The Tragedy of the Commons structure represents a situation where, to produce growth, two or more Reinforcing Loops are dependent on the availability of some common limited resource.
A's activity interacts with the resources available adding to A's results. A's results simply encourage more of A's activity. The same sequence plays out for B's activity. And, the more resources used the greater the results. This simply encourages A and B to use more resources.
A's activity and B's activity combine to produce some total activity. This total activity subtracts from the resources available. The extent of the resources available being defined by the resource limit.
Total activity continues until it completely depletes the resources available. When this happens A's results and B's results stop growing as there are no more resources to use. What makes this structure even worse is that whoever figures out the structure first, A or B, wins because they use all the resources before the other has a chance to. This structure is often referred to as "All for one and none for all."
This structure repeatedly appears in organizational contexts where a service organization supports the success of multiple departments who fail to support the service organization in return. There are two strategies for dealing with this structure, one more effective than the other.