CASE STUDY: Tragedy of the Commons
The phenomenon known as the "tragedy of the Commons" is a classic environmental event that can be applied to a number of different situations and locations throughout the world. It is best know as the "tragedy of the Sahel", referring to the area in Africa below the Sahara Desert. The Tragedy of the Sahel is an instance of a more general phenomenon, The Tragedy of the Commons as articulated by Garrett Hardin. The Tragedy of the Commons occurs when people individually perform rational acts intended to further their immediate self-interest, but the combination of every ones behavior hurts the long-term, collective interest. Hardin offers the following example to illustrate the problem. Imagine a cattle pasture, or common, open to all. If any single herdsman increases his herds size, then he benefits directly from that increase. It is the individual who profits from the extra milk and meat produced by additional animals. Yet increasing the number of cattle also yields negative consequences: additional food, water, and space -- all provided by the common pasture -- are required to support the extra animals. Should these resources become strained, overgrazing will occur. However, the individual herdsman himself does not pay the costs of overgrazing. Instead, such costs affect all herdsmen as well as the community at large. For example, if all of a pastures grass gets eaten, then no food remains for the cattle and they will die, creating a famine. Such famines affect not only the herdsmen who caused the destruction, but also the people who depend on the cattle for food. Each individual in the community, including the herdsman, pays the costs of overgrazing. Thus it is in the herdsmans self-interest to increase his cattle holdings to the point of overgrazing because he receives the benefits of the increase and pays only a fraction of the costs. In this discussion, the term tragedy is used in a precise and philosophical manner. The Tragedy of the Commons is not tragic because overgrazing and famine occur due to factors beyond human control. The situation is tragic because famine results from people acting in accordance with the incentives presented to them by their community. The very process of receiving the benefits of additional cattle and the division of their costs lies at the root of the Tragedy of the Commons. Thus, a link is created from human desires and motivations to unpleasant, physical consequences. It is the inevitableness of these consequences that most embodies the term tragic. Building the Model:
For purposes of this model, we're only going to concern ourselves with three main components: 1.
Building the Cattle model:
Cattle population changes from simple births and deaths. The average birth rate for cattle is 30% of the cattle population. Initially in this model there are 3,000 head of cattle. Cattle can die in one of two ways: they are either killed by people (known as the "offtake" percentage), or die of natural causes. The total death rate depends on three factors: cattle mortality rate (which depends on the amount of grass eaten), the cattle offtake percentage, and a "medical factor". The medical factor, a dimensionless number between 1 and 2, takes into account improvements in technology, which can lower the cattle mortality rate (cattle mortality rate = cattle mortality/medical factor). Cattle mortality (a separate variable from cattle mortality rate) depends directly on the amount of grass eaten per cow. The table below shows this relationship: Grass eaten per cow
| Cattle Mortality (percentage)
REMINDER: don't forget to chance percentages to decimal numbers (100% = 1.0). Also remember that grass eaten per cow is the independent...
Please join StudyMode to read the full document