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Chapter 16

BIOLOGY 2F03 Chapter Notes - Chapter 16: Log-Normal Distribution, Species Evenness, Ecological Niche

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D R.Kajura

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Chapter 16- Community Structure & Function
Community – an association of potentially interacting species inhabiting some defined area, at
some particular scale, over some particular span of time
Community Structure – attributes such as the number of species, the relative abundance of
species and the kinds of species comprising a community
Species Abundance & Diversity
Species Diversity Combination of the number of species and
their relative abundance
Species Richness (s) The number of species within a community
Species Evenness Relative abundance of the different species
within a community
Gamma Diversity Measuring diversity of the entire region
(grassland, wetland, forest, etc.)
Alpha Diversity Measuring diversity of local area (Grassland)
Beta Diversity Differences between communities within the
region of landscape
Guild Group of organisms making their living in a
similar way
- Seed-eating animals in the boreal
- Fruit-eating birds in a tropical rain
- Filter-feeding invertebrates in a
Dominant species – substantially more common than other species
oCalculated through counting, biomass estimations, basal area estimations
oFrank Preston – viewing populations as relatively abundant to each other
oRobert Whittaker – plotted abundances using coverages rather than number of
individuals representing how much in percentages these organisms cover
oRobert May – proposed that the lognormal distribution is the product of many
random environmental variables acting upon the populations of many species
oGeorge Sugihara - suggested that lognormal distribution is a consequence of the
species within a community subdividing niche space, and this alone could result
in there being relatively few very abundant species
Ecologists use two factors to define diversity: (1) species richness and (2) species
oEvenness can be thought of as inverse of dominance
oEven community has no dominant species
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oCommunity with 20 species is obviously less diverse than one with 80 species
So the first picture and the second have the same species richness
But, (b) has more diversity because it has more species evenness
o84% of individuals in community (a) are only one species
oCommunity (a) has a dominant single species!!!
oBasically if you walked around in the second forest you’d see a lot more stuff
that’s different so it has to be more diverse, even though technically it has the
same species richness as the first forest
oShannon-Wiener Index:
H = value of the diversity index
Pi = proportion of the “i”th species
S = the number of species in the community
The minimum value of H is 0, which is the value of H for a community
with a single species, and increases as species richness and species
evenness increase
oPielou’s measure of evenness (J)
H = the value of the Shannon-Wiener diversity index
Hmax = the total possible H for the number of species in the sample
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