Community Ecology

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Community Ecology

• What is a community?

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Community Ecology Outline

• Community concepts
• Succession in communities
• The process
• Mechanisms and models

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Historical ideas of the community
Community Concepts

• Josias Braun-Blanquet (1932)
• The floristic-sociological approach,
  phytosociology
• Communities discrete
• Defined by associations of indicator species
• accidental taxa removed
• Concerned with pattern not process

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Historical ideas of the community
Community Concepts

• Frederick Clements (1916, 1928)
• Organismal view, or super-organism concept
• Communities discrete
• Species interdependent
• Emergent properties
• Climax community determined by climate

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A Clementsian Community Concept
Community Concepts
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Historical ideas of the community
Community Concepts

• Henry Gleason (1917, 1926, 1927)
• Continuum view, or individualistic concept
• Communities intergrade
• Interactions not specific
• a community is not an organism, but a
  coincidence
• Succession a dynamic process

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Tree IVs across a moisture gradient
Community Concepts
Fig 9.1
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Tree frequency along an altitudinal gradient
Community Concepts
Fig 9.2
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Modern Community Concept
Community Concepts

• Mostly Gleasonian, with a few Clementsian touches
• Pattern Communities are somewhat recognizable
  entities, but they intergrade
• Process? How do communities assemble?

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Biogeoclimatic zones as communities
Community Concepts

• Krajina was a student of the European school
• Used the relevé method, which is subjective
  placement of plots into discrete community
  associations
• Our zones reflect the European view that
  communities are discrete, and are more focussed
  on pattern than process

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Succession
Succession the process

• Directional change in community composition or
  structure over time
• Non-seasonal, continuous pattern of local
  colonization and extinction by different plant
  populations

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Definitions
Succession the process

• Sere unit of succession
• Seral stage distinct community within sere
• Pioneer community initial seral stage
• Climax community final seral stage
• Climax dynamic equilibrium, stable species
  composition
• Old growth complex forest, beyond climax

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Primary succession
Succession the process

• Succession on land not previously influenced by a
  community because soil must form, estimated to
  take 1000-5000 yr
• Classic example glacial retreat in Glacier Bay,
  Alaska
• Retreat documented since 1760, since 1900 retreat
  is gt 30 km

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Primary succession at Glacier Bay, AK
Succession the process
Means with the same letter are not significantly
different
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Glacier bay succession considered fast
Succession the process

• Sandstone, limestone, weather quickly
• Constant presence of N fixers

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Secondary succession
Succession the process

• Succession in an area where vegetation has been
  removed, but soil, propagules, etc. remain e.g.
  after cultivation, fire, storm
• Expected to take 100-200 yr
• Extensively studied in eastern, central US, less
  so here

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Old field succession in New Jersey
Succession the process
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Secondary succession in the CWH
Succession the process

• A typical sequence of species after logging or
  other disturbance
• Fireweed
• shrubs (esp. Rubus)
• Alder
• bigleaf maple, Douglas fir
• cedar and hemlock are climax

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Succession at Mt. St. Helens
Succession the process

• Many biological legacies allowed rapid
  recolonizationsimilar to secondary succession
  despite lava flows
• One of the first colonists lupine. N-fixing,
  self pollinating, abiotic dispersal

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Succession the process
Bare Lupine
Soil Nitrogen
mg/kg soil
J. Halvorsen, J. Bishop, et al.
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Succession in the Douglas Fir-Western Hemlock
Sere
Succession the process

• A recent review Franklin et al. 2002. Forest
  Ecology Management 155399-423
• Structural attributes of forest stands important,
  especially legacies after disturbance

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Succession the process
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Summary of changes during forest succession
Succession the process

• Establishment/initiation
• Competitive exclusion
• Maturation/diversification
• Dynamic climax

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Changes over progressive succession
Succession the process
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Other types of Seres
Succession the process

• Retrogressive succession habitat becomes more
  hydric or xeric, e.g. bog succession
• Cyclical succession, e.g. shrub communities

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Cyclical succession
Succession the process
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Summary Succession processes

• Distinction between primary and secondary
  succession can be unclear, but there are
  differences between processes occurring that
  appear repeatable
• General changes during succession can be
  described, although there is some debate about
  their timing