Community Organization and dynamics

Ecological Communities can be open or closed.

  • a cave is a closed community because of the distinct physical boundaries. Islands and peninsulas are other examples of closed communities
  • most communities are open, their boundaries blend and interact with neighboring communities.


Habitats within a community afford organisms the necessary energy and materials for survival. They provide the ecological niche each organism needs to play its role in the web of life. The ecological niche is the sum total of the organism's use of the biotic and abiotic resources in its environment.

By using software and data presented at the BioQUEST Curriculum Consortium ( you can use images of organisms in their habitat and your computer to process and quantify the ecological niche of these organisms

Niches have the following properties:

  • they are the organism's job description and define how that organism utilizes its resources
  • they vary in scope -- some are broad some narrow
  • fundamental niches include all the ways an individual can potentially interact with its surroundings
  • the realized niche is the limited set of actual interactions between an organism and its environment.
  • only one species can reside in a given niche at a time. This is known as the principle of competitive exclusion. It seems "that two species with similar needs for the same limiting resources cannot coexist in the same place." This is known as Gause's Principle

 A couple of examples of niches are provided by Gondar Design Ecology Web.

Community Structure

A community's structure is defined by the activities and abundances of its diverse organisms. The species richness of a community refers to the number of different species found in a community.


The general ways in which species interact are listed in the table below.

Interspecific Interactions


Predation (+ -) (includes parasitism)

The interaction is beneficial to 1 species & detrimental to the other

Competition (- -)

The interaction is detrimental to both species

Commensalism (+ 0)

One species benefits from the interaction but the other is unaffected

Mutualism (+ +)

The interaction is beneficial to both species

The factors that structure communities include:

  1. competition
  2. predation
  3. environmental patchiness

Visit a community Mt. Graham in Arizona demonstrating applied conservation biology.

1. The role of competition in community structure.

competition is any struggle for survival between two individuals for the same resource. Competition may occur between or within species. Competition between different species is most important in forming community structure.

For a thorough discussion of competition and cooperation visit Virginia Tech. - Department of Entomology.

Two types of interspecific competition exist:

  • exploitative competition occurs between organisms struggling for the same limited resource.
  • interference competition involves one organism blocking another's access to a resource, and involves actual fighting over those resources.

Example: Two species, A and B, occupy adjoining environmental patches that differ in several abiotic factors. When species A is experimentally removed from a portion of its patch, species B colonizes the vacated area and thrives. When species B is experimentally removed from a portion of its patch, species A does not successfully colonize the area. From this one might conclude that:

species A is limited to its range by abiotic factors and species B is limited to its range because it cannot compete with species A

Through resource partitioning, slight variations in niches allow closely related species to coexist in the same habitat. Such partitioning may be the "ghost" or telltale clue of past competition between the closely related species.

Members of the same species can avoid destructive competition by either of two types of behaviors: Territoriality or Dominance Hierarchies.

  • Territoriality involves the establishment of boundaries inclosing areas for mating or other purposes.
  • Dominance hierarchies include an initial struggle to establish a pecking order among the members of a close-nit community.

Both behaviors generally involve males, but is not limited to them. In times of shortages, social organization can lead to a more equal distribution of resources but, reproduction may be restricted to dominant individuals.

2. Community structure can be influenced dramatically by disease, parasitism, or predation.

  • Disease and parasitism can cause selective weakness and death in segments of the population. Other members of the community unaffected by the pestilence can move in and populations expand as they take advantage of the other's weakness.


  • Predation influences both predator and prey numbers. According to Lotka-Volterra theory increases or decreases in prey numbers are closely tracked by the numbers of predators. Thus the prey numbers have a negative feedback on predator numbers.

Visit the preditor-prey simulation at CSC 171 : Problem Solving With Computers.

You may want to go to a lecture by Thomas J. Herbert, University of Miami, which details some problems with, and solutions to the Lotka-Volterra theory

The use of natural predators or parasites by farmers and gardeners to reduce insect pests is called biological control.

Defenses against predators

Coevolution has produced a dynamic equilibrium between species in a well established climax community. Coevolution is marked by reciprocity (give and take) and specificity (high degree of likeness)

A change in one species acts as a selective force on another species followed by a counter adaptation by the first species, and so on. Coevolution has been studied both in predator-prey relationships and in mutualism


The following are examples of coevolution

  • adaptations of flowers to attract their exclusive pollinators
  • a parasite that is specific for one host


Aposematic coloration -- warning coloration -- of monarch butterflies and predators that learn not to eat them is not an example of coevolution because specificity in color is not a warning factor, rather only that the color is bright and memorable. This form of warning must be backed up with some form of chemical or physical defense which traumatizes or injures the predator.

Cryptic coloration defends an organism by camouflaging it -- making it more difficult to detect against its background.

Deceptive marking such as eye spots on butterfly wings are another form of adaptive coloration. This is often coupled with specific behaviors to fool the predator into thinking (and striking) the prey at a less vulnerable spot on its body.

Mimicry by either predator or prey is an attempt to appear, in terms of coloration, shape or behavior, like a different organism. The following is an example of a predator using mimicry &emdash; a snapping turtle whose tongue resembles a wriggling worm.

Examine each butterfly pair. The various morphs of the single species in the top row have mimicked the local variations of a different species from the bottom row. For more excellent examples of mimicry click the picture below.

This comparison is found on James Mallet's web page

See the article in Science News pg. 42, Vol. 160, July 21, 2001

3. The role of Environmental Patchiness in community structure.

Habitats that are more diverse (heterogenous) can support a wider variety of organisms making a richer community. Biodiversity is greatest in tropical rainforests because of the 4 levels of plant life capable of living there. Each level can sustain a wider variety of animals because of the layering of plant life and the enormous potential for resource partitioning.

The environmental heterogeneity can be both spatial and temporal (seasonal changes).

Modified Nov. 3 2001