Getting started with STELLA
To build a model in STELLA, one simply drags in stocks (the square representing “population of animals” in the example in Figure 1), flows (the taps going in and out the stock, in this example animals being born and dying each year) and converters (the circles in this example representing birth and mortality rate) from the menu bar. While doing so, STELLA automatically creates the mathematical equations in the equation level. To connect the stocks, flows and converters one simply pulls ‘action connectors’ (the red arrows in Figure 1) from one attribute to the other.
Figure 1. Population of animals
For example, the birth of animals per year is calculated by multiplying ‘birth rate’ by ‘population of animals’, therefore action connectors (red arrows) have to be drawn from the converter ‘birth rate’ to the flow ‘birth’ and from the stock ‘population of animals’ to the flow ‘birth’. To enter how the birth of animals per year will be calculated, double-click on the flow ‘birth’ after which the window in Figure 2 is opened. Once we’ve drawn the action connectors, under required inputs we now see the ‘population of animals’ and ‘birth rate’ and thus we specify we want the flow ‘birth’ to be calculated by: ‘birth rate’ * ‘population of animals’. For the converter ‘birth rate’ and ‘mortality rate’ values have to be entered and for the stock ‘animal population’ and initial population size has to be entered. The values entered in the model are: birth rate= 0.6; initial population size=1000 and mortality rate= 0.5
Figure 2. Calculation of birth rate
After building the model, the user can drag in a so-called graph pad from the menu bar to the model level to see what happens with the features in our model over time. The X-axis displays the years and the user places on the Y-axis the feature of interest, in this example ‘population of animals’. After running the model STELLA generates the output graph shown in Figure 3. In this example, the population of animals shows exponential growth. By adding more features in the model (e.g. carrying capacity, hunting pressure on population of animals) we can now get closer to a more realistic scenario for the animal population.
Figure 3. Population growth of animals
For those in possession of a version of STELLA on
their computer, you can download the following pdf file containing a
step-by-step example of how to start building a landscape model.
Download (PDF size 332 KB)
