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Lecture 33: Community Change
Reading: Economy of Nature, pp. 521-545.
Community Change
Cyclical: Seasonal
Temporal: Succession
Spatial: Environmental Gradients
Seasonality
Terrestrial Communities
Cold - Warm
Wet - Dry
Leaf drop (deciduous plants) in
the adverse climatic season Flowering Times: Biological seasons,
non-overlapping flowering times may be driven by competition
between plants for pollinators (a limited resource).
Aquatic or Marine Communities
Nutrient Rich - Poor
Cold - Warm
Succession
Succession is directional change in community
species composition in ecological time (102 years maximum time
span).
Primary: Starting from bare rock, simultaneous
formation of soil with community development and change.
Factors:
climate, time, topography, organisms available,
parent (rock) substrate
Abiotic and biotic weathering: erosion
of parent material
Soil formation involves accumulation
of organic matter along with weathering of the parent
substrate.
Soil types and vegetation types
(community formations)
Close parallels except among
forests and rain forests
Secondary: Starting with soil already present,
results when existing vegetation is destroyed leaving soil
intact.
In either type of succession, vegetation changes
until a stage of equilibrium is reached, the climax stage, which is the
final stage of succession at a given site.
Successional equilibrium or Climax Community
Self-perpetuating, stable community
Monoclimax: Climate alone important, a given
area has only one climax type, so similar sites in a given area will have
similar climax communities.
Polyclimax: Different climax communities possible
in a given area, soil moisture, nutrients, and biotic factors determine
the kind of climax community at a given site in addition to climate.
Climate is not a constant, so succession can
be viewed as a constant process.
Mechanisms of successional change
Facilitation (Monoclimax) Relay Floristics
- Species replace one another due to sequential
modification of the environment. The environment is changed by resident
species making it less suitable for residents and more suitable for
colonists.
- Fixed orderly sequence of replacements
- Competition with dominants (new colonists)
prevailing
- Early succession necessary for later
succession
- Facilitation applies best to primary succession.
Tolerance (Monoclimax)
- Early successional species are not necessary
to start the successional sequence. Any species (for community type)
can start succession), but some species are competitively dominant to
others. The dominant species predominate in the climax community.
- Species replaced by others more tolerant
to limiting resources
- Species replacement not a function of
present residents
- Residents tolerate colonists
Tolerance applies best to secondary succession,
large scale (old field, clear-cut, forest fire recovery).
Inhibition (Polyclimax)
- Species replacements are not orderly
as residents attempt to exclude new colonists. Succession depends on
which species arrive and become established first.
- Species replacements result from residents
being damaged or killed
- Any replacement is possible
- Succession is generally from short lived
to long lived species
Inhibition applies best to secondary succession,
small scale (tree fall in a mature forest, small patch colonization in
a rain forest).
General trends in plant communities in early
and late successional stages (after Ricklefs, 1996, p 537, Table 23.2).
|
Characteristic
|
Early Succession
|
Late Succession
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Number of seeds
|
many
|
few
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Seed size
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small
|
large
|
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Seed dispersal
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wind, stuck to animals
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gravity, eaten by animals
|
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Seed viability
|
long
|
short
|
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Root:shoot ratio
|
small
|
large
|
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Growth rate
|
rapid
|
slow
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Mature size
|
small
|
large
|
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Shade tolerance
|
low
|
high
|
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Species richness
|
low
|
high
|
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Abundance uniformity
|
low
|
high
|
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Spatial heterogeneity
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low
|
high
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Specialization
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little
|
much
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Production
|
"quantity" (r)
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"quality" (K)
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Community stability
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low
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high
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