Community ecology part 1

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Community ecology (part 1)
--What is a community? --How to measure
community structure? --What factors determine the
number of species? --How do species interactions
influence community structure
function? --How do communities change over
time? ?disturbances ?intrinsic processes
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--What is a community?

• A community
• is defined by spatial boundaries
• includes all popns within
• But
• ? migratory animals?
• ? other linkages across boundaries?

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Communities often named for dominant species
ponderosa pine forest
deciduous forest
riparian forest
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Holistic vs. individualistic concepts of community
holistic view
each species contributes to the dynamics of the
whole community interspecies relationships
enhance stability and nutrient cycling
individualistic view
species distributed at random, according to
limits and tolerances of individual species (not
organized above species level)
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• Intermediate view of communities
• communities not organized
• many/most species interactions (SI) are
  antagonistic
• (i.e. consumer-resource)
• some attributes of communities arise from SI
• SI reinforced by coevolution biological
  selection

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• ecotones occur where (1) abrupt local physical
  change,
• (2) range of one species dictates distribution of
  other spp.

cooler
hotter
wetter
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• Examples of factors that maintain ecotones
• conifer needles ? soil pH
• dominant grasses reduce soils moisture (exclude
  shrubs)
• dominant shrubs shade grass seedlings (? growth)
• prairie fires kill tree seedlings (grasses
  rebound)
• moist forests resist fire disturbance

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but other community boundaries are more gradual
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and most species have different geographic
ranges
12 tree species, but no broadly overlapping
ranges
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--How to measure community structure?

• count species richness
• partition species into
• guilds (assemblages)
• based on method or
• location of forage

Ex forest on Barro Colorado Island, Panama
Ex forest on island in Bay of Fundy, NB, CA
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• organize feeding relationships into food web

limpet
sea urchin
chitin
chitin
limpet
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? two communities with similar diversity may have
very different structures
--how many omnivores?
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? communities with greater species richness
typically have more trophic levels and more
feeding guilds
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• rank-abundance plots describe pattern of
  relative abundance

most species are rare
ex plant species in peat bog sampling areas (25
x 0.1 m2)
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? rankabundance plots are used to compare
different guilds, assemblages, or communities
Ex1 forest birds in WV
Ex2 vascular plants in fir forest, Great Smoky
Mtns., NC
Ex3 vascular plants in deciduous forest, GSM,
NC
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• species-area plots

--Why would bigger islands have more species?