Editing Lichen (section)

== Ecology and interactions with environment ==

===Substrates and habitats===
[[File:Der Heilige Dominikus am Nordturm des Regensburger Doms.jpg|thumb|upright|Lichens on a limestone statue on a tower of [[Regensburg Cathedral]]]]
Lichens grow on and in a wide range of substrates and habitats, including some of the most extreme conditions on earth.<ref>{{Cite news|url=https://www.nytimes.com/2019/11/19/science/lichens-plants-evolution.html|title=In the Race to Live on Land, Lichens Didn't Beat Plants|first=JoAnna|last=Klein|newspaper=The New York Times |date=19 November 2019}}</ref> They are abundant growing on bark, leaves, and hanging from [[epiphyte]] branches in [[rain forest]]s and in [[temperate woodland]]. They grow on bare rock, walls, gravestones, roofs, and exposed soil surfaces. They can survive in some of the most extreme environments on Earth: [[arctic tundra]], hot dry [[desert]]s, rocky coasts, and toxic [[slag heap]]s. They can live inside solid rock, growing between the grains, and in the soil as part of a [[biological soil crust]] in arid habitats such as deserts. Some lichens do not grow on anything, living out their lives blowing about the environment.<ref name=WIL/>

When growing on mineral surfaces, some lichens slowly decompose their substrate by chemically degrading and physically disrupting the minerals, contributing to the process of [[weathering]] by which rocks are gradually turned into soil. While this contribution to weathering is usually benign, it can cause problems for artificial stone structures. For example, there is an ongoing lichen growth problem on [[Mount Rushmore National Memorial]] that requires the employment of mountain-climbing conservators to clean the monument.<ref name="WaPo11Jul2005">{{cite news |title=For Mount Rushmore, An Overdue Face Wash |url=https://www.washingtonpost.com/wp-dyn/content/article/2005/07/10/AR2005071000754.html |access-date=20 August 2022 |newspaper=[[The Washington Post]] |agency=Associated Press |date=11 July 2005}}</ref>

Lichens are not [[parasites]] on the plants they grow on, but only use them as a substrate. The fungi of some lichen species may "take over" the algae of other lichen species.<ref name=LLHE/><ref name=ABGPPU>{{cite web |url=https://www.anbg.gov.au/cryptogams/underworld/panel-6/index.html |title=Pollution, The Plant Underworld |publisher=Australian National Botanic Gardens |access-date=10 October 2014 |archive-url=https://web.archive.org/web/20140217122343/https://www.anbg.gov.au/cryptogams/underworld/panel-6/index.html |archive-date=17 February 2014 |url-status=dead }}</ref> Lichens make their own food from their photosynthetic parts and by absorbing minerals from the environment.<ref name=LLHE/> Lichens growing on leaves may have the appearance of being parasites on the leaves, but they are not. Some lichens in ''Diploschistes'' parasitise other lichens. ''Diploschistes muscorum'' starts its development in the tissue of a host ''Cladonia'' species.<ref name=Honegger1988/>{{rp|30}}<ref name=Dobson/>{{rp|171}}
[[File:Lichen A 01.jpg | thumb | 220x124px | right | Lichen on a fallen branch]]

In the arctic tundra, lichens, together with [[mosses]] and [[liverworts]], make up the majority of the [[ground cover]], which helps insulate the ground and may provide forage for grazing animals. An example is "[[reindeer moss]]", which is a lichen, not a moss.<ref name=LLHE/>

There are only two species of known permanently submerged lichens; ''[[Hydrothyria venosa]]'' is found in fresh water environments, and ''[[Verrucaria serpuloides]]'' is found in marine environments.<ref>{{cite journal |last1=Ahmadjian |first1=V. |title=Lichens are more important than you think |journal=BioScience |date=1 March 1995 |volume=45 |issue=3 |page=1 |doi=10.1093/bioscience/45.3.124 |url=https://academic.oup.com/bioscience/article-abstract/45/3/124/252273?redirectedFrom=PDF |access-date=14 September 2021}}</ref>

A crustose lichen that grows on rock is called a ''[[saxicolous lichen]]''.<ref name=ASLGPZ/><ref name=VMBMLF>''Mosses Lichens & Ferns of Northwest North America'', Dale H. Vitt, Janet E. Marsh, Robin B. Bovey, Lone Pine Publishing Company, {{ISBN|0-295-96666-1}}</ref>{{rp|159}} Crustose lichens that grow on the rock are [[epilithic]], and those that grow immersed inside rock, growing between the crystals with only their fruiting bodies exposed to the air, are called ''[[endolithic lichen]]s''.<ref name=LNAILV/><ref name=VMBMLF/>{{rp|159}}<ref name=ASLGAF/> A crustose lichen that grows on bark is called a ''[[corticolous lichen]]''.<ref name=VMBMLF/>{{rp|159}} A lichen that grows on wood from which the bark has been stripped is called a ''[[lignicolous lichen]]''.<ref name=ASLGGO>{{cite web|url=http://www.lichens.lastdragon.org/faq/glossary2.html|title=Alan Silverside's Lichen Glossary (g-o), Alan Silverside|access-date=10 October 2014|archive-url=https://web.archive.org/web/20141102105836/http://www.lichens.lastdragon.org/faq/glossary2.html|archive-date=2 November 2014|url-status=live}}</ref> Lichens that grow immersed inside plant tissues are called ''[[endophloidic lichen]]s'' or ''[[endophloidal lichen]]s''.<ref name="LNAILV" /><ref name=VMBMLF/>{{rp|159}} Lichens that use leaves as substrates, whether the leaf is still on the tree or on the ground, are called ''[[Epiphyllous lichen|epiphyllous]]'' or ''[[foliicolous lichen|foliicolous]]''.<ref name=UNOLP/> A ''[[terricolous lichen]]'' grows on the soil as a substrate. Many squamulose lichens are terricolous.<ref name=VMBMLF/>{{rp|159}} ''[[Umbilicate lichen]]s'' are foliose lichens that are attached to the substrate at only one point.<ref name="LNAILV" /> A ''[[vagrant lichen]]'' is not attached to a substrate at all, and lives its life being blown around by the wind.

===Lichens and soils===
In addition to distinct physical mechanisms by which lichens break down raw stone, studies indicate lichens attack stone chemically, entering newly chelated minerals into the ecology. The substances exuded by lichens, known for their strong ability to bind and sequester metals, along with the common formation of new minerals, especially metal [[oxalate]]s, and the traits of the substrates they alter, all highlight the important role lichens play in the process of chemical [[weathering]].<ref name=catena>{{cite journal |doi=10.1016/S0341-8162(99)00085-5 |title=Weathering of rocks induced by lichen colonization – a review |journal=CATENA |volume=39 |issue=2 |page=121 |year=2000 |last1=Chen |first1=Jie |last2=Blume |first2=Hans-Peter |last3=Beyer |first3=Lothar |bibcode=2000Caten..39..121C |url=http://www.geo.mtu.edu/~raman/papers2/ChenetalCatena.pdf |access-date=21 March 2015 |archive-url=https://web.archive.org/web/20150402091727/http://www.geo.mtu.edu/~raman/papers2/ChenetalCatena.pdf |archive-date=2 April 2015 |url-status=live }}</ref> Over time, this activity creates new fertile soil from stone.

Lichens may be [[Lichens and nitrogen cycling|important in contributing nitrogen]] to soils in some deserts through being eaten, along with their rock substrate, by snails, which then defecate, putting the nitrogen into the soils.<ref>{{cite journal |doi=10.1038/346839a0 |bibcode=1990Natur.346..839J |title=Fertilization of the desert soil by rock-eating snails |journal=Nature |volume=346 |issue=6287 |page=839 |year=1990 |last1=Jones |first1=Clive G. |last2=Shachak |first2=Moshe|s2cid=4311333 }}</ref> Lichens help bind and stabilize soil sand in dunes.<ref name="WIL"/> In deserts and semi-arid areas, lichens are part of extensive, living [[biological soil crust]]s, essential for maintaining the soil structure.<ref name="WIL"/>

===Ecological interactions===
Lichens are [[pioneer species]], among the first living things to grow on bare rock or areas denuded of life by a disaster.<ref name=WIL/> Lichens may have to compete with plants for access to sunlight, but because of their small size and slow growth, they thrive in places where higher plants have difficulty growing. Lichens are often the [[ecological succession|first to settle]] in places lacking soil, constituting the sole vegetation in some extreme environments such as those found at high mountain elevations and at high latitudes.<ref>{{cite journal | last = Walker |first = T. R. | date = 2007 | title = Lichens of the boreal forests of Labrador, Canada: A checklist | journal = Evansia | volume = 24 | issue = 3| pages = 85–90 | doi=10.1639/0747-9859-24.3.85|s2cid = 129100097 }}</ref> Some survive in the tough conditions of deserts, and others on frozen soil of the Arctic regions.<ref name=Oksanen2006/>

A major ecophysiological advantage of lichens is that they are [[poikilohydry|poikilohydric]] (''poikilo''- variable, ''hydric''- relating to water), meaning that though they have little control over the status of their hydration, they can tolerate irregular and extended periods of severe [[desiccation]]. Like some [[moss]]es, [[Marchantiophyta|liverworts]], [[fern]]s and a few [[resurrection plant]]s, upon desiccation, lichens enter a metabolic suspension or stasis (known as [[cryptobiosis]]) in which the cells of the lichen symbionts are dehydrated to a degree that halts most biochemical activity. In this cryptobiotic state, lichens can survive wider extremes of temperature, radiation and drought in the harsh environments they often inhabit.
[[File:Moss_lichen.jpg|thumb|Lichens suppress the growth of mosses and higher plants around them]]
Lichens do not have roots and do not need to tap continuous reservoirs of water like most higher plants, thus they can grow in locations impossible for most plants, such as bare rock, sterile soil or sand, and various artificial structures such as walls, roofs, and monuments. Many lichens also grow as [[epiphyte]]s (''epi''- on the surface, ''phyte''- plant) on plants, particularly on the trunks and branches of trees. When growing on plants, lichens are not [[parasite]]s; they do not consume any part of the plant nor poison it. Lichens produce [[allelopathy|allelopathic]] chemicals that inhibit the growth of mosses. Some ground-dwelling lichens, such as members of the subgenus ''[[Cladoniaceae|Cladina]]'' (reindeer lichens), produce allelopathic chemicals that leach into the soil and inhibit the germination of seeds, spruce and other plants.<ref>{{cite book|chapter=Lichen Allelopathy: A Review|author=Lawrey, James D. |pages=26–38| doi=10.1021/bk-1995-0582.ch002|title=Allelopathy. Organisms, Processes, and Applications|volume=582 |editor1=Inderjit |editor2=K. M. M. Dakshini |editor3=Frank A. Einhellig |year=1994| publisher=American Chemical Society|series=ACS Symposium Series |isbn=978-0-8412-3061-3 }}</ref> Stability (that is, longevity) of their [[Substrate (biology)|substrate]] is a major factor of lichen habitats. Most lichens grow on stable rock surfaces or the bark of old trees, but many others grow on soil and sand. In these latter cases, lichens are often an important part of soil stabilization; indeed, in some desert ecosystems, [[vascular plant|vascular (higher) plant]] seeds cannot become established except in places where lichen crusts stabilize the sand and help retain water.

Lichens may be eaten by some animals, such as [[reindeer]], living in [[arctic]] regions. The [[larva]]e of a number of [[Lepidoptera]] species feed exclusively on lichens. These include [[common footman]] and [[marbled beauty]]. They are very low in protein and high in carbohydrates, making them unsuitable for some animals. The [[Northern flying squirrel]] uses it for nesting, food and winter water.

=== Effects of air pollution ===
[[File:Lobaria pulmonaria 010108c.jpg|thumb|Some lichens, like the foliose ''[[Lobaria pulmonaria]]'', are sensitive to air pollution.]]
If lichens are exposed to air pollutants at all times, without any [[deciduous]] parts, they are unable to avoid the accumulation of pollutants. Also lacking [[stomata]] and a [[plant cuticle|cuticle]], lichens may absorb [[aerosol]]s and gases over the entire thallus surface from which they may readily [[diffusion|diffuse]] to the photobiont layer.<ref name=Nash2008/> Because lichens do not possess roots, their primary source of most [[chemical element|elements]] is the air, and therefore elemental levels in lichens often reflect the accumulated composition of ambient air. The processes by which atmospheric deposition occurs include [[fog]] and [[dew]], gaseous absorption, and dry deposition.<ref name=Knops1991/> Consequently, environmental studies with lichens emphasize their feasibility as effective [[bioindicator|biomonitors]] of atmospheric quality.<ref name=Nash2008/>

Not all lichens are equally sensitive to [[air pollution|air pollutants]], so different lichen species show different levels of sensitivity to specific atmospheric pollutants.<ref name="hogan">{{cite encyclopedia|last=Hogan |first=C. Michael |date=2010 |url=http://www.eoearth.org/article/Abiotic_factor?topic=49461 |title=Abiotic factor |encyclopedia=Encyclopedia of Earth. |publisher=National Council for Science and the Environment |location=Washington, D.C. |access-date=27 October 2013 |url-status=dead |archive-url=https://web.archive.org/web/20130608071757/http://www.eoearth.org/article/Abiotic_factor?topic=49461 |archive-date=8 June 2013 }}</ref> The sensitivity of a lichen to air pollution is directly related to the energy needs of the mycobiont, so that the stronger the dependency of the mycobiont on the photobiont, the more sensitive the lichen is to air pollution.<ref name=Beltman1980/> Upon exposure to air pollution, the photobiont may use metabolic energy for repair of its cellular structures that would otherwise be used for maintenance of its photosynthetic activity, therefore leaving less metabolic energy available for the mycobiont. The alteration of the balance between the photobiont and mycobiont can lead to the breakdown of the symbiotic association. Therefore, lichen decline may result not only from the accumulation of toxic substances, but also from altered nutrient supplies that favor one symbiont over the other.<ref name=Nash2008/>

This interaction between lichens and air pollution has been used as a means of monitoring air quality since 1859, with more systematic methods developed by [[William Nylander (botanist)|William Nylander]] in 1866.<ref name=WIL/>