Communities

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Communities
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I. Patterns in local and regional diversity

• Number of species (or richness) of invertebrates
  in one location depends on the size of the sample
• Rarefactiontechnique to alleviate this problem

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I. Patterns in local and regional diversity

• Number of species of invertebrates depends on the
  area of the drainage basin sampled
• S CAZ, Z 0.2 to 0.4
• Why?larger
• basins include
• more habitats
• Increase S as
• move downstream

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I. Patterns in local and regional diversity

• Few common species
• Lots of rare species

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II. Community structure

• What species are in a locality
• Is the community a tightly interwoven network of
  strongly interacting species, or loose
  assemblages that persist in an area because
  individual species are adapted to that
  environment?

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II. Community structure

• Three hypotheses that address this question
• Harsh-benign
• Biological interactions dominant
• Patch dynamics

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II. Community structure

• Harsh-benign
• Environments vary from harsh (lots of
  environmental variation in space and time) to
  benign (little variation)
• Relative importance of biotic interactions varies
• High importance in benign environments
• Low importance in harsh environments

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II. Community structure

• Biological interactions structure communities
• Biological interactions permit only superior
  organisms to exist in constant environment, but
  some physical disturbance prevents dominance by a
  small number of superior species
• Are biological interactions strong?
• Competition probably occurs episodically
• Trophic linkages strongpredation trophic
  cascades
• But predation effects detected infrequently in
  fast flowing streams with organisms that have
  high mobility

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II. Community structure

• Patch dynamicsencompasses much of the other two

Extreme space unoccupied
Lotka- Volterra in a bottle
No competition
Resource partitioning