A Tragedy of the Commons situation exists whenever two or more activities, each, which in order to produce results, rely on a shared limited resource. Results for these activities continue to develop as long as their use of the limited resource doesn't exceed the resource limit. Once this limit is reached the results produced by each activity are limited to the level at which the resource is replenished. There is a also a video
for this insight which is a component of the Systems Archetypes Course
We begin with growth being produced by [a activity] and [b activity], though it's difficult to grow from nothing.
Both [a activity] and [b activity] consume resources from [resources available]. These resources are critical for producing results. And here's where the wicket gets sticky.
In time using the resources depletes the [resources available], there is likely to be a [resource limit] and the resources may or may not be replenished. Because R1 and R2 are reinforcing structures you can be rather certain that sooner or later they will run into limits.
In the simulation version of this structure [a factor] and [b factor] are added to enable control of the [a activity] and [b activity] rates. The [resource replenishment] and [replenishment rate] were added to allow for simulating with renewable resources. [min res] is the resource limit that [resources available] is not allowed to go below.
With [replenishment rate] = 0 and [min res] = 0 resources are quickly consumed. Note that [a result] and [b result] are the same as are their activity rates.
If [b result] starts out larger it just grows faster with a higher activity rate.
Even if there is a replenishment of [resources available] they will still end up being consumed. It just takes a little longer because they're feeding reinforcing structures R1 and R2.
If we set [min res] = 20 to control the extent of the resource use then resources will be used down to that level and then growth will be limited to the replenishment rate.
Strategy: There are only two strategies for this structure; 1) Collaboratively manage resource usage or 2) Have someone else manage resource usage. One could attempt to promote resource development though faced with R1 & R2 usage this only buys time. The other non-viable option is simply a race to the bottom and when the resources are all gone then do something else. Sounds like oil consumption to me.
I would encourage you to interact with the parameters of this model and do a number of simulations until you're comfortable with the realities of this structure.