Ecology
TEMPORAL PATERNS IN COMMUNITIES:SUCCESSION
I. GENERAL CONCEPTS ABOUT SUCCESSION
A. Definition-Succession is the non-seasonal, directional and continuous pattern of colonization and extinction on a
site by species populations.
B. Primary vs. Secondary succession
1. Primary succession: the establishment of vegetation on land not previously occupied by vegetation.
2. Secondary succession: the establishment of vegetation on a site where previous vegetation has been destroyed
by some disturbance (fire, hurricane, agriculture).
C. Autogenic vs. Allogenic succession
1. Autogenic succession: Biotic succession. Both the environment and plant communities changes through time,
but the changes are due to the activities of the organisms themselves.
2. Allogenic succession: The condition where new habitat or substrate is opened up for invasion by green plants
by non-biological sources.
D. Concept of Climax-
1. Climax: the cessation of directional change in presence and abundance of species over time; a stable equilibrium
will occur if individuals which die are replaced on a one-to-one basis by young of the same species.
a. Small changes may occur that deviate from the long-term mean, or pattern, but this is part of the dynamic
equilibrium of the system.
E. Typical trends in succession.
1. Soils- Increase in nutrients, increase in soil depth, increase in organic matter and development of soil horizons.
2. Inorganic nutrients- the pool of inorganic nutrients decreases in the soil and increases within the plants
increases. (Compare field to forest; young forest to old forest)
a. Early successional vegetation depends upon abiotic sources of nutrients- mineral soil.
b. Late successional vegetation acquires nutrients from organic sources -decomposed organisms.
3. Vegetation sequence from annual to perennial herbs, to woody plants (not necessarily shrubs -> trees.)
4. Species diversity increases from simple communities of early succession to richer communities.
a. Diversity increases throught time on xeric and intermediate sites.
b. Diversity on mesic sites is maximized in early stages, then decreases from late stages to climax.
5. Productivity โ the total biomass of the site increases from early to late successional stages.
a. Gross primary productivity decreases (GPP) โ GPP is expressed as the rate of formation of organic matter per
unit area per unit time.
b. GPP decreases because most of the biomass is not photosynthetically productive, but supportive (woody).
6. Niche Breadth โ of organisms decreases from early to late successional stages. Organisms become more
specialized.
III. EXAMPLES OF SUCCESSION
1. Seral stage: The sequence of communities, e.g., from grassland to shrubland to forest, is known as a sere (from
the word series), and each of the changes (i.e. each different community) is called a seral stage.
PRIMARY SUCCESSION
A. Bog succession- hydrarch succession- an example of primary succession, but process is autogenic.
1) Open water is full of organic matter and humus, therefore pH is low and so is content of inorganic nutrients.
2) Around water, flourishing mat of vegetation formed by sedges and peat mosses (Sphagnum spp.). Sphagnum
absorbs cations and releases H+ ions, decreasing the pH of the bog.
3) The mat of vegetation that you can walk on is composed of roots and sphagnum moss.
a. Combination of low root temperature, acidity, and high temperatures at leaf level (xeric conditions) favors
plants with morphological traits that are adapted for water conservation.
4) The thickening mat of roots from herbaceous plants permits small shrubs of the heath family, leatherleaf and
Labrador tea (acid tolerant) to invade, followed by larger species.
5) Shrub stage is invaded by several trees, including tamarack (Larix laricina).
6) Tamaracks replaced by spruces, white pine, beech and maples from the surrounding forests.
B. Michigan Dunes- xerarch succession. Description is given in your book.
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