Community structure

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Community structure and species diversity
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• Community an assemblage of species in a
  particular location
• Two central questions
• Does a community have emergent properties? That
  is, is a community more than the sum of its
  parts?
• How might changes involving individual species
  affect community function?

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Community structure the number of species,
their relative abundance, and the interactions
between them
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Species diversity has two components Species
richness (number of species s) Species evenness
(relative abundance numbers of individuals in
each species)
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• Causes of species diversity
• Habitat heterogeneity

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The Paradox of the Plankton Whatever happened
to competitive exclusion (or why are there so
many coexisting species) ?
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Freshwater phytoplankton
Asterionella
Cyclotella
Which species requires more silicate (key
component of outer casing)?
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Resource partitioning by marine phytoplankton
Stomp et al., 2004, Nature 432, 7013, p. 104
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Divergence in wavelength of light absorbed
allows stable coexistence
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• Causes of species diversity
• Habitat heterogeneity
• Intermediate disturbance

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Sousas experiment boulders in the intertidal
zone
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(unwitting agents of diversity)
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Pocket gopher creator of islands of plant
diversity on California grassland
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Grassy field with patches of other
vegetation the work of pocket gophers
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• Causes of species diversity
• Habitat heterogeneity
• Intermediate disturbance
• Large area

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Where is the greatest diversity in land plants?
The wet tropics with the largest contiguous land
masses (uninterrupted by oceans, mountains or
deserts)
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• Causes of plant diversity in
• tropical rainforests
• Habitat heterogeneity
• Intermediate disturbance
• Large area
• Long time
• High rainfall
• High solar radiation
• Non-freezing temperatures
• 8. Pest-pressure hypothesis

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