Stella Activity for Population Growth (in Secret Readings.)
Modeling is the process of simplifying a system we wish to better understand. Systems that can be modeled range from the very simple to the extraordinarily complex. The reason we are using dynamic computer models in this text is to gain a working knowledge of, and an insight into the processes of global change. Because of the complexity of these processes, it is often not enough to read about the workings of these phenomena. We must think about the dynamics of a system, extract critical functioning parts, and attempt to build a model that captures its "essence" by making assumptions to account for external variables.
Modeling should be approached in a logical and ordered manner. Though following guidelines may seem tedious and unnecessary at times, it greatly increases the chances that your model will (a) work and (b) be a relatively accurate representation of the actual behavior of a real-life system. At its essence, modeling is a 5 part process:
This is where most people get into trouble. Aside from programming problems, most models give incorrect results because (a) the system under study was not understood well enough or (b) the modeler did not have a clear idea of what the model was actually supposed to show. Make note of any assumptions needed to simplify the model at this stage.
Figure out the crucial components of the system. (These are represented as stocks, flows, converters and connectors in STELLA.) Exclude any components that are unnecessary. Keep the model as simple as possible, as unnecessary complexity will make it difficult to understand and modify later on.
Sketch out the connections between model variables. Once you have made all of the relevant connections, use STELLA icons to build the model.
Run the model with different data in order to find out if it works, what its limitations are, and where and when it breaks down.
5. Verify the Model
Though not always possible, try to test your model against real world data. Although we will not be strictly following every step of this guideline in the development of the models in this text, it is important that you keep them in mind as you develop them now and especially in the future.
Moai Figure. More Moai are
here
For this exercise we will explore a model based on the inhabitants of Easter Island. Easter Island was first settled by a small party of Polynesians. For most of its history it was one of the worlds most-isolated places. Its inhabitants are called the "Rapanui". Their population numbers have wildly fluctuated because of growing populations that completely denuded the island of trees and other native plants. In addition their arable land was rendered useless which resulted in civil war, famines, and epidemics. To this day the island is desolate and unable to support more than a few inhabitants. It serves as a reminder of how overpopulation can destroy the environment.
Easter Island Environmental Disaster Movie
More on Easter Island is HERE. This is a REQUIRED reading assignment.