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

Barlow J et al. PNAS 2007;104:18555-18560

primary forest

secondary forest

plantation forest

Species Accumulation Curves

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 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

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

Species abundance (or biomass)

Species Ecological Effect

RARE

KEYSTONE DOMINANT

UMBRELLA

INDICATOR

Not all species within a community are equivalent

OTTER RECOLONIZATION

Kelp recovery in some cases

But not in others (hysteresis)

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

Clements

Gleason

Ecotones ARE easily recognizable breaks in community structure, but often arise from abiotic factors, rather than biotic interactions among species

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