BIO226 : Community : Community Structure : Lesson

Glossary terms that are important in this lesson: Association, continuum, diversity index, ecotone, emergent properties, food web, gradient, guild, log-normal distribution, power function, relative abundance, species richness, trophic level

I. Community Structure

1. Hypotheses
   • Populations are integrated into biological communities of interacting organisms
   • A biological community is an assemblage of independent species

2. Concepts
   • Holistic concept
     • A community is analogous to a superorganism: populations are like the organs in a body
     • Community has emergent properties: it is more than the sum of its parts
   • Individualistic concept
     • Community structure and function express interactions of individual species in local associations
     • Natural selection acts on individuals but not on communities
   • Mixed concept
     • Species evolve out of "self-interest"
     • Communities are assembled by chance
     • Attributes of communities result from interactions among species reinforced by coevolution
   • Community properties
     • Species richness: the number of species
       • More species live in tropical than in temporal and boreal zones
       • 1.5 million species have been described, 10-30 million exist; human activities are causing species to go extinct faster than they can be described
       • Trophic level: feeding level (from the sun) within a community
       • Guild: method or location of foraging within a trophic level
     • Relative abundance: a few species are abundant and many more are rare

3. Definition of community
   • An association of plants and animals occurring in a particular locality and dominated by one or more prominent species or by a physical characteristic: ponderosa pine community, pond community
   • Functional definition based on organism interactions
   • Migratory animals tie communities in different locations together: songbirds
   • Intricate web of structure resulting from the functional interactions among component species

4. Natural units of organization
   • Closed community: species belonging to a community are closely associated with one another: their distributions coincide (Clements)
   • Open community: each species is distributed independently: community limits are arbitrary
   • Ecotone: region of rapid replacement of species along an environmental gradient
     • Physical environment changes abruptly: transition from terrestrial to aquatic, drastic changes in substrate (e.g., serpentine soils)
     • Change from a dominant species or life form: transition from deciduous to coniferous vegetation, grassland to forest
     • Climate influences: frost line
   • Continuum concept
     • Populations gradually replace one another along gradients of physical conditions
     • In eastern U.S., there is a N-S gradient of temperature, E-W gradient of rainfall
     • Each species has a unique evolutionary history with a variable degree of association with others
   • Gradient analysis
     • Abundances of species related to physical conditions
     • Species occupied unique evolutionary distributions, with peaks of abundance scattered along the environmental gradient: consistent with open community structure
     • Examples: red oak, white oak, and beech in Smoky Mountains

5. Trophic structure and food webs
   • Analysis includes species information as well as functional interactions
   • Hypothesis 1: Increasing complexity of community organization leads to increasing dynamic stability
   • Hypothesis 2: Greater complexity creates inherent time lags which decrease stability
   • Keystone predator: controls community structure by pressure on the dominant competitor in the trophic level below
   • Attributes of communities and food webs relevant to stability: diversity, connectance, food chain length, omnivory, compartmentalization

6. Relative abundance
   • Raunkiaer found a regular pattern in which there are few dominant species with high abundance but most species are represented by relatively few individuals
   • Log-normal distribution
     • Population sizes result from many independent factors, each multiplying its small effect
     • Veil line hides species too rare to be found in a sample
     • Can predict the species present in the community but not found in the sample
   • Diversity indices
     • Total number of species in a sample varies with sample size
     • Contribution of a species to diversity depends on its abundance
     • Simpson's Index
     • Shannon-Weaver Index
   • Sample size and species richness
     • Use rarefaction to statistically create equal-sized subsamples for comparison

7. Species and area
   • Species-area relationship can be characterized by a power function
   • Hypothesis 1: Values of z might result from the statistical character of the log-normal distribution
   • Hypothesis 2: Values of z result, at least partly, from biological processes
     • Continental areas have lower values than islands of comparable size

When you have completed this lesson, go on to Review Questions