Transcript Thoughts on Garden/Tomato Curriculum Unit

Feedback Loops in Flower Gardening
Paul Newton, Linda Tompkins,
Marianne Krasny, and Karl North
October 29, 2004
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• Diagramming feedback loops can help us to
solve practical problems. It can help us to
understand how different factors influence
each other. Use the scenario on the slides 39 to introduce feedback loops to students.
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Behavior-Over-Time-Graph (BOTG)
maximum
possible
Desired
Future
Feared
Future
Flower
production
(flowers/year)
0
2000
2004
2008
Year
The year is 2004. Students have observed declining flower production in their school flower garden (the
acreage of the garden is constant). The students are concerned that their garden’s production rate might
continue to decline in the future. They would like to reverse the downward trend.
Their Problem Statement: What can we do to create the desired future?
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flower production
Dynamic Hypothesis
What caused this decline in production? You can develop a dynamic hypothesis for changes in
flower production. Let’s Begin with flower production, which is the variable of concern graphed in
the Behavior-Over-Time-Graph (BOTG) on the previous slide…
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seeds
flower production
+
Dynamic Hypothesis
The more flowers produced per year, the more seeds are produced…
The (+) sign signifies that a change in flower production causes a change in seeds in the same direction as the change in
flower production. This would also indicate that a decline in flower production would result in a decline in seeds
produced.
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+
seeds
R
flower production
+
Dynamic Hypothesis
And the more seeds, the more flowers are produced.
These two links form a Reinforcing [R] feedback loop. A reinforcing loop acts to reinforce a change in any variable in
the loop, in the same direction as the original change. That is, if any variable in the loop is increased, each circuit
around the loop acts to increase the variable above what it would have been had the change not occurred. (Conversely, if
any variable in the loop is decreased, each circuit around the loop acts to decrease the variable below what it would
have been had the change not occurred.) The plus arrow thus indicates the two variables change in the same direction.
The more flower production, the more seeds; the more seeds, the more flower production, ad infinitum, producing
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continuously increasing growth in both seeds and flower production.
+
seeds
R
flower production
soil quality
+
Dynamic Hypothesis
But more flower production, over time, acts to decrease soil quality…
Hence the (-) sign signifies that a change in flower production causes a change in soil quality in the opposite direction
from the change in flower production. As flower production increases, soil quality decreases. If fewer flowers were
grown, the quality of the soil would decrease less than had fewer flowers not been grown. Thus the (-) sign indicates the
two variables change in the opposite direction.
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+
seeds
R
flower production
B
soil quality
+
+
Dynamic Hypothesis
Reductions in soil quality cause corresponding reductions in flower production.
These two new links form a Balancing [B] feedback loop. A balancing loop acts to reverse a change in any variable in the
loop. That is, if any variable in the loop is increased, each circuit around the loop acts to decrease the variable below what
it would have been had the change not occurred. (Conversely, if any variable in the loop is decreased, each circuit around
the loop acts to increase the variable above what it would have been had the change not occurred.) The minus arrow thus
indicates the two variables change in the opposite direction. Flower production, in the absence of other influences, always
causes soil quality to decrease. Decreased soil quality causes a decrease in flower production.
Shifts in feedback loop dominance cause changes in behavior. Refer back to the graph on slide 3. What caused the growth
the first couple of years? What caused the decline? Looking at the diagram above, the first loop dominated early on, and
the second after that. (See more discussion on the following slide.)
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Why Feedback Loops?
+
Feedback loops cause behaviors over time.
Shifts in feedback loop dominance cause shifts
in behaviors over time. By drawing feedback
loops, you can understand how behaviors
change over time.
seeds
flower production
B
soil quality
+
+
For example, the reinforcing [R] loop could be
responsible for the increasing rate of flower
production beginning in 2000 (more flowers 
more seeds  more flowers  more seeds 
ad infinitum).
Then, sometime in 2001, the curve reverses.
Even though flower production is still
increasing, it increases more and more slowly.
This declining rate of flower production
indicates that, with declining soil quality, the
balancing [B] loop is now stronger than the
reinforcing [R] loop. The balancing [B] loop
initially acts to slow increase in flower
production, and then causes flower production
to decline beginning around 2002.
R
Feedback loops
cause
behavior over time.
Flower Production
(flowers/ year)
Desired
Future
[B] Loop Dominant
Feared
Future
[R] Loop Dominant
2000
2004
Year
2008
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Student Activities
1. Now that your students have been introduced to feedback loops, they
can work in small groups to draw a second behavior-over-time-graph
(BOTG). This graph should show changes in soil quality for the past
four years, and should project desired and feared future soil quality
over the next four years.
Student BOTGs should look something like the following BOTG
sketch, showing a decline in soil quality after a year or two of flower
production, with continued decline to the present (2004). It should also
show a feared future of continuing decline in soil quality causing
continued decline in flower production, and a desired future of
improvements in both soil quality and flower production.
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Behavior-Over-Time-Graph (BOTG)
maximum
possible
Desired
Future
Soil Quality
Soil quality at
which the B
Loop begins to
dominate
Flower
production
(flowers/year)
Feared
Future
B Loop Dominant
R Loop Dominant
0
2000
2004
2008
Year
11
2.
Students should then draw at least one feedback loop that could act to
create the Desired Future.
Two potential feedback loops for the teachers’ use are shown on the
following slides (through slide 15)
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+
seeds
R
flower production
B
soil quality
+
+
Dynamic Hypothesis (previous slide sketch repeated here for continuity)
Reductions in soil quality cause corresponding reductions in flower production.
These two new links form a Balancing [B] feedback loop. A balancing loop acts to reverse a change in any variable in the
loop. That is, if any variable in the loop is increased, each circuit around the loop acts to decrease the variable below what
it would have been had the change not occurred. (Conversely, if any variable in the loop is decreased, each circuit around
the loop acts to increase the variable above what it would have been had the change not occurred.) The minus arrow thus
indicates the two variables change in the opposite direction. Flower production, in the absence of other influences, always
causes soil quality to decrease. Decreased soil quality causes a decrease in flower production.
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desired flower
production
seeds
R
-
B
+
flower production
B
+
soil quality
+
+
gardening practice quality
(fertilizing, watering,
weeding, etc.)
+
Dynamic Hypothesis
Flower production decreasing to less than desired flower production causes gardening practice quality to increase.
Increased gardening practice quality in the areas of soil enrichment, especially fertilizing (whether organically or not),
causes increased soil quality. Increased soil quality causes increased flower production, completing the description of a
new balancing feedback loop. Over time, this balancing feedback loop causes flower production to move toward
desired flower production.
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desired flower
production
seeds
R
-
B
+
flower production
B
soil quality
+
+
+
gardening practice quality
(fertilizing, watering,
weeding, etc.)
B
+
+
Dynamic Hypothesis
Improved gardening practices also cause increased flower production via mechanisms in addition to enhancement of
soil quality. This creates another balancing feedback loop that, over time, tends to cause flower production to increase
toward desired flower production.
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3. As a third exercise, students could be asked to draw other
feedback loops that they come up with. These loops may, or
may not, relate to the BOTGs already drawn. The teacher
may choose to give the students a hint that the new loops
could relate to variables such as customers, disease, and
diversity.
The following slides show some, but certainly not all,
possibilities that might be useful to the teacher in reviewing
student work.
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desired flower
production
seeds
R
-
B
+
flower production
B
soil quality
+
+
+
gardening practice quality
(fertilizing, watering,
weeding, etc.)
B
+
+
Dynamic Hypothesis (previous slide repeated here for continuity)
Improved gardening practices also cause increased flower production via mechanisms in addition to enhancement of
soil quality. This creates another balancing feedback loop that, over time, tends to cause flower production to increase
toward desired flower production.
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desired flower
production
customers
+
seeds
R
-
B
+
flower production
B
soil quality
+
+
+
gardening practice quality
(fertilizing, watering,
weeding, etc.)
B
+
+
Dynamic Hypothesis
The more flower production, the more customers may be solicited to purchase them, and therefore
the more customers.
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desired flower
production
customers
+
R
+
seeds
R
+
flower production
-
B
B
soil quality
+
+
+
gardening practice quality
(fertilizing, watering,
weeding, etc.)
B
+
+
Dynamic Hypothesis
And the more customers, the more flowers they demand, yielding more flower production. These 2 links
form another Reinforcing (R) feedback loop acting to increase both customers and flower production.
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+
customers
desired flower
production
R
+
R
+
seeds
R
+
flower production
-
B
B
soil quality
+
+
+
gardening practice quality
(fertilizing, watering,
weeding, etc.)
B
+
+
Dynamic Hypothesis
The more customers we have, the more our desired flower production increases so we can meet the
demand. Increased desired flower production spurs us to improve our gardening practice quality, which
then causes an increase in flower production. And more flower production gives us the opportunity to
solicit and capture more customers. Thus is formed yet another Reinforcing (R) feedback loop.
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+
+
customers
desired flower
production
R
+
R
+
seeds
R
+
flower production
-
B
B
soil quality
+
+
+
gardening practice quality
(fertilizing, watering,
weeding, etc.)
B
+
+
+
diseases
Dynamic Hypothesis
More flower production will also mean more diseases, a positive link.
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+
+
customers
desired flower
production
R
+
R
+
seeds
R
+
flower production
+
B
soil quality
-
+
+
gardening practice quality
(fertilizing, watering,
weeding, etc.)
B
+
B
-
B
+
+
diseases
Dynamic Hypothesis
But, an increase in disease will cause a decrease in flower production. These last two links form
another Balancing (B) feedback loop.
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+
+
customers
desired flower
production
R
+
R
+
seeds
R
+
-
flower production
+
-
B
soil quality
B
-
+
+
gardening practice quality
(fertilizing, watering,
weeding, etc.)
B
+
B
+
B
R
+diseases
Dynamic Hypothesis
Our gardening practice quality also influences the incidence of diseases. The addition of this loop adds
two more loops, one reinforcing and one balancing. Can you find and trace out both loops?
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+
+
customers
desired flower
production
R
+
R
+
diversity
seeds
R
+
-
flower production
+
soil quality
B
-
+
+
gardening practice quality
(fertilizing, watering,
weeding, etc.)
B
+
B
+
B
-
-
B
R
+diseases
Dynamic Hypothesis
Research supports that diversity of flowers acts to reduce diseases.
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+
+
customers
desired flower
production
R
+
R
+
diversity
seeds
-
R
+
-
flower production
+
soil quality
B
B
+
B
-
+
+
gardening practice quality
(fertilizing, watering,
weeding, etc.)
B
+
R
-
B
R
+diseases
Dynamic Hypothesis
And fewer diseases enable more diversity. (Also, more diseases reduce diversity, as diseases often are
species-specific.) Thus a reinforcing loop is formed that can act either to increase or decrease both
diversity and disease.
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+
+
+ customers
R
+
R
B
diversity
seeds
-
R
+
R
desired flower
production
+
-
flower production
+
soil quality
B
B
+
B
-
+
+
gardening practice quality
(fertilizing, watering,
weeding, etc.)
B
+
R
-
B
R
+diseases
Dynamic Hypothesis
The more diversity, the more potential customers will be interested in our flowers, and therefore the
more customers we will have. Less diversity will likewise reduce our stock of customers. Adding this
link creates three new feedback loops (only two loop symbols were added). Two feedback loop symbols
were added here, but actually, adding this link creates three new feedback loops, one reinforcing and
two balancing; can you find them?
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+
+
+ customers
R
B
R
+
seeds
-
R
-
flower production
+
soil quality
B
B
+
B
-
+
+
gardening practice quality
(fertilizing, watering,
weeding, etc.)
B
+
R
-
B
+
+
diversity
R
+
R
desired flower
production
R
+diseases
Dynamic Hypothesis
And finally (because we’re running out of space!), more customers means that more diversity will be
demanded. Customers will want a variety of flowers to choose from for different occasions. This creates
another reinforcing feedback loop that can act, over time, to either continually increase, or decrease,
both customers and diversity.
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