Lecture 18: Community change

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Lecture 18 Community change

• EEES 3050

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Succession

• Succession
• Definition
• The process of directional change in vegetation
  during ecological time.
• Primary Succession
• Succession on new areas
• Secondary Succession
• The recovery of disturbed new sites.

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Example Dune succession

• Henry Cowles (1899)
• Studied sand dune vegetation on the shores of
  Lake Michigan.
• Brief history of Lake Michigan Lake levels.

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Historic Lake Levels
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Example Dune succession

• ideal system for studying succession.
• Why?
• Same initial substrate.
• Same relief
• Same available flora and fauna.
• Thus, only differences between dunes should be
  time, biological processes of succession, and
  random events.

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Example Dune succession

• Primary Assumption
• Exchanging space for time.

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Stages of dune succession

• Bare soil
• Bare sand produced by drop of lake and blowouts
• Wind continually shifts sands.

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Stages of dune succession

• 1) Bare soil
• Bare sand produced by drop of lake and blowouts
• Wind continually shifts sands.
• 2) Colonization
• Establishment of grasses (marram grass).
• Usually by rhizomes
• Likes moving sands
• Not found in areas after about 20 years.

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Stages of dune succession

• 3) Colonization of additional grasses.
• Help stabilize dunes.

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Stages of dune succession

• Colonization of additional grasses.
• Help stabilize dunes.
• 4) Colonization by woody species
• 1st usually cottonwoods
• Also willow and sand cherry.
• Dune becomes stabilized

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Stages of dune succession

• 5) After stabilization
• Jack pine white pine
• 100 to 150 years later
• Black oak trees.

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• What other systems can be used by exchanging time
  with space?
• Retreating glaciers.
• Examples of succession
• Old farm field
• Post-burn
• Tree gaps
• Volcanic lava flows
• Disease
• Human disturbance

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Succession Happens How?
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Succession an historical perspective.

• 4 major hypotheses
• Relay floristics/monoclimax hypothesis
• Initial floristic composition
• Tolerance Model
• Random colonization

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Succession an historical perspective.

• Clements view Monoclimax Hypothesis
• Called relay floristics by Egler

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• Clements view Monoclimax Hypothesis
• Key assumption
• Species replace each other through each stage
  because they change the environment such that it
  is more suitable for the next species.
• Climax community in any region is determined by
  climate.

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Nature and Structure of the Climax by Frederic
Clements (1936)

• Has a category for everything!
• Climax state
• Proclimax
• Disclimax
• Subclimax
• Climax community definition
• Final or stable community in a successional
  series. It is self-perpetuating and in
  equilibrium with the physical and biotic
  environments.

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Succession

• Initial floristic composition
• Proposed by Elger (1954)
• Succession is heterogeneous.
• Development depends on who gets there first.

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Succession

• Initial floristic composition
• Still species have no competitive advantage.
• Dominant community merely a matter of who gets
  there first and lives the longest.
• Succession proceeds from short-lived to
  long-lived species.

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Succession

• Tolerance model.
• Could lead to Tilmans resource ratio hypothesis.
• Species are replaced by other species that are
  more tolerant of limiting resources.

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• Random Colonization

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• Random Colonization
• Succession involves only the chance survival of
  different species and the random colonization by
  new species.
• Thoughts?
• Not a realistic scenario, but established a null
  model to test against.
• If not random, than some process must be at work.

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Back to the dunes
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What model fits the dune scenario?

• Monoclimax?
• Initial floristic composition?
• Tolerance?
• Random Colonization?
• We only examined the dominate species!
• Perhaps a mix of climax and random colonization.

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Example 2 Glacier Bay
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Glacier Bay Movie
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Changes in pH
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Changes in Nitrogen
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Types of species

• pioneers (r-species)
• short-lived
• high reproduction
• poor competitors
• emphasize dispersal
• climax species (K-species)
• long-lived
• low reproduction
• strong competitors
• emphasize survival

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Type of Succession

• primary
• new site, never before home to a community
• Examples ash flow, new sediments, retreat of
  glacier
• secondary
• disturbed site
• most common
• Examples abandoned farm field, flood, forest
  fire, disease/insect outbreak.
• regenerative
• replacement with same species
• Examples Periodic disturbance

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Community Change

• Patch Dynamics
• Small-scale changes in a community
• Examples
• Degeneration of plants
• Tree fall gap
• Dispersal of organisms
• Wave action
• Burrowing animals

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Treefall gap