2.7 Representing feedback through system dynamics diagrams
System dynamics diagrams, also sometimes called ‘stock flow’ diagrams, can be derived from causal diagrams, although in some cases it might be easier to start directly with the system dynamics diagramming technique, especially if you need to explore around one particular object’s attribute, such as population number.
System dynamics diagrams are drawn using four symbols: boxes representing attributes or ‘stocks’ of objects (e.g. level of water in a tank); valves representing the rate of increase and decrease of the object’s attribute, which are drawn on the left-hand side (increase) or right-hand side (decrease) of the boxes; circles representing factors or variables which influence the rate; and finally, arrows showing the links between their stocks, rates and variables.
There are different representations of the object variables according to the system dynamics and sign graph diagrams.
In the water hyacinth example, the reproduction rate of the water hyacinth (represented by the left-hand side valve in Figure 4.7) is influenced by the amount of dissolved nutrients and the water surface area. The death rate (represented by the right-hand side valve in Figure 4.7) is influenced by the number of herbivores eating the plants and by the availability of water surface area. The variables depicted in Figure 8 can in turn be represented as stocks.
You can see how using the system dynamics diagramming technique can very rapidly develop a complex web of interrelated objects, so complex in fact, that you can lose sight of the purpose of the original diagramming activity. System dynamics diagramming is at its most powerful when you need to focus on a single attribute and explore the main factors which may determine the quality or quantity of the attribute. For example, the diagramming technique was successfully used by a group of basket weavers in Zimbabwe to explore the causes in the decline of the main grass species that they harvested in order to weave their baskets (Standa-Gunda et al., 2003). The project’s facilitators took advantage of diagramming since they did not have to be too concerned with different levels of literacy among participants.
As with sign graph diagrams their development may require several cycles of analysis and synthesis before settling down on a final version. Many systems practitioners then go on to mathematically model the relationships identified in the system dynamics diagram using software programs such as Simile. These programs allow you to drag-and-drop system dynamics diagram components onto a blank window and then to introduce mathematical equations that determine the rates of increase and decrease of each state. Yet, many people are happy to stop at the diagramming stage. The mere fact that the situation is looked at with a relational and multidisciplinary focus, using a visual mode of communication, is sometimes enough to produce new insights.or