Energy for the growth of this system comes from two sources, one a steady-flowing renewable source, like nutrients flowing into a pond from a stream (J). The other source is a temporary nonrenewable source, like one-time runoff of fertilizer into the pond (N). This model (Figure II-4a) is a combination of the last two models. Renewable and Nonrenewable. At first the two sources of fertilizer nutrients stimulate a surge of growth of plants (Q). When the nonrenewable source is used up, the growth declines back to a steady state based on the renewable source (Figure II-4b).
The equations are a combination of the renewable model and the nonrenewable model.
Examples of Two-Source Models
Another example is the growth of a population of microbes, which decompose leaves. This autumn the forest leaves fall and also the household rakes the lawn and puts a pile of leaves in the forest. The microbes grow up quickly on both sources of food, but when the pile of leaves is used up, their population decreases to the number that can live on the steady leaf-fall.
We sometimes call this model, "the future model" because represents one view of the past and prediction of the future of society. The world economy grew up on both fossil fuels and renewable sources. As the nonrenewable fossil fuels run out, the economy will have to come down to a level using only renewable resources. Where do you think the world is on this curve? How about the United States?
"What If" Experiments
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