What are examples of the tragedy of the commons?

A case counts as a tragedy of the commons when the resource passes two tests. It is rival, meaning one person's use leaves less for everyone else, and it is non-excludable, meaning nobody can practically be stopped from using it. When both hold, each user gets the full benefit of taking a little more but bears only a small share of the cost that use imposes on others, so the resource ends up overused. A resource with both properties is a common-pool resource. The cases below are described in general terms, and each is paired with the fix that fits it.

Ocean fisheries. A fish caught by one boat cannot be caught by another (rival), and on the open sea no single owner can keep boats out (non-excludable). Each crew gains from a bigger catch, while the smaller stock left for the future is shared by everyone, so stocks get fished down. Fixes: total catch limits, fishing seasons, and individual tradable quotas that give each holder a secure share of the catch. Groundwater. Many farms pump from one aquifer. Water one farm pumps is water another cannot use (rival), and any landowner above the aquifer can drill a well (non-excludable in practice). Each pump lowers the water table for every neighbor. Fixes: metered pumping permits or tradable water rights that cap the total withdrawn.

Shared grazing land. The classic example: herders on an open pasture each add animals, because each herder keeps the full value of another animal while the thinner grass is shared by every herd. Fixes: fencing and private ownership, or community rules on how many animals each family may graze, which local groups have often set and enforced themselves. Roads at rush hour. A free highway is rival when it is crowded, since each extra car slows every other car, and it is non-excludable because anyone can drive on it. Each driver weighs their own travel time but not the delay they add for everyone behind them. Fix: congestion pricing or tolls that charge more at peak hours.

The atmosphere. Emitting greenhouse gases uses up the atmosphere's limited capacity to absorb them without changing the climate, a capacity everyone shares, and no one can be shut out of emitting. Each emitter keeps the benefit of the activity while the climate cost is spread across the world, which also makes emissions a negative externality. Fixes: a carbon tax, which is a Pigouvian tax, or cap and trade permits. Antibiotic effectiveness. Each prescription helps one patient but adds a little to bacterial resistance, which weakens the drug for everyone later. The shared resource is the drug's effectiveness, and any prescriber can draw on it. Fix: prescribing rules and stewardship programs that limit use to cases that need it.

Testing both conditions also rules cases out. A toll road is not a commons, because the toll makes it excludable. An empty public beach on a weekday is not one either, because one more visitor crowds nobody, so use is not rival at that moment; the same beach on a holiday weekend can become one. National defense fails the rival test entirely, which makes it a public good, whose problem is too little being produced rather than too much being used. The full model, with a worked fishing example showing how open access doubles the number of boats, is in the tragedy of the commons explained, and common-pool resource vs tragedy of the commons separates the resource from the outcome.