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Community Structure (Chapter 21) Defining communities Patterns of community organization Patterns of diversity PowerPoint Presentation
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Community Structure (Chapter 21) Defining communities Patterns of community organization Patterns of diversity. Community – association of interacting populations Different ways that communities are organized….

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Presentation Transcript
slide1
Community Structure (Chapter 21)
  • Defining communities
  • Patterns of community organization
  • Patterns of diversity
slide2
Community – association of interacting populations
  • Different ways that communities are organized…
slide3
Because there are so many species, it’s useful to group species by how they use resources
  • Guild – group of species that use resources in a similar way
slide4
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…
slide5
Why are guilds useful?
    • simplifies analysis of communities
      • a few guilds vs. 1000’s of species
    • allows comparison across locations
    • organisms that use the same resources will respond to environmental changes in similar ways
slide6
Biome – community type distinguished by dominant plant form
  • Ecotone – a zone of transition between two habitat types
slide7

Sharp ecotone

Soft ecotone

slide10
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
slide12
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
slide13
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
slide14
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
slide16
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
slide18
The continuum concept is the idea that plant and animal species continually replace each other along environmental gradients
  • temperature
  • precipitation
  • soil types
  • soil moisture
slide23
For any different type of organism in a community
  • a few species will be very abundant (dominant species)
  • most species will be relatively rare
slide24
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
slide26
Because species can differ in abundance, richness may be a poor measure of diversity
  • Evenness – how evenly abundances are distributed among species
slide28
Two diversity indices that account for evenness
  • Simpson’s 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
slide29
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