AEC 360  Ecology

Lecture 24:  Competition -- classic studies

I.  Background
    A.  Definition:  Any use or defense of a resource by one individual that reduces the availability of that resource to other individuals

        1.  These interactions can be either intra-specific (within a species) or inter-specific (between different species).

        2.  An increase in N1 causes a decrease in the growth rate of N2.

        3.  Indirect evidence for competition includes character displacement.

   B.  Types of competition

        1.  Exploitation Competition - individuals compete indirectly through their mutual effects on “shared” resources.

        2.  Interference Competition - direct interactions between competitors over shared resources

            - Classic experiment with barnacles (Fig. 16.10)
        3.  Other examples of interference competition -- plant chemicals and fire

        4. Apparent competition   (Fig. 16.18)

        5.  Exploitation Competition and Gause's experiments with protozoans (Fig. 16.4)
      C.  Intensity of competition related to limitation of resources

        1. Resources are consumed; thus their amounts are reduced.
        2. A consumer uses the resource for its own maintenance and growth
        3. Lower availability of resources leads to lower population growth rates


    D.  How do many species that are competitors coexist (that is, live together in their environments)

        1. In simple systems, the best competitor for a resource wins
        2.  The best competitor can persist at the lowest resource level
        3.  Competitive exclusion principle: Two species can not coexist on the same limiting resource
        4.  In early ecological theory, the number of coexisting species was considered to be limited by the number of resources
        5.  The problem: there are only a a small number of limiting resources

        6.  Classic experiments with flour beetles

                - shows how environmental conditions can alter the outcome of exploitation competition 

   E.  How can many different competitors coexist?

        1.  There are strong evolutionary tradeoffs between the ability to grow rapidly & exploit resources and the ability to tolerate stress

        2.  Examples with barnacles and chipmunks show how competitors and stress tolerators divide the environment

        3.  These tradeoffs extend to the entire community

            -  ants

            -  plants

        4.  Apparent competition (that is, competition mediated by a predator) can also promote species coexistence, as consumers may reduce the abundance of the dominant competitor (Fig. 16.12 and 16.17)

        5.  Coexistence can also be explained by environmental variation for multiple resources (Fig. 16.2, 16.9)
             -- Each species persists on a different combination of resources
             -- Thus, different parts of the environment are suitable for different species