Meadows Chapter 2: A Brief Visit to the Systems Zoo

One Stock Systems:
A thermostat is an example of a stock with two competing balancing loops.
-In this example the heating loop dominates the cooling loop.
Competing Balancing loops often result in feedback loops.

Important General Principle: The information delivered by a feedback loop can only affect future behavior: it can't deliver the information, and so can't have an impact fast enough to correct behavior that drove the current feedback.

There will always be delays in responding.

A stock-maintaining balancing feedback loop must have its goal set appropriately to compensate for draining or inflowing processes that affect that stock. Otherwise the feedback process will fall short of or exceed the target stock.

Every balancing feedback loop has its breakdown point.


Whenever the inflow rate falls behind the outflow rate, the temperature falls.
Shifting dominance refers to the change in dominance between inflows and outflows.
Complex behaviors of systems often arise as the relative strengths of feedback loops shift, causing first one loop and then another to dominate behavior.

Questions for testing the value of a model
1. are the driving factors likely to unfold this way?
2. If they did would the system react this way?
3. What is driving the driving factors?

Model utility depends not on whether its driving scenarios are realistic but on whether it responds with a realistic pattern of behavior.
Systems with similar feedback structures produce similar dynamic behaviors
A delay in a balancing feedback loop makes a system likely to oscillate.
Delays are pervasive in systems and they are strong determinants of behavior. Changing the length of a delay and the relative lengths of other delays make a large change in the behavior of a system.

Two Stock Systems
Any physical, growing system is going to run into some kind of constraint sooner or later.
In physical, exponentially growing systems there must be at least one reinforcing loop driving the growth, because no physical system can grow forever in a finite environment.
The limits on a growing system can be temporary or permanent.
A quantity growing exponentially toward a constraint or limit reaches that limit in a surprisingly short time.
Nonrenewable resources are stock-limited. The entire stock is available at once at can be extracted at any rate. But since the stock is not renewed the faster the extraction rate, the short the lifetime of the resource.
Renewable resources are flow-limited. They can support extraction or harvest indefinitely, but only at a finite flow rate equal to their regeneration rate. If they are extracted faster than they regenerate, they may eventually be driven below a critical threshold and become for all practical purposes nonrenewable.


This chapter was ultimately confusing for me! I have trouble seeing the charts and graphs literally which makes it difficult for me to understand.