Feedback Why is it that a higher level of system organization is more stable? Definition of a system: keyword interact If one part has an effect on the rest of the system and the system has an effect on that one part , then a ‘circular’ relationship or ‘loop’ has been created. Example: you and a bicycle (two part system) combined, do more than single parts actions influence parts simple system which creates stability riding a bicycle ‘constant adjustments to correct errors’

Feedback example

Feedback A feedback loop exists when decisions change the state of the system, changing the conditions and information that influence future decisions.

Feedback example

Feedback Positive loops Symbolized by Negative loops Symbolized by self-reinforcing growth producing destabilizing accelerating even number of –’s Symbolized by Negative loops counteracting goal seeking stabilizing balancing odd number of –’s Symbolized by

Negative feedback Negative feedback is universal and makes all sorts of different systems behave in certain, similar ways. Things in common: Active systems - self stabilizing systems make an active response to change. Applies to living systems - population, object systems - bicycle, as well as social systems - economy. System limitations - limit to amount of change which active system can deal with Loose system - negative feedback does not prevent change, it responds to change.

Examples of negative feedback loops

Examples of negative feedback loops

Exponential decay (workers.mdl)

Negative feedback Reaction time - every negative feedback loop has time limits which affect its behavior. minimum amount of time necessary for one complete circuit around the loop. target

Negative feedback

Negative feedback Counter-intuitive systems - negative feedback often produces counterintuitive behavior. For example, killing the wolf is not good for the deer. Also, killing off pests on the farm kills the predators too, (pests breed faster so the next generation will arrive and there will be no predators to control them). The ‘obvious’ solution does not work because the negative feedback is set up to cancel direct interventions. For negative feedback systems it is much better to change the way the pieces interact than to try to ‘out-muscle’ the system. The first step is always to figure out what the system is and how it works.

Negative feedback Hidden systems - all feedback loops are seldom out in plain sight

Positive feedback Most change comes from different (positive) feedback process Change in one part of the system produces changes in the whole system which then “feeds back” and affects the original part ( amplification of change )

Examples of positive feedback loops

Exponential growth (money.mdl)

Feedback

Putting pieces together We started by finding out what the systems are made of, and studied the pieces individually. Gradually, we learned that it was just as important to study how these pieces were organized (still thinking that the organization of any kind of system is unique). Finally we have began to realize that all complex systems have many things in common, in the way they are organized. Every complex system is built out of the same two simple elements: positive and negative feedback loops ( basic building blocks ).

Putting pieces together When positive and negative loops combine, any behavior is possible! Basic Types: Exponential Growth Exponential Decay S-Shaped Growth Oscillation Combined Behaviors S-Shaped Growth with Overshoot Overshoot and Collapse

Putting pieces together

Putting pieces together Identifying the positive and negative loops is very important in order to distinguish between things which are going to affect the system temporarily or permanently. Any change which does not change the important positive and negative loops will be temporary. Any change that affects the relationship between the positive and the negative loop will have long term impact on the system.

What have we learned?

Causal loops For each link, determine the effect of an increase in the variable at the tail of the arrow: If the variable at the head increases, assign a plus. If the variable at the head decreases, assign a minus. For each loop, count the number of negative signs: An even number of negative links is a reinforcing (R) loop. An odd number of negative links is a balancing (B) loop. Most important: For each loop, tell a self-reinforcing or balancing/counteracting story, and check that the story matches the loop polarity.

Causal loops Handout 1 (a) Population and economic growth loop (b) Population and land-use loop

Causal loops (a) Sentence 1

Causal loops (a) Sentences 2 and 3

Causal loops (a) Sentence 4

Causal loops (a) Sentences 5 and 6

Causal loops (b) Sentence 1

Causal loops (b) Sentence 2

Causal loops (b) Sentences 4 and 5

Causal loops (a + b) Combined