Download Biol3700 Lec3 Succession

Succession: a term used to refer to
temporal changes in
biological communities
Definition: A directional change in the species
composition of a community over time
History and Review of Ideas
Two Basic Types of Succession
• Primary succession
establishment and development of
communities in newly formed habitats
Eg. land exposed by glacier retreat
• Secondary Succession
return of an area to its natural vegetation
following a major disturbance
Eg. fire, land clearing for agriculture
Mechanisms: What causes succession to occur?
Successional change is a pattern.
What processes or mechanisms cause the pattern?
Mechanisms of Succession
• Differences in plant life history characteristics
- growth rate, seed size, height, longevity
• Changes in resource availability over time
• Interactions among species
- Competition
- Herbivory
• Facilitation
- ‘earlier’species alter the environment
and make it more favorable for ‘later’ species
Chance events can alter the course of succession in
any one specific location
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Case Study: Primary Succession in Glacier Bay, Alaska
Grand
Pacific Gl.
1940
Canada
Alaska
1912
1948
1941
1899
1907
1935
1879
1949
1948
1879
1931
1911
1900
1892
1879
Johns Hopkins
Gl.
1913
1860
Reid Gl.
1879
0
Glacier
Bay
Miles
5
0
10
5 10 15
Kilometers
1830
1780
1760
Pleasant Is.
McBride glacier retreating
Pioneer stage, with fireweed dominant
Life History Characteristics
Dryas stage
60
Soil nitrogen (g/m2)
50
40
30
20
10
0
Pioneer
Dryas
Alder Spruce
Successional stage
Nitrogen fixation by Dryas and alder
increases the soil nitrogen content.
Spruce
stage
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Life History Characteristics
Species Interactions: Competition
Species Interactions: Herbivory
Short-lived dominants in early succession
inhibit the development and dominance of
spruce trees later in succession
Preference of herbivores for broad-leaf species
speeds up the transition to spruce trees
Facilitation in Glacier Bay Succession?
Change in soil nitrogen concentration during succession after glacial retreat
in Glacier Bay, Alaska
Greenhouse bioassay with spruce seedlings:
27 g/seedling in pioneer soil
43 g/seedling in alder soil
147 g/seedling in spruce soil
•
•
Soil gets better for spruce over time.
Effect of time or facilitation by alder?
Need a test with and without alder,
while time was held constant.
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Overall Glacier Bay story:
• Life history characteristics indicate spruce will dominate.
If there was no competition, the vegetation trend would be
the same: dryas > alder > spruce, but the sequence
would occur faster.
• Competition causes spruce dominance to occur more slowly.
Competition among species slows the sequence, as short-lived
‘early’species inhibit the dominance by long-lived spruce.
• Facilitation by alder results in higher productivity in the final
spruce forest. Facilitation happens, but the effect is secondary.
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