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Loop Analysis
Once a student has identified a feedback loop, the next step in analysis is to determine whether it’s a positive or negative feedback loop. If, like Kendra, they have assigned positive (+) signs to cause-and-effect relationships that change in the same direction (i.e., if one goes up the other goes up, and if one decreases, the other decreases) and negative (-) signs to the relationships that change in opposite directions (i.e., if one increases the other decreases, and vice versa), then they can simply count the number of minus signs around the loop. If it’s an odd number, the loop is a negative feedback loop. If it’s an even number, the loop is a positive feedback loop.
Negative feedback loops tend to stabilize systems, holding state variables at intermediate values. Positive feedback loops tend to destabilize systems, prompting state variables to accelerate toward extreme values, either high or low. In real systems, positive and negative loops work in tandem, each modifying the effects of the other. Negative feedback loops typically put limits on the runaway behavior of positive feedback loops, and positive feedback loops often shift the equilibrium point of variables stabilized by negative feedback.
Kendra’s revised diagram has three feedback loops. All three pass through coral reef health and tourist visits. Both of the upper loops (located above coral reef health) contain three positive (+) arrows and one negative (-) arrow. Since this is an odd number of negative signs, both these upper loops are negative feedback loops. The lower loop contains no negative signs. Zero is an even number, so this is a positive feedback loop.
Teachers should encourage their students to try to identify at least one negative feedback loop and one positive feedback loop in their system, though this may be difficult to do in some cases, particularly for beginners.
Though she knew it was based on some solid systems theory, Kendra was not immediately convinced by this plus-and-minus hocus-pocus. To her credit as a critical thinker, she worked her way around each loop until the conclusions of the simple loop analysis made sense to her.
She knew that negative feedback loops should drive the state variable toward a stable intermediate value. As a thought experiment, she considered how the two upper feedback loops would react to either an increase or decrease in reef health. An increase in reef health would presumably increase the beauty of the reef and increase the number of tourists. The increased tourist activity would support more coastal development (hotels, golf courses, etc.) and more diving and snorkeling boat trips to the reef. These increases in turn would likely produce more water pollution, sediment in the water (e.g., mud from construction sites), and anchor damage. Thus any initial increase in coral reef health would be counteracted by a subsequent decrease in coral reef health, confirming the presence of a negative feedback loop. Conversely, a significant decline in reef health would make the reefs less attractive to tourists, reducing the likelihood of additional coastal development and boat trips, thereby reducing the pollution and anchor damage and ultimately allowing a gradual increase in reef heath. Again, the negative feedback would counteract the change. The balance between these two opposing tendencies would drive the reefs toward an intermediate level of health – they probably would not die off completely, but they would be chronically sick.
With this knowledge, she reviewed the systems behaviors associated with negative feedback processes and revised her initial graph to be more realistic and to show a more realistic goal for her project:
Kendra recognized that this decaying exponential curve was not the only possible behavior that could be produced by the negative feedback loop. Overshoots, Decaying oscillations and even chaotic behavior were also possible, but this simple graph seemed to capture the essence of what she hoped to accomplish, and she didn’t yet know enough about systems analysis to predict exactly what behavior would happen.
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