Community Organization and Structure

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Community Organization and Structure
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Ecotones
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Boreal Forest/Tundra Ecotone
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Community properties Abundance
Abundance ( individuals)

• Log Normal distribution of two species

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• Moths caught in light traps
• Number of species behind the veil
• Number of species found increases as the number
  of individuals sampled increases.

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Unpolluted stream
Number of diatom species
Polluted stream
Number of individuals per diatom species
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Community properties Species richness
Abundance of Bird Species found in tin-mining
areas in Indonesia
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Community properties Species diversity

• Species diversity is comprised of two separate
  items
• Evenness (how evenly distributed among the
  species are the individuals in the community?
• Species richness (number of species)
• The two measures together are referred to as
  heterogeneity

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Heterogeniety

• 5 species in this forest community (richness)

See Fig. 16.5
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Community properties Species diversity
Ways to measure Species Diversity

• Shannon-Wiener index (H)
• H -? pi loge pi
• Sums from 1 to the number of species (S)
• p proportion (decimal) of number in species i
  out of all individuals in the sample.
• negative value compensates for log of decimal
  (negative)
• Simpsons index
• A measure of dominance, but the reciprocal can be
  used as a diversity index
• Diversity or 1/?pi2
• Emphasizes the more abundant species

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Calculation examples

• Five species in the community
• Species A B C D E
• Community A 20 20 20 20 20
• Community B 60 10 10 10 10
• Community C 25 25 25 25 0
• Richness is the same for the first two
  communities, but species diversity is higher in
  Community A.
• Shannon Index
• A - .2(-1.61).2(-1.61).2(-1.61).2(-1.61).2(
  1.61) 1.61
• B - .6(-.511).1(-2.30).1(-2.30).1(-2.30).1(
  -2.30) 1.22
• C - .25(-1.39).25(-1.39).25(-1.39).25(-1.39)
  1.39
• Simpsons diversity index
• A 100(99)/20(19)20(19)20(19)20(19)20(19)
  5.20
• B 100(99)/60(59)10(9)10(9)10(9)10(9)
  2.52
• C 100(99)/25(24)25(24)25(24)25(24)
  4.12

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Richness gradients

• North to South sampling shows an increasing
  number of species (richness)

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• Mammals show similar trends

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Bird Richness
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Tree richness
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25
25
Snails
Tube worms
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20
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Species diversity
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10
5
5
Coast
Deep Sea
Coast
Deep Sea
0
0
0
2,000
4,000
6,000
0
2,000
4,000
6,000
Depth (meters)
Depth (meters)
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Intermediate disturbance hypothesis

• The highest species diversity is found at the
  intermediate level of disturbance.
• High disturbance kills off too many species and
  low disturbance limits them through dominance and
  competition.

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Coastal boulders as habitat
High disturbance
Low disturbance
Intermediate disturbance
Text Fig. 16.19
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Keystone species

• Species that help to determine the composition of
  the community
• Generally the top predator
• The predator tends to prefer the most abundant
  prey species
• Competition lowered predator regulates prey
  population sizes
• If keystone species is removed, extinction of
  other species through competition often occurs

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Community properties Interactions

• Change over time - Succession
• Energy transfer (food webs) and nutrient cycling
• Invasive species affect ecosystems through these
  means in addition to competition both directly
  and indirectly

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