The "systems" approach starts by analyzing an environmental problem by breaking it down into sources/sinks, stocks, flows and information connectors. The "systems" diagram generated this way will show several important features;
what's considered in the system and what's not - i.e. defining the boundaries
the units of the stocks and the flow of those units from one stock to another per time (as flows)
the information required to control the flow rates
At the second level of analysis, one of the most important features is to identify the feedbacks. These maybe a simple feedback from a stock to the flow or they may be loops. There are two types, positive and negative.
definition: Increased concentration of the stock results in increased flow into that stock.
example: exponential phase growth of a micro-organism (before resource limitation sets in)
diagram:
Negative feedback
definition: Increased concentration of the stock inhibits the flow into that stock
example: as trees become more crowded in the forest the rate of new tree growth slows down
diagram:
You can't tell negative from positive feedback unless you know the relationhip between the flow and the stock.
Also, remember the terms used here "negative" and "positive" are not synonymous with bad and good. You can have a positive feedback for the spread of a disease and you can have a negative feedback in the health gains of children in cities. These are specific terms in the "systems" view.
John Rueter
November 24, 2007
