Subject: Loop of the Week

Posted by Steve Kipp on 11/4/2003
In Reply To:Loop of the Week Posted by John Sterman on 11/1/2003

Message:

Enjoyed your rant, John H, interesting stuff about the small standing biomass ratio of producers to consumers in plankton-based ecosystems.....thankfully, decomposers ARE starting to show up as a return loop for nutrients in newer k-12 textbook pictures of energy pyramids. The mental model that is sometimes tough for kids to see is that after the sunlight energy is “injected” into the system via photosynthesis, the energy is CARRIED BY the matter (food) from level to level, with huge energy losses to heat at each level, until the matter is out of nutritional energy after the decomposers are through with it and some of the matter (CO2, water, minerals) gets re-energized and caught in the cycle again with the next round of photosynthesis.



I’ve been using a very simple STELLA representation of the photosynthesis/cellular respiration cycle to help 7th graders grasp this concept that “materials cycle” and “energy flows one way”. The need to teach in this way is huge. Just looking at the equations for photosynthesis and respiration does not allow for many of them to make this connection, especially when so many textbooks wrongly represent “sunlight energy” as one the REACTANTS for photosynthesis instead of the ENERGY SOURCE that drives the conversion flow of CO2 + water to glucose + oxygen. They do love learning that food is chemically stored solar power, that this stored energy may only be used one time (so do something good with it!), that nature does not waste matter and neither should we, and that they are made of recycled atoms billions of years old.



I do have a question for the group re: closed loop thinking. Could we come up with some clear illustrations of the difference between closed (conserved flow) material loops like the water cycle, photosynthesis/respiration cycle, and rock cycle; and dynamic loops that show causality like archetypes/causal loop diagrams/influence diagrams? Thanks to John S for starting this thread.





Steve Kipp

Follow Ups:

Loop of the Week - Della Robertson 11/4/2003 
Loop of the Week - Elisabeth Roberts 11/5/2003