Many of the most common lichens I find here in Europe seem to have a very large area of distribution. Most are at least holarctic, many are also present in other places. Here is a table of the 15 species of lichens (for simplicity only Lecanoromycetes) observed most often in Europe, their distribution, and their statuses according to iNat:
Species
Distribution
Status (Europe)
Status (N.-America)
Status (New Zealand)
Xanthoria parietina
temperate
native
native
native
Parmelia sulcata
holarctic + NZ + AU
native
native
—
Evernia prunastri
holarctic
native
native
not present
Hypogymnia physodes
holarctic + NZ + AU
—
native
—
Physcia adscendens
temperate
native
native
native
Protoparmeliopsis muralis
mostly holarctic
—
native
?
Flavoparmelia caperata
global
native
native
—
Phlyctis argena
global
native
—
—
Cladonia fimbriata
temperate
native
native
native
Pseudevernia furfuracea
holarctic
—
—
not present
Ramalina farinacea
mostly holarctic
native
native
—
Lecidella elaeochroma
holarctic + NZ + ZA
native
native
—
Phaeophyscia orbicularis
holarctic + NZ
native
native
—
Lobaria pulmonaria
temperate – South America
—
native
?
Peltigera praetextata
holarctic
—
native
?
My first guess was that lichens (or their diaspores) are simply very likely to be hitchhikers on plants (especially because many of the above are epiphytes). However, that does not seem to be the case, judging at least by the fact that many are labeled “native” in both North America and Europe, some even in New Zealand. And in any case, I know that lichens are extremely sensitive to their environment, so it seems like all of them getting established in all these other places is unlikely, maybe?
My second guess is that most of these species of lichen must be very old. Assuming, the statuses are applied correctly, and these species haven’t therefore been distributed through human activity, these lichens must date back to when all the landmasses were connected, right?
What are your thoughts on this? :)
Notes on the table:
“temperate” has been simplified here to mean present on both hemispheres, with a gap in the middle (usually in the tropics)
“mostly holarctic” means there is a noticeable number of records in other places, but very dispersed
“—” means no status was applied
“?” means fewer than 5 observations in New Zealand
X. parietina is interestingly labeled introduced in Australia, which kinda makes me question whether it really is native to New Zealand
Maybe it’s just that the splitters have been so busy making sure that no mushroom species occurs on more than one continent that they haven’t gotten around to doing the same for lichens yet.
I would think the reason these are the most observed is actually because they are very widely distributed. A species which is native to a small area would have only very few observations. I would think that includes a majority of lichen, but I could be wrong - I definitely have seen some lichen endemic to just a single small place.
And then what really makes lichen special is, most of them are restricted to pollution-free areas. And so they naturally only occur in areas with few people, away from polluted cities and agriculture. And that also means, away from potential observers. The 15 species above are mostly atypical ones which are tolerant to pollution - except maybe Lobaria pulmonaria, but that one just is really showy!
True, but I only looked at the top observed in Europe, so how often they are seen somewhere else shouldn’t matter.
And that doesn’t explain how a lichen came to be native in Europe, North America, and New Zealand all at once.
They have to have made it across the ocean somehow or these species already existed before the current geography (but at those time scales and with the time they’d have spent physically separated in that case, that much similarity and there not being a speciation event seems almost impossible).
I’m assuming this is meant mostly jokingly, but I did think of the possibility of the lichens not actually being the same species (though again, the similarity seems far too strong for that level of isolation from each other).
In the fungal world, many species currently thought to be globally distributed are being split into American and European species. See Amanita pantherina and Amanita pantheroides, for example. I wouldn’t be surprised if we saw this in lichens as well.
Spores are also much lighter than than plant seeds, so many fungi are able to travel further, unrelated but many bryophytes/ferns have ranges based on wind currents.
Specieation isn’t mandatory after a certain amount of time apart, some taxa are just really stable bc they just work.
I thought I knew the answers to your questions, but I found myself saying bc of the way it is, so I did some reading and research
I think these should answer a lot of your questions:
Science has no universally applicable standard for how to delineate species, and (as has been discussed on this forum several times, including here: https://forum.inaturalist.org/t/why-do-some-genera-have-so-many-species/18452 ) is unlikely to ever do so. A species is functionally “A group of related organisms considered (in publication) by a taxonomist of that group to be a species, using whatever standard or definition that taxonomist preferred at the moment the analysis was performed.”
This being unfortunately true, discussions about distributions of species are extremely prone to being mostly semantics.
Looking at the papers above, I think for this specific questions the species delineation is not that relevant since “genetic distance” of sampled lichen can be measured, to infer things like dispersal patterns. With limitations of course, but the currently assigned name or species rank doesn’t seem to matter for the results.
but the specific question is “Distribution of Lichen-Species.” Species delineation is very relevant to that question. A taxonomist could easily (and some likely will at some point) decide that the New Zealand populations, the American populations, the populations in Europe, etc. are different sets of species, and that would very much affect the results of any species-based distribution analysis.
Many of the papers cited above ask a different question. Not why species are distributed as they are, but how genetic diversity is distributed. This is (in my view) a more productive way to ask the question because it does not depend on (necessarily arbitrary) species labels.
Because the question they are asking, and the data in their table, are about species. I’m glad that the papers you shared mostly don’t focus on species distributions, and I think it is worth making people aware of the difference in approach between species distributions and phylogeography.