Transcript Describing communities

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
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
Species Rank - Abundance Curves
Species Accumulation Curves
primary forest
secondary forest
plantation forest
Barlow J et al. PNAS 2007;104:18555-18560
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
17.1 The Number of Species and Their
Relative Abundance Define Diversity
• Simpson's index (D) = (ni/N)2
• Simpson's index of diversity = 1 – D
– The greater the value of D, the lower the
diversity
• 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)
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
Not all species within a community are equivalent
KEYSTONE
DOMINANT
Species Ecological
Effect
RARE
UMBRELLA
INDICATOR
Species abundance (or biomass)
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: Organismic concept of
community organization and structure
• Gleason: Individual concept
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Quantifying Ecology 17.1 Community
Similarity
• Distinguishing between communities
based on differences in species
composition is important to understanding
the processes that control community
structure
• Sorensen's coefficient of community
(CC) is based on species presence or
absence
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Quantifying Ecology 17.1 Community
Similarity
• CC = 2c/(s1 + s2)
– 2c = number of species common to both
communities
– s1 = number of species in community 1
– s2 = number of species in community 2
• CC ranges from 0 to 1 (= 1.0 when the
species composition of the two
communities is identical)
© 2012 Pearson Education, Inc.
Quantifying Ecology 17.1 Community
Similarity
• Percent similarity (PS) is based on the
relative abundance of species within the
communities being compared
• PS ranges from 0 (when the communities
have no species in common) to 100 (when
the relative abundance of the species in
the two communities is identical)
© 2012 Pearson Education, Inc.