When species are counted in a forest or other ecosystems, a graph may be plotted of species found and area of land counted. The resulting graphs look like that in Figure III-16b. It was generated by the model in Figure III-16a, which expresses a theory relating species to their resources. The idea is that more energy is required for the support of more species.

Because of the tendency for competitive exclusion, adding species requires some special resources to be used to keep the species cooperating or at least in different niches so they do not compete destructively. The energy required for interactions is represented by the probabilities of their interactions.

The program has a time iteration within an area iteration. For each value of area A the program builds up the value N until it is near its limit. Then it plots the point, increases the area, resets time and N to 0 and builds up N to its carrying capacity again.

Other Examples

The model may apply to the number of occupations in human societies in relation to the resource found and the area of cities sampled have a similar shape to the species-area graph.

The model may represent information and resources required more generally. The ability to organize, retrieve, and maintain information may go up as the square of the number of units. As one adds more resources, the number of books that can be supported and well used in a library may follow a curve like that in Figure III-16.

"What if" Experiments

1. What happens if the budget of energy is greater? Double E.
   


2. Suppose pathway K2*N represents the rate of loss of information due to the depreciation of the structures on which information is stored. What is the effect of increasing the depreciation rate. Increase E.
   


3. What would be the shape of the graph of species and area if the horizontal scale were put on a logarithmic scale. Substitute a plot statement with log function; multiplying by 50 spreads the scale as needed:
   

   220 PSET (50*log(A), 180-N*N0)
   

   When species-area curves are plotted with a semi-log scale (horizontal scale on log paper and vertical scale regular) they curve upward. How does the model compare with data in this respect?

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