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: 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) =  (n i /N) 2 –  = summation for all species –n i = 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) =  (n i /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 = -  (p i )(log e p i ) –Relative abundance of each species = p i = n i / N –p i = proportion of species i In the absence of diversity, where only one species is present, H = 0 H max = ln S, occurs when all species are present in equal numbers

Species abundance (or biomass) Species Ecological Effect RARE KEYSTONEDOMINANT 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

© 2012 Pearson Education, Inc.