This article shortly documents the system dynamics plugin for the Freestyler software. As of Feb 2019 the program is still working under Windows §10 and Ubuntu 10. However, it requires some extra installation efforts described in the Freestyler article. Also, mouse behavior is fairly erratic which is probably due to the fact that Java programs age very badly, even if adapted for the latest Java Runtime. Anyhow, apart from these shortcomings, the program does its job and in addition runs within an environment that includes other interesting educational applications.
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The system dynamics palette allows creating system dynamics models from the following graphical elements:
As of Feb 2019, there are some problems with the interface. It is difficult to grab Stock icons. Only the ASCII character set is supported. We therefore also suggest to look at alternative technology, such as the free NetLogo simulator. That being said, getting the model to run is easier, since no programming is required, neither for launching the model nor for data collection.
FreeStyler remains interesting if you plan to currently use its other modules, at least the ink annotation.
The following example (badly) models a system for cat growth, using yearly increase:
After 10 years we got almost 36'000 cats. That is impressive. A real model would include food for cats (e.g. mice) and food for mice (e.g. grass) and interactions between these variables. For example, cats starve without mice, mice would eat all the grass if not eaten by cats and starve too.
The following model taken from a NetLogo library [1] simulates growth of mice and cat populations with so-called Lotka-Volterra equations.
The following picture shows the popup when clicking on "Births".
Since four variables are connected to this rate, we then can type the simple equation:
Births = Efficiency * CatPop * MicePop * PredationRate. Mouse Death = CatPop * MicePop * PredationRate.
This model still needs improvement, e.g. we should simulate food for mice, i.e. the effect of population density. Also, cat population could also be modeled by starvation. Right now, the mouse population only affects the birth rate of cats.