Environmental Science

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How does lichens contribute to primary succession?

BSCI 124 Lecture Notes
Undergraduate Program in Plant Biology, University of Maryland
I. Plant succession - a series of predictable changes over time in the kinds of plants growing in an area

* A. Ecosystems mature and change with time

o 1. Always determined by the physical parameters of the environment
B. As ecosystems age, the kinds of organisms found in them changes until some stable type of community forms

o 1. Initial stages- high rate of replacement, unstable (prone to erosion and wind damage)

2. Later stages- low rate of community change, more stable
C. The relatively stable community at the end of succession is called a climax community

o 1. Climax community is thought to be in equilibrium with the environment

2. Permanent until there is some type of environmental change (flood, fire, wind, climate change)
3. Climax varies depending on the local conditions
4. Climax in some Maryland habitats is a hardwood forest, in some California habitats is a grassland, and in some Arizona habitats is a desert
D. In characterizing succession, we only list the dominant plants (the ones that shape the ecosystem)

E. Two kinds of succession, primary succession and secondary succession

II. Primary succession

* A. Primary succession occurs when plants become established on land completely devoid of soil and vegetation

o 1. Soil is that portion of the earth's surface consisting of disintegrated rock and organic materials or humus. Primary succession is the development of soils. The plant communities will generally change as the soil develops.

B. Succession on barren rock or lava (example: Mount St. Helens)

o 1. Possible primary succession for a forest habitat:

Lichens (pioneer species) --> mosses & ferns --> grasses --> shrubs --> trees
+ a. bare rock is first colonized by lichens and bacteria

b. small amount of soil formed by the lichens is colonized by mosses, which do not have roots and require little soil, and ferns
c. as the seedless plants live and die, the soil continues to develop to the point that grasses can successfully grow and a grassland community forms
d. over time, the soil level increases to the point that shrubs can grow in the grassland. The grassland is replaced by a shrub community
e. shrub community may be replaced by a forest 2. Each stage alters the habitat in such a way that it prepares the way for the next invasion of species
3. As succession proceeds, soil is formed and thickens - the result of decomposition
4. When the changes in the composition of plants stop and the plant community remains generally the same for many years, the community is mature or at climax. A climax community is the relatively stable community at the end of succession.

Last revised: September 1, 1998 - Browning
updated Dec. 2000 - Straney

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