Chapter 16
• Most species are moderately abundant
• Species diversity is a combination of species richness and species relative
abundance
• Both abiotic and biotic factors jointly influence community structure
• Environmental complexity increases diversity
• Disturbance and diversity are linked
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What is a Community?
Classic definition says a community is a group of interacting organisms
– What defines the interactions?
– How strong are the interactions?
– How big is the group?
Species Abundance
There are regularities in the relative abundance of species in communities that
hold irregardless of the ecosystem.
Preston developed concept of distribution of commonness and rarity.
Lognormal Distribution (Fig. 16.3, 16.4)
Preston graphed abundance of species in collections as frequency
distributions.
– Lognormal Distributions
– Bell-shaped curves.
– In most lognormal distributions, only portion of bell-shaped curve is
apparent.
• Sample size has large effect.
• Significant effort to capture rare species.
Defining Community Structure
Two important parameters:
– Number of different types of species
• Equals number of species in the area of interest
• Also known as species richness
– Relative abundance of species
• Species i relative abundance equals the percentage of all individuals in
the area that are species i
• The distribution of species relative abundance is one way of quantifying
species evenness.
Richness and Evenness (Fig. 16.5)
• Community A
– 5 species, a total of 100 individuals
– Species A has 96% of individuals (high relative abundance)
– Species B, C, D, and E each have 1% of remaining individuals (low relative
abundance)
• Community B
– 5 species, a total of 100 individuals
– all 5 species have 20% of individuals (equal relative abundance
• Which community is “richer”?
• Which community is “more even”?
• Which community is more “diverse”?
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Species Diversity
Species Diversity Calculations
To “quantify” species diversity, we combine evenness and richness.
Shannon-Weaver Index (H) is most common:
Formula on Page 374
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Individuals or Aggregations?
Two contrasting views of communities:
– Clements
• Community as highly co-evolved “superorganism”
• Based view on consistent co-occurrence of similar species
– Gleason
• Community is simply loose aggregation of individual species
• Consistent co-occurrence due to similarities in environmental
requirements
Clements
Gleason
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Spatial Variation and Zonation
Zonation is simply spatial change in the physical and biological structure of a
community.
Who would predict clear boundaries between zones in species occurrence?
– Clements?
– Gleason?
– Why?
Heterogeneity in Tropical Forests (Fig. 16.14)
Variation in Algal Resources (Fig.16.11, 16.12)
• Pyramid Lake fed by Truckee River
• Nitrate and Silicate concentrations vary with distance from river inflow
• Would algal response support Gleason or Clements?
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Niches and Diversity of Algae and Plants
Algal niches appear to be defined by their nutrient requirements.
– Tilman found coexistence of freshwater diatoms depended upon ratio of
silicate and phosphate.
– Found conditions allowing coexistence.
– Diatoms held different trophic niches.
– Thus different diatoms would dominate different areas.
Given Tilman’s findings…
• How could we approach the analysis of algal communities in Pyramid Lake?
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Environmental Complexity (Fig. 16.9, 16.10)
In general, species diversity increases with environmental complexity or
heterogeneity.
MacArthur found warbler diversity increased as vegetation stature increased.
Measured environmental complexity as foliage height.
Disturbance and Diversity
Disturbance difficult to define as it involves departure from “average
conditions.”
– Average conditions may involve substantial variation.
Sousa defined disturbance:
– Discrete, punctuated
– Involves killing, displacement, or damaging of one or more individuals that
directly or indirectly creates an opportunity for new individuals to be
established.
Disturbance and Diversity
White and Pickett defined disturbance:
– Any relatively discrete event in time that disrupts ecosystem, community, or
population structure and changes resources, substrate availability, or the
physical environment.
– Two major characteristics:
• Frequency
• Intensity
Intermediate Disturbance Hypothesis (Fig. 16.18)
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Disturbance and Diversity in the Intertidal Zone (Fig.16.19)
Sousa studied effects of disturbance on diversity of algae and invertebrates
growing on boulders in the intertidal zone.
– Predicted level of disturbance depends on boulder size.
– Large boulders require more force to move.
– Boulders supporting greatest diversity of species were those subject to
intermediate levels of disturbance.