Subject: Closed Loop Problems

Posted by Karl North on 11/10/2009
In Reply To:Closed Loop Problems Posted by Tim Joy on 11/9/2009

Message:

I teach an upper level undergraduate course in ecological agriculture to a class limited to 20 students, at least half of whom are environmental science majors. I start with Sterman's chickens-eggs-roadcrossings CLD because it fits the subject matter of the course, demonstrates both types of feedback, and gets a laugh (except from squeamish vegetarians, of which there is always a contingent).

Then I replace 'roadcrossings' with 'chicken sales' to give an example of another negative feedback loop in the same model. Then I immediately give them other CLDs to practice reading and explaining orally in class. Like the one that explains a counterintuitive forest fire control policy. Or one on how sheep internal parasites are gaining immunity over time to all the commercial sheep wormers. Not being shepherds, or even farm raised for the most part, the class has a hard time with this one, and I end up telling them some parts of the story of it.

I use a CLD of the poverty trap phenomenon that is common in tropical semi-subsistence agriculture to teach the idea of shifting loop dominance. Toward the end of the semester when the class has become familiar through numerous examples with these ways of modeling problem behaviors, I ask groups of students to create their own CLDs of causal structures that might explain existing trends and possible scenarios of change in the nature of farming in the US. They usually enjoy this and do a pretty good job.

I create my own demonstration models of the examples described above, with some help from the ST literature, and would submit them here for critical comment and hopefully improvement from professional modelers (I am not one) if the listserve would permit graphics or attachments.

Karl North

Follow Ups:

Closed Loop Problems - John F. Heinbokel 11/12/2009