Two populations may compete by means of negative interactions. As shown in Figure III-5, each population may have a negative effect on the other population. The competiting populations are shown without food limitations (E is constant) and each population has a self-crowding limitation, which was shown, in section III.2 on Logistic growth. Without interactions the two populations may coexist as in the simulation curve in Figure III-5b.

With negative interactions neither can maintain as much growth nor as high a population at steady state. With sufficient negative effects, one population may drive out the other, as shown in Figure III-5b, a process called competitive exclusion. See another example of competitive exclusion in Section III.3.

Examples of Competitive Interactions

An example of competitive exclusion is competition between two species of grain beetles in a jar in which adequate food (grain) is added each day. If a few of either species are put into the jar alone, the number of beetles increases up to a steady level. If a few of both species are put into the jar together, they both increase, but soon the more efficient species consumes all the food and the other species dies out. This example is appropriate if the action of one species negatively affects the other through waste products, disturbing egg production, or other means.

Another example is the competition between two species of weeds, where each secretes chemical substances that inhibit the root growth of the other.

"What if" Experiments

Study the way the two logistic-growing populations interact by making changes and running the simulation. Then make a change by retyping a line. Then run the simulation. After each simulation run, type? Q,Q2 and the last values of populations Q and Q2 will appear on the screen. In this way you can see which population curve is Q and which is Q2.

1. First, set the two interaction coefficients (K5 and K6) to equal zero. Then run. What happens to each population? Is there competitive exclusion? How does your result compare with Figure III-2b.
   


2. Next, with interactions still set at zero, decrease the available food supply (Energy concentration maintained = E) to be half of what it was in simulation # 5.1. How was growth and steady state level changed?
   


3. Next set the interaction coefficient K5 = 0.0005 and run again. What happens to populations now? Is there competitive exclusion?
   


4. Next set the second interaction coefficient K6 = 0.002 and run again. How is competition affected now? Can you increase K6 to a high enough value to displace the other species? (Change the coefficient and run again).

COMPUTER MINIMODELS AND SIMULATION EXERCISES FOR SCIENCE AND SOCIAL STUDIES

Howard T. Odum* and Elisabeth C. Odum+
* Dept. of Environmental Engineering Sciences, UF
+ Santa Fe Community College, Gainesville

Center for Environmental Policy, 424 Black Hall
University of Florida, Gainesville, FL, 32611
Copyright 1994

Autorização concedida gentilmente pelos autores para publicação na Internet
Laboratório de Engenharia Ecológica e Informática Aplicada - LEIA - Unicamp
Enrique Ortega
Mileine Furlanetti de Lima Zanghetin
Campinas, SP, 20 de julho de 2007