A common resource is a resource readily accessible to all members of the public who wish to obtain benefits from it. Some examples are natural, like forests, rivers, and lakes. Others are made by humans, as in the case of irrigation canals and reservoirs. With a common resource, limiting availability would be difficult and members of the public theoretically can enjoy unrestricted use. Such resources are vulnerable to overuse, a concern sometimes addressed with legislation and other steps designed to protect common resources.
A big problem with common resources is that greed on the part of individuals can ruin the resource for the community. In an example using public pasturelands, if one farmer chooses to pasture more animals than his fair share, they will deplete the pasture, harming everyone's livestock. On the other hand, if a sustainable number of livestock can be determined and the people who share the land agree to limit their livestock to this number, everyone can enjoy the common resource, in addition to preserving it for future generations.
In a situation known as the tragedy of the commons, common resources are destroyed by acts of greed or a poor understanding of the limitations of that resource. People may take too much water from a river, for example, not realizing that they are basing their usage on water levels from flood years. In a situation known as the free rider problem, some people use more than their fair share of a common resource and everyone suffers because their over-exploitation of the resource diminishes the amount available to the community.
Humans have been taking advantage of common resources for thousands of years and numerous studies have been conducted to learn more about how people interact with common resources. To this day, arguments among different groups over resource usage can be very contentious, as seen among nations with disputed resources along shared borders. The Colorado River in the United States, for example, is a heavily exploited common resource disputed by multiple states, as well as Mexico. All of the claimants to the river's water rely on it, forcing them to negotiate a fair division of its contents.
Privatization of common resources is also a topic of concern in some areas of the world. While some resources are protected for use by members of the public, others may be purchased by private firms. People who previously enjoyed a resource for free or at low cost may resent having to pay for it, and privatization can limit access to the wealthiest individuals.