Before the era of our fossil fuel civilization, agricultural-based human economies had to rotate their land back and forth between a resting state and farming. During years of farming the soil usually loses structure, organic matter and nutrients so that farm productivity declines. In the resting (fallow) state plant succession was allowed to run for several years, rebuilding the vegetation and soil.

The simulation model has the human economic assets (Q) depending on the human productive activity based on the land in economic use (A2) and the stored products of environmental work such as wood and soil (S). The environmental support system has assets (B) which depend on the environmental sources 110) and the fallow land (A1). Land rotation is modeled by letting land cycle back and forth between fallow and economic use. When human assets increase, they cause more land to go into developed use (pathway L6*/A1*Q). Land is returned to succession in proportion to the seeding (K4*B and L1*A2*B) from the environmental stocks (B).

The simulation starts with a natural landscape, such as a forest cover with large storages of trees (B) but low economic assets (Q). As the economic system develops, land shifts from A1 to A2, and forest stocks are used up as they are transferred into trees ready to be processed (S) into economic assets (Q). Because of the large initial storages, there is a surge to a higher economic development than can be sustained. A steady state at a lower economic level results later as some land (A2) goes back to fallow state (A1).

In the diagram (Figure IV-10a) only the coefficient is labeled on each line. To find out what the whole expression is for a line, find the coefficient in its equation below the diagram.

Examples of Land Rotation Systems

This model illustrates what happens when a new area is colonized without much outside imports. It is appropriate for early slash-and-burn agriculture in the rain forest. There a very small part of the forest was in agriculture while most was in very slow succession.

The model also represents the whole human civilization in relation to environmental resources. In this example B is products of geologic production such as fuels and minerals. We are using up these products much faster than the small amount of resting land can produce them.

"What If" Experimental Problems

1. What would happen to the land rotation if so much of the original vegetation was destroyed that there were no seeds for reseeding?
   


2. What is the effect of starting with larger resource reserves? Set S = 100.
   


3. What is the effect of having less total land area? Set A and A1 = 50.

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