Community Structure

***The following notes are taken directly from –Species Interactions and Community Structure, Chapter 17 of  Ecology:  Concepts and Applications, 5th edition, by Manuel C. Molles Jr.***




*Some of the most easily documented examples of interactions within communities are feeding relationships.


*A food web is a summary of the feeding interactions within a community.

*Direct interactions between two species, including competition, predation, herbivory and mutualism, involve positive or negative effects of one species on another without the invovement of an intermediary species.

*In indirect interactions, one species affects another through

*A commensalism is an interaction between two species in which one species is benefited and the other is neither benefited nor harm a third, intermediary species.

*Indirect commensalism occurs when the activities of one species indirectly–that is, through an intermediary species–benefits another species without itself being helped or harmed.

*Apparent competition occurs when negative results are the result of two species sharing a predator or herbivore or by one species facilitating populations of a predator or herbivore of the second species.

*The feeding activities of a few keystone species may control the structure of communities.

*A keystone species is a species that has an influence on the health of its ecological community which is inordinate to its biomass.  (This concept is further explained and discussed in the following article:

*Dominant, or foundation species are those species that have substantial influences on community structure as a consequence of their high biomass.

*People have long manipulated food webs both as a consequence of their own feeding activities and by introducing or deleting species from existing food webs. –Many of these manipulations have been focused on keystone species.

*The following article discusses the fundamental importance of specific keystone species and their relationship to humans:


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