1. Simulating the Tragedy of the Commons Using Agent-Based Modeling
Josh Lee
Computer Systems Lab 08/09

2. Abstract Tragedy of the Commons Experimental Economics
-autonomous individuals
-communal resources
Experimental Economics
-conventional economic wisdom
Agent-Based Modeling
-agent types
-NetLogo

3. The Tragedy of the Commons
Example: Waste
-individuals contributing to group problem
-finite resources
Sahel
-three-tier model
-more complex, but more realistic/practical

4. Background “Understanding the Tragedy of the Sahel,” Corey L. Lofdahl
-original Tragedy of the Commons ABMS
“The Tragedy of the Commons,” Garrett Hardin
-max goods v. max population
-stabilization

5. System Dynamics v. Agent-Based Modeling
Sahel, overshoot-and-collapse
Individual agent behavior
-agent cooperation (or lack thereof); experimental economics
-emergent behavior, dominant behavior types

6. Fig. 1: Model, Upon Opening

7. Model Overview Adjustable Parameters “grass-growth-rate”
“grass-energy”
“cattle-energy”
Likelihood of finding resources
Example:
-greater grass-growth-rate
>greater cattle population
>lower grass count
>greater competition
-long-term, greater instability

8. Fig. 2:
Population
Fluctuations
(Instability)

9. Further Development Drought Length, frequency Demonstrates instability
Emergent Behavior
Dominant behavior type
Population stability

10. Fig 4: Unique Emergent Behavior

12. - systematically varies parameters - extensive data output
BehaviorSpace
- systematically
varies
parameters
- extensive
data output

13. BehaviorSpace
-Stability decreases with increases in grass-energy; begins to have irreversible ramifications around ~17
-Human populations (higher level tier) less stable than cattle populations (lower level tier)

14. Behavior Space, cont.
-Raised cattle-threshold/people-threshold leads to greater stability
-Greater stability leads to more clearly delineated population patterns

15. “Greater stability leads to more clearly delineated population patterns”V.

16. Conclusions
-Greatest stability: minimal competition, ability to sustain at a steady population rate
-Stability: ability to counteract perturbances
-Balance between resources, population size, and reproduction rate