Thinking Systems
Class 10
Matt Cohen, PhD
A Rat Infestation
• Gainesville home built in 1928
-
– No rats when we moved in
-
• Lived there for just under 2
years
– “Massive” control efforts by the
end
• Owners of 2 large dogs
– Exceedingly poor hunters
+
• Neighborhood of cat owners
– Every direction (E, W, N, S) had
one or more felines
– Drove the dogs crazy…evervigilant border patrols
Elements of Systems
•
•
•
•
Boundary (the yard, canine patrolled)
Inputs and outputs (cats, dead rats)
Internal components (rats, dogs, cats)
Interactions
– Positive interactions (rats breeding)
– Negative interactions (cats on rats, dogs on cats)
Why Systems?
• Interactions create complexity
– Emergent behavior
• Water is “wet”
• Traffic snarls (even without accidents)
• The Rise of Fall of Pet Rocks
• Thresholds (tipping points) exist
$3.95 each (!)
– Predicting these is enormously important
• Global climate change, business cycles, disease epidemics
• Epileptic seizures, landslides, fisheries collapse
• Systems aren’t more complex than we think, they
are more complex than we can think.
– But…we have to try!
Key Attributes of Systems I.
• Mutual causality
– Components affect each other,
obscuring linear cause-effect
A
B
• Popularity → sales → popularity
• Poverty → soil erosion → poverty
• Chicken → Egg → Chicken
• Indirect effects
– Component A exerts control
over Component B via its action
on Component C
C
A
B
Indirect Effects - Aleutian Islands
• Nutrients are essential for plant
and animal production
– Phosphorus (P) is often limiting
nutrient
– Was mined for fertilizer for years
Croll et al. (2005) - Science
Depleted P
• Grassland production of Aleutian
islands is P limited
• Sea bird guano is a rich P source
Abundant P
• Essential for ribosomes and
metabolism
• Limited geologic source in the
region
• Amount of P controls the
productivity of the ecosystem
Nutrients and Sea Birds
• Seabirds eat fish from
the sea but poop on
land
• Major flow of P from
sea to land that
supports productive
grasslands
+
Fish
Marine
Birds
+
Soil
P
+
Grassland
Production
Predator Control of Ecosystems
• Introduce Arctic Foxes
– Top-predator
– Seabirds never had a
terrestrial predator
– Decimated the sea-bird
populations
Arctic
Foxes
+
Fish
Marine
Birds
+
Soil
P
+
Grassland
Production
Roughly 300% more soil P AND
biomass on fox-free islands
than on fox-infested islands
Key Attributes of Systems II.
• Consist of processes at
different space/time scales
A
B
– Fast and slow variables
• Humans and viruses
• Evolution and extinction
• Supply and demand
• Systems are historically
contingent
– Deep dependence on what
happened in the past
• The Great Unfolding
• Beta-max, Bacteria, Base 10
B
A
C
Fast and Slow: Beer and the
Business Cycle
• There exists a cycle of boom (bull) and bust
(bear) periods in economic systems…WHY?
A Systems View of Boom and Bust
1. The structure of a system influences
behavior. Systems cause their own problems,
not external forces or individual errors.
– Distribution chains (and economies) contain fast
and slow moving parts
– Communication between parts is LAGGED
2. Human systems include the way in which
people make decisions.
3. People tend to focus on local optimization
NOT global optimization.
Consider a Typical Supply Chain
• Retailer: Sells products, varying consumer demand, orders
to wholesalers for next weeks delivery
• Wholesalers/Distributors: Distribute beer to multiple
retailers, orders to brewery for two weeks in the future
• Brewery: Make beer, adjust production to demand
• ALL
– Avoid the costs of excess and insufficient inventory
J. Sterman at MIT http://web.mit.edu/jsterman/www/SDG/beergame.html
Beer Game Simulator
Brewery
Wholesaler
Distributor
Retailer
Team 1
ORDERS
Oscillation
EXCESS/
BACKLOG
Amplification
Changing Demand
Lag
Team 2
Team 3
Team 4
Dependence on History: Algae,
Nutrients, and Shallow Lakes
• Shallow lakes (< 10 m deep)
• Two alternative “states”
– Rooted vegetation (macrophytes)
– Algae
• Shifts between the two occur
catastrophically, and BOTH can
occur under the same
environmental conditions
• Where you are depends on where
you’ve been
Self-Reinforcing Feedbacks
in Shallow Lakes
• Rooted Plant State
– Plants require clear water
– Plants stabilize sediments
– Stable sediments keep
water P concentrations
low AND limit stirring
– Low P limits algae and high
clarity favors rooted plants
• Algae State
– Algae makes ooze
– Ooze is easily stirred up,
making the water turbid
and recycling P
– More P makes algae grow
faster AND sediments
looser via loss of plants
• Regime shifts due to combined effects:
– Too much P (human pollution)
– Disturbances (pollution affects vulnerability)
Environmental Change and
Ecosystem “State” Shifts
Typical Models of Nature
Emerging Model of Many
Complex Systems
Scheffer et al. (2001) - Nature
Thinking for Managing Complex
Systems
• The “state” of a system is controlled by
external forces AND internal interactions
• Indirect effects lead to surprising behavior
• Fast and slow variables interact to create
instability
– Spatial variability (local vs. global variable) also
• Managing for ONE THING often creates bigger
problems later (discussion section)
The End
Matt Cohen
mjc@ufl.edu