genetic data.
Patterns of distribution in lichens
Lichen ranges span from local endemics to taxa recorded across multiple continents, with recurring patterns at both small and large spatial scales. Many lichen-forming fungi occur across multiple climatic zones or continents, a breadth of distribution that is uncommon in vascular plants. Wide ranges that track climate and habitat are common among lichen-forming fungi, whereas truly cosmopolitan taxa are few.
Latitudinal gradients Studies of latitudinal diversity patterns in lichens use both specimen-backed occurrence data (vouchered records) and sequencing-based surveys. In specimen-backed datasets, heavily collected areas can appear artificially species-rich. In sequencing surveys, the DNA often comes from propagules (spores or tiny fragments) rather than from established lichens. In the western United States, richness does not simply increase or decrease with latitude when models (MaxEnt) account for uneven collecting. A DNA metabarcoding survey of lichen-forming fungal propagules from settled dust across the contiguous United States found a significant but weak reverse latitudinal gradient: propagule richness was higher at higher latitudes. In Spain, epiphytic communities on European beech (Fagus sylvatica) showed higher lichen richness towards the north across the study gradient. Species turnover along the gradient was linked to higher dry-season rainfall at higher latitudes.
Widespread and cosmopolitan patterns
Even the widest-ranging lichens are rarely found everywhere: many have clear geographic limits and track broad climatic gradients. Some lichen-forming fungi have circumpolar ranges in the Northern Hemisphere, including arctic–alpine variants. Examples of these distribution types include Cladonia coniocraea (circumpolar) and Montanelia tominii (circumpolar arctic–alpine). Some lichens described as cosmopolitan are recorded from all major landmasses and from many oceanic islands. Within the clade (Arctoparmelia, Brodoa, Hypogymnia and Pseudevernia in family Parmeliaceae), only a few species appear truly cosmopolitan. These are the taxa that reproduce using vegetative propagules that disperse both symbiotic partners together. Phylogenetic evidence in Parmeliaceae suggests that truly cosmopolitan species are rarer than earlier assumed.
One analysis compared foliicolous (leaf-dwelling) lichens using presence/absence across six lichenogeographical regions, and it defined 22 distribution patterns within that framework. In that analysis, many foliicolous lichen species were widely distributed, including cosmopolitan or pantropical species (21%) and species disjunct on at least two continents (19%).
Endemism and restricted ranges
Regional lichen floras commonly include taxa regarded as endemic, sometimes even at the level of genera. Endemic ranges are consistent with dispersal limitation in some lichen fungi, and reported centres of endemism include Mediterranean climate regions, Macaronesia, and the Galápagos Islands. Restricted ranges do not always imply poor dispersal; proposed explanations include narrow habitat requirements, limited time since speciation, or strong photobiont specificity when compatible algal strains are unavailable locally. In lichens (a group of mostly crustose species), some taxa previously regarded as Antarctic or Sub-Antarctic endemics (or as bipolar) are also known from montane regions of western North America, raising questions about how common truly endemic Antarctic lichen-forming fungi are.
Approximately 60% of foliicolous lichens are restricted to one of three major tropical areas, whereas vascular plants were almost entirely restricted to a single one of those areas. A 2003 estimate placed 539 foliicolous lichen species in the Neotropics, including 252 endemic species (47%), while the African Paleotropics were estimated to include 309 species, including 56 endemic species (18%).
Oceanic islands and archipelagos
Oceanic archipelagos are natural tests for island biogeography because their isolation lets researchers ask two basic questions: do lichen floras mainly reflect repeated long-distance colonization from continents or diversification after arrival, and how often do lineages fail to persist because of local extinction? Classic island-biogeography models treat richness as a balance between immigration and extinction that varies with island area and isolation. Later "general dynamic" approaches also emphasise how islands change through time (ontogeny) and how niche opportunities shift as islands grow and erode. In lichen studies, these questions often become: are lichens dispersal-limited, do populations establish after rare arrivals (including founder effects), and does isolation promote endemic lineages rather than repeated recolonization? In Macaronesia, endemic taxa of Nephroma are interpreted as neo-endemics that originated after island emergence, forming two sister clades distinct from widespread relatives. In Hawaii, the neotropical genus Lobariella forms a single clade (monophyletic) of three presumably endemic species, described as a local "micro-radiation" (a small, local burst of diversification). A Gran Canaria endemic in Xanthoparmelia falls within an East African clade and was dated to the Pliocene, consistent with long-distance colonization followed by isolation rather than deep shared geological history.
Disjunctions and southern-hemisphere patterns