Community Structure (Chapter 21)

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• Community Structure (Chapter 21)
• Defining communities
• Patterns of community organization
• Patterns of diversity

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• Community association of interacting
  populations
• Different ways that communities are organized

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• Because there are so many species, its useful to
  group species by how they use resources
• Guild group of species that use resources in a
  similar way

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• Types of guilds
• foraging use the same food resources or feed in
  the same locations
• nesting reproduce in the same places
• Guilds can include diverse, unrelated species
• seed-eating animals in the desert
• ants, rodents, birds
• cavity-nesting animals in forest
• woodpeckers, squirrels, raccoons, wasps, bees,
  ants, snakes

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• Why are guilds useful?
• simplifies analysis of communities
• a few guilds vs. 1000s of species
• allows comparison across locations
• organisms that use the same resources will
  respond to environmental changes in similar ways

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• Biome community type distinguished by dominant
  plant form
• Ecotone a zone of transition between two
  habitat types

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Sharp ecotone
Soft ecotone
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• Ecotones are often caused by underlying
  environmental gradients

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• Ecotones often have more species than either of
  the two habitats does alone
• Some species are specifically adapted to edge
  conditions
• Ecotones are maintained by plants themselves,
  fire, or competition

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• Two ways of looking at a community
• Holistic view
• species in a community act as a superorganism,
  coevolved to act as one unit
• can only understand the community by
  understanding the species together, not
  separately
• distributions of species in a community coincide

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• Two ways of looking at a community
• Individualistic view
• communities are aggregations of populations that
  happen to be in the same place
• each population has its own independent dynamics
• distributions of species are individualistic, in
  response to environmental conditions

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• Holistic view leads to the idea of a closed
  community one in which each species
  distribution coincides with the boundaries of the
  community
• Individualistic view leads to the open community
  concept each species has its own limits, so the
  boundaries of a community are abitrary

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• The continuum concept
• Ecotones tend to be soft, with overlap between
  communities across the ecotone
• Distributions of plant species tend to be
  independent of one another

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• The continuum concept is the idea that plant and
  animal species continually replace each other
  along environmental gradients
• temperature
• precipitation
• soil types
• soil moisture

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• Within any community species differ in how
  abundant they are

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• For any different type of organism in a community
• a few species will be very abundant (dominant
  species)
• most species will be relatively rare

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• Diversity
• Diversity the variety of taxa in a particular
  place
• Species richness the number of species in a
  community
• varies widely from place to place
• can be very high in some locations

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• Because species can differ in abundance, richness
  may be a poor measure of diversity
• Evenness how evenly abundances are distributed
  among species

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• Two diversity indices that account for evenness
• Simpsons index
• pi the proportional abundance of species
• (count of species i)/(total sample
  size)
• varies from 1 to S (the species richness)
• larger numbers indicate more diversity

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• Two diversity indices that account for evenness
• Shannon-Wiener Index
• pi the proportional abundance of species i
• maximum ln (S)
• larger numbers indicate more diversity
• often reported as eH

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