chapter 17 community structure a community has attributes that differ from those of its components...
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Chapter 17 Community Structure
• A community has attributes that differ from those of its components– Number of species– Relative abundance of species– Nature of species interactions– Physical structure
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17.1 The Number of Species and Their Relative Abundance Define Diversity
• Species richness (S) is the count of the number of species occurring within the community
• Relative abundance represents the percentage each species contributes to the total number of individuals of all species
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Species Rank - Abundance Curves
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Barlow J et al. PNAS 2007;104:18555-18560
primary forest
secondary forest
plantation forest
Species Accumulation Curves
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17.1 The Number of Species and Their Relative Abundance Define Diversity
• Diversity indexes provide a way to quantify the relationship between species number and relative abundance
• Simpson's index (D) = (ni/N)2
= summation for all species
– ni = number of individuals of species i
– N = total number of individuals of all species– D ranges between 0 and 1 and as both
species' richness and evenness increase, the value approaches 0
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17.1 The Number of Species and Their Relative Abundance Define Diversity
• Simpson's index (D) = (ni/N)2
• Simpson's reciprocal index or Simpson's diversity index = 1/D– The lowest possible value is 1, representing a
community containing only one species– The maximum value is the number of species
in the community (species richness: S)– Maximum value with perfect even-ness of
relative abundance
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17.1 The Number of Species and Their Relative Abundance Define Diversity
• Shannon (or Shannon-Weiner) index = H = -(pi)(loge pi)
– Relative abundance of each species =
pi = ni / N
– pi = proportion of species i
• In the absence of diversity, where only one species is present, H = 0
• Hmax = ln S, occurs when all species are present in equal numbers
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Species abundance (or biomass)
Species Ecological Effect
RARE
KEYSTONE DOMINANT
UMBRELLA
INDICATOR
Not all species within a community are equivalent
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OTTER RECOLONIZATION
Kelp recovery in some cases
But not in others (hysteresis)
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17.9 Two Contrasting Views of the Community
• Clements: Organismal concept of community organization and structure: communities are sets of species that are co-adapted to biotic interactions with one another
• Gleason: Individual concept of community organization and structure: each species largely responds to environmental gradients independently
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Clements
Gleason
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Ecotones ARE easily recognizable breaks in community structure, but often arise from abiotic factors, rather than biotic interactions among species
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