Vulnerability to Extinction

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Vulnerability to Extinction
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Communities
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Definition - what is a community?

• An assemblage of plant species which interact
  among themselves and with their environment
  within a time-space boundary.
• John S. Beard - "A formation is a major kind of
  plant community on a given continent,
  characterized by physiognomy and a range of
  environments to which that physiognomy is a
  response. (Ordination and Classification of
  Communities, p. 358)

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Evolution of Community Concepts

• Natural History
• Plant Geography - von Humbolt (1850)

• Plant Sociology - Zurich and Montpellier - J.
  Braun-Blanquet.
• Uppsala - quadrat sampling and similarity
  measures.

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Succession

• 1899 - Cowles- dunes along lake Michigan.

Space-for-time-substitution
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Succession

• 1899 - Cowles- dunes along lake Michigan.

• Clements - Monumental work, Plant Succession
  (1916) showed change a part of the community.
  Developed into the organism view.
• Gleason and others contested this view.
• Whitaker - concept of the community as a dynamic
  grouping of populations, interacting, and
  somewhat distinctive in composition and structure.

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Whitaker - definition of community

• "a combination of plant, animal and bacterial
  populations, interacting with one another in an
  environment, thus forming a distinctive living
  system with its own composition, structure,
  environmental relations, development and
  function."

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Clements

• First real quantitative community ecologist
• Organism concept - based on the interactions of
  organisms in a community.
• Concept first proposed by Stephen Forbes, a
  limnologist. He stated, A group or association
  of animals or plants is like a single organism.
• "As an organism, the climax formation arises,
  grows, matures, and dies. Its response to the
  habitat is shown in processes or functions and in
  structures which are the record as well as the
  result of these functions." - 1916
• Succession ontogeny
• Plant competition is the most important element
  in causing succession.

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Community compostion over space

• As you move along a gradient, composition of a
  community tends to change.

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Clements - cont

• His concept would be what we term a closed
  community

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Sharp ecotones

• Salt marsh gradation

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Clements - cont

• Monoclimax theory - large areas should eventually
  reach the same basic species mix.
• local conditions may make modifications

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Clements concept could be summarized in four
points

• A community occurs over a large area and is
  shaped by the climate of the region.
• It takes long periods of time to reach the climax
  stage
• Areas or pockets will be found with a different
  community
• The community is like an organism - maturing and
  dying.

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Henry A. Gleason (1882-1975)

• "The vegetation unit is a temporary and
  fluctuating phenomenon, dependent, in its origin,
  its structure, and its disappearance, on the
  selective action of the environment and on the
  nature of the surrounding vegetation."

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Gleason community composition results from
random factors

• Dispersal is basically random.
• The local environment determines which plant will
  live.
• Similar environment leads to similar vegetation
• Environment of each spot of ground varies in time
• Environment varies in space
• No two communities are exactly alike or have
  genetic or dynamic connection.

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