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WHAT IS IT?

This is a model of the tragedy of the commons. First popularized by Hardin (1968), the tragedy is a metaphor for the overuse and depletion of shared environmental resources. Because such common resources are a public good--in which no one can be excluded from consuming them--each individual has an incentive to exploit the resources, and no one has an incentive to protect them. As a consequence, self-interested individual behavior makes society worse off over the long run. The tragedy of the commons is thus of the importance of civic responsibility and enlightened self-interest in the management of shared resources.

The metaphor refers to the English practice of the middle ages in which villages provided a common grazing area for livestock. If each member of the village tried to maximize his or her wealth by grazing as much livestock as possible, the commons would quickly be depleted. The village as a whole ends up worse off. But if villagers forego short-term gain from the commons, they can sustain the resource over the long run.

This model replicates the tragedy of the commons by creating a grassy common area and a small population of cattle farmers.


HOW IT WORKS

The model contains a shared area of grass, a population of farmers, and cows. The model contains five parameters: the number of farmers; their greediness; the rate at which cows degrade the grass; the rate at which the grass regrows; and the metabolism rate of the cows.

Farmers add cows to the commons in order to maximize their profit. For simplicity's sake, the model assumes that a farmer measures his profit by the average health of the cows on the commons. Each farmer has knowledge of all the cows on the commons. As long as the cows stay healthy (i.e. their average energy increases) farmers keep adding cows. (The model also assumes that farmers have an unlimited number of cows that they can add to the commons.) As cows get "sick", however (that is, the average energy of all cows declines) farmers stop adding cows until their average energy level once again begins to climb.

The model allows the user to vary the "greediness" of farmers. Greedy farmers are insensitive to the health of the cattle, and will keep adding cows even as the health of the cattle begins to decline. Charitable farmers will stop adding cattle as soon as the cows begin to lose energy.

Cows graze the commons and eat grass. Their metabolism rate determines how efficiently they convert grass into energy. Hence a high metabolism means cattle gain energy more quickly and are healthier. As cattle graze, however, they degrade the grass. A higher rate of degradation means a single cow consumes more of the grass in a given patch. The grass regrows at a variable rate as well. The higher the rate, the quicker a given patch of grass will grow back after a cow eats it. Hence the commons replenishes itself at a rate that varies both with the degradation and the rate of regrowth.



HOW TO USE IT

Press the "setup" button and then "go". You will see farmers adding cows to the commons, the cows grazing, and the grass depleting and regrowing. The plot to the right measures the total number of cows on the commons, the amount of grass, and the average energy of the cows (which by assumption is equivalent to the wealth of the farmers).

You can manually adjust the five variables by moving the sliders for each parameter, then clicking setup and go. You can also let the program select random parameters by clicking the "jumble" button and then pressing go.


THINGS TO NOTICE

The user interface includes buttons that configure the parameters for three different scnearios on the commons.

1. By clicking the "crash" button and running the model, you can observe the farmers exhaust the commons.
2. The "sustain" button configures the parameters to illustrate how the commons can support a limited number of cattle.
3. The "peril" scenario illustrates how the population of cattle may fluctuate as greedy farmers suffer through booms and busts on the commons.
4. The POLS 100S button configures the model to illustrate some scenarios for my Political Science 100 class. It illustrates a commons ecology that teeters on the verge of collapse, often reduced to a single grazing cow. When greediness is set to 70, the commons is quickly exhausted. When greediness is set to 10, farmers can sustain the commons for longer. Under this enlightened scenario, the commons may sometimes become exhausted, though sometimes farmers can sustain it with a few cattle. Nevertheless farmers periodically add cows as the grass regrows and the commons recovers.


THINGS TO TRY

When does enlightened farming make a difference on the commons? Try this: click on the "crash" button and then go. Notice how quickly the cows deplete the commons. Now change the "greediness" of farmers to 10, click setup and go. Notice how less greedy farmers are able to sustain the commons. Now change the greediness value to 30, click setup and go. Repeat this with greediness set to 40. At what value does the greed of farmers lead to collapse of the commons?


EXTENDING THE MODEL

I welcome ideas about how to extend the model. Email me at dearnest@odu.edu.


NETLOGO FEATURES

None.


RELATED MODELS

The NetLogo library includes a HubNet model of the tragedy of the commons (Wilensky 2002). In the HubNet model multiple users can participate as farmers in the commons.


CREDITS AND REFERENCES

Model developed by:

David C. Earnest, Ph.D.
Assistant Professor of Political Science and
International Studies
Old Dominion University
BAL 700
Norfolk, VA 23529
dearnest@odu.edu



Alcock, Frank. 2002. "Bargaining, Uncertainty, and Property Rights in Fisheries." World Politics 54, 4 (July): 437-461.

Hardin, Garrett. 1968. "The Tragedy of the Commons." Science v. 162. 1243-1248.

Hope, Christine A. and Ronald G. Stover. 1982. "The Commons Game: An Exercise in Resource Allocation." Teaching Sociology 9, 4 (July): 383-399.

Rapoport, Anatol. 1988. "Experiments with N-Person Social Traps II: Tragedy of the Commons." The Journal of Conflict Resolution 32, 3 (September): 473-488.

Wilensky, U. 2002. NetLogo HubNet Tragedy of the Commons model. http://ccl.northwestern.edu/netlogo/models/HubNetTragedyOfTheCommons. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.