Weber State University       Department of Botany

                                                    BOTANY  LS1203 - PLANT  BIOLOGY

The fungal partner in the lichen is unique and used to ID the lichen.  The algal partners can be found in several different lichens.  The fungus is rarely found alone in nature; the alga can often be found living independently of the fungus.

Some think that the fungal/algal relationship is parasitic by the fungus:  the fungus forms haustoria that can be seen penetrating the algal cells; some algal cells are destroyed in the process.

Most view the relationship as mutualistic:
In nature:  the lichen can grow in habitats where neither the fungus not the alga can grow alone
In lab:  alga and fungus cultured separately will form a lichen if placed together in a medium in which neither can grow alone.

Lichens grow everywhere, from arctic and alpine tundra regions to desert, on a variety of substrates:  concrete, tombstones, rocks, roof shingles, tree trunks and branches.  A wide variety of colors is seen:  shades of green,  shades of gray, orange, red, yellow, black.

Importance of lichens:
pioneer organisms in xerosere plant successions
    can grow on bare rock and initiate soil formation
very sensitive to air pollution (sulfur dioxide) ==>  indicator species
reindeer moss, other fruticose lichens == fodder for reindeer, caribou, deer, and cattle
source of dyes == Harris tweed, litmus paper

Plant Successions

Population:   a group of individuals of a single species in a particular location, generally viewed as capable of interbreeding

Community:   the various populations that share a particular location

Ecosystem:  the community members + abiotic factors (light, temperature, precipitation, soil) that interact with each other in a particular location

Plant communities change over time, with certain communities generally following each other because of the nature of the abiotic factors.  These gradual changes in plant communities as environmental factors change over long spans of time result in successions.

Primary Succession:  an area that has not been occupied before is colonized by organisms.  The first organisms that establish themselves on either a bare substrate or in water form a pioneer community.  The members of the pioneer community initiate soil formation.  As these organisms grow and change their environment,  they are replaced by successive communities called seral stages.  Eventually, a long lasting community called a climax community can be reached.  The climax community replenishes  itself rather than giving way to another community as time goes by.  Two types of primary successions are recognized:  xerosere and hydrosere.

Primary succession in the Uinta Mountains in the present day Hudsonian Life Zone (timberline forest of Subalpine Fir and Engelmann Spruce) might be diagrammed like this:

Xerosere Primary Succession:

Rock --> Lichens (Pioneer Community) --> Mosses --> Annual Grasses --> Perennial Grasses and Forbes --> Shrubs --> Spruce/Fir (Climax Community)

Moisture:  xeric to mesic

Hydrosere Primary Succession:

Glacial Lake --> Phytoplankton (Pioneer Community) --> Floating Aquatic Macrophytes --> Water Lilies --> Sedges/Rushes --> Grasses --> Shrubs --> Spruce/Fir (Climax Community)

Moisture:  hydric to mesic

Secondary successions occur when a climax community is disturbed (fire, bulldozer, etc.)  The plant community that arises following the disturbance will be of one of the earlier seral stages.  A climax community is generally reached faster in a secondary succession than in a primary succession, often with some seral stages skipped.  Also, the populations that make up the climax community might not be exactly the same in a secondary succession compared to a primary succession.

Review

What is a lichen?  
Know the three growth forms of lichens (foliose, crustose, fruticose)
Who first described the dual nature of lichens?  Who first proposed that the organismal relationship in a lichen is a mutualistic symbiosis?   What are the benefits to the organisms involved?
What evidence supports the view that the relationship is mutualistic?
What evidence supports the view that the relationship is parasitic?

biotic vs. abiotic
community
ecosystem
xeric  vs. hydric
What are the steps in a primary plant succession?  What are the differences between a xerosere succession and a hydrosere succession?  What are the similarities?  What are the pioneer organisms in each type of succession?  What happens to the moisture levels as the successions proceed?
What is the difference between a primary plant succession and a secondary plant succession?
In what part of a succession would you find:  pioneer organisms, seral stages, climax communities?

What uses have been made of lichens?

Suggested Online Reading

The Microbial World:  Lichens     (by Jim Deacon at the University of Edinburgh)

Symbiosis: Mycorrhizae and Lichens    (by George Wong at the University of Hawai`i at Manoa) 

Biological Diversity:  Fungi    (from the Online Biology Book by M.J. Farabee at Estrella Mountain Community College)  Go to the section on lichens.  

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Revised 15 November 2004.  Links checked 15 November 2004.