Figure 6 also shows additional variables to find the total food requirement. When the land is cultivated with sufficiently long fallow period, the Tsembaga obtain a yield of around 4.4 Mcal/year for each acre under cultivation. Let's use this normal yield to determine the acres under cultivation and the fraction of the total arable land that would be under cultivation.
Figure 6. Adding the cultivation requirements to the previous model.
The cultivation requirements associated with the growing populations are shown in Figure 7. In the early years of the simulation, there are around 150 humans, 150 pigs and a total requirement of 200 Mcal/yr. Since each acre can yield 4.4 Mcal/yr, cultivated land is only around 50 acres, 5% of the total arable land. At this utilization, the slash and burn practices would not rotate back to the same area for 20 years. This rotation interval is sufficient to allow the normal yield to be sustained indefinitely.
Figure 7. Simulated food and cultivation requirements if warfare is prohibited.
Now, consider the cultivation requirements near the end of the simulation. By this time, the human population has climbed to over 600, and the pig population is varying between 150 and 600. The total food requirements would average 600 Mcal/yr, around three times larger than at the start of the simulation. To meet this need, the Tsembaga would have to maintain 150 acres of land under cultivation. This is 15% of their total land, so the rotation period would be down to less than 7 years.
Shantzis and Behrens argue that this rapid rotation would not allow sufficient fallowing time. They raise the prospects of a vicious circle of declining yields, increased cultivation, still shorter rotation periods, and still further declines in yields. They use their model to demonstrate that the end result could be starvation, first for the pigs and then for the humans.
Discussion
These introductory simulations help us appreciate some of the complexities of the Tsembaga system. The simulations also reveal the dynamic problem of interest to Shantzis and Behrens. They used their model to study the sustainability of the Tsembaga population with and without warfare. They concluded that warfare was an integral part of the overall system. They believed that the system had evolved into a homeostatic mode of behavior that controls the population at levels that could be sustained indefinitely. Their simulations without warfare showed an unsustainable situation, and they questioned the wisdom of those who would intervene to disallow warfare.