Community Dynamics

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Community Dynamics
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Colonization

• ARRIVAL
• ESTABLISHMENT

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Initial floristic composition model

• Proposed by Egler (1954)
• All the plants arrive or germinate near the same
  time.
• Second stage of succession the perennials
  overtop the annuals.
• Third stage of succession shrubs overtop the
  perennials
• Fourth stage trees overtop, etc.

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Connell and Slatyer

• Facilitation
• invasion depends on conditions created by earlier
  colonists
• Tolerance
• later successional species probably
  competitor-stress tolerators
• help reduce the level of resources to where only
  they can tolerate it
• Inhibition
• species displaced only by death or damage by
  factors extrinsic to competition

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Tilman resource-ratio hypothesis

• Successional sequence depends primarily on three
  things
• 1) interspecific competition for resources
• 2) long-term pattern of a supply of limiting
  resources, especially nutrients and light
• 3) a small group of other life history factors.
• Early in sequence Light is high and nutrients
  low. These factors have reversing trends over
  time.

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Resource - Ratio
A
B
C
D
Light
Nutrients
Relative abundance
Nutrient or light availability
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Probabilistic view of species replacement

• Plots were mapped on glacial outwash in Alaska
• 617 plants were initially recorded
• 417 died during a 5 year period
• 535 new plants became established during 5
  years
• Density increased by 20, but more than 60 of
  the original individuals were replaced by others.

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Horn replacement probabilities

• Studied saplings under a mature tree. If 60 of
  the saplings under aspen trees were beech, the
  probability of a beech replacing an aspen would
  be .6
• Forest studied beech replacement of aspen .6
  and aspen had only a .03 chance of replacing
  itself. Beech had a .8 probability of self
  replacement.

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General Postulates - succession

• Biomass
• As supported biomass increases, net productivity
  decreases
• total resources available to higher trophic
  levels are about the same
• Nutrients
• increasingly tied up in biotic pools. Rate of
  loss decreases
• Life history
• r-selected species are replaced by K-selected
• Species diversity
• increases (dominance declines) until just before
  climax growth, then reverses some

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Species diversity over time
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General Postulates - succession

• Biomass
• As supported biomass increases, net productivity
  decreases
• total resources available to higher trophic
  levels are about the same, but heterogeneity is
  increasing
• Nutrients
• increasingly tied up in biotic pools. Rate of
  loss decreases as they are held in standing
  biomass
• Life history
• r-selected species are replaced by K-selected
  ecological specialists as resources become more
  limiting
• Species diversity
• increases (dominance declines) until just before
  climax growth, then reverses some
• Stability - to ecological perturbations increases

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Alternation of species

• James Fox
• Amer. Nat. 111(977)69-88
• Kerry Woods (reciprocal replacement)
• Saplings (not suckers) counted maple more common
  under Beech and vice versa
• Mature trees within 10m of a sapling counted,
  measured

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Alternation of species

• James Fox (1977)
• Amer. Nat. 111(977)69-88
• Kerry Woods (reciprocal replacement)
• Saplings (not suckers) counted maple more common
  under Beech and vice versa
• Mature trees within 10m of a sapling counted,
  measured
• b beach m maple

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Alternation of species

• James Fox (1977)
• Amer. Nat. 111(977)69-88
• Kerry Woods (reciprocal replacement)
• Saplings (not suckers) counted maple more common
  under Beech and vice versa
• Mature trees within 10m of a sapling counted,
  measured
• b beach m maple
• Found Igt0 for high frequency Maple, Therefore
  Beech has greatest canopy influence
• More dead saplings under conspecific canopies

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Alternation of Generations

• Larry Forcier
• Found maple with many beech saplings, but not the
  reverse. He found a trend towards beech
  dominance.
• Replacement tended to be in a series when a gap
  was created (minor disturbance).

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Reasons for Alternation

• Competition
• Intraspecific stronger than Interspecific
• Chemical antagonism
• Allelopathy canopy and root exudates
• Seed predation
• Microhabitat differences
• relates to facilitation
• fungal pathogens
• saplings the most susceptible to attack - most
  mortality is related to this
• most easily obtained near the host species

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