Disturbance, Succession, and Stability

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Disturbance, Succession, and Stability
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What is ecological succession?

• Progressive change in the composition of a biotic
  community.

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Why does succession occur?

• Because different species have different
  characteristics
• resource needs
• environmental tolerances
• competitive abilities
• And because organisms modify their environments
• e.g., trees create shade and leaf litter, which
  lowers ground surface temperatures, which
  decreases evaporation, which increases soil
  moisture

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Species Characteristics

• Lets look at light and moisture.
• Species vary in their tolerance for shade and
  drought
• Those tolerances tend to be inversely correlated

drought tolerance
shade tolerance
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Species Characteristics

• Lets look at light and moisture.
• Species vary in their tolerance for shade and
  drought
• Species vary in their ability to get light (be
  tall) and ground water (have deep roots).

Height
Root depth
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Environmental Modification

• Light availability at surface
• Soil organic matter
• Nutrient availability
• Moisture availability (soil, root zone)

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All of these factors contribute to ecological
change, or succession.Succession comes in two
flavors.

• Primary succession
• Secondary succession

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Primary Succession

• Development of biotic communities where none
  existed previously, e.g. on bare rock.

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Primary succession
Bare rock
Etc.
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Where have we seen this?
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Secondary Succession
Trees 10 m. 10 m Low High
Bare 0 0 High Low
Weeds 10 cm 10 cm Med Med
Grasses 1 m 1 m Low High
Shrubs 3 m 3 m Low High
Trees 10 m. 10 m Low High
Vegetation Canopy ht. Root depth Surface light
Soil moisture
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Where have we seen this?
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Secondary Succession Where does it lead?

• In the absence of anything to stop or slow it
  down, succession leads to a stable end state,
  sometimes called the climax community
• There can be more than one possible end state
• Usually something happens to slow it down or stop
  it
• Disturbance

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Disturbance

• Any change that causes resources to become
  available that were formerly fully utilized
• Disturbance may lead to stability
• Disturbance can foster diversity

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Simulation