The 'tragedy of the commons' is a fairly well known ecological dilemma, which arises from the fact that people will exploit a shared resource to exhaustion before they will deplete a resource that they own themselves. This concept was first described by the ecologist Garrett Hardin in 1968 and is often used to help plan for sustainable development in an area.
The basic principle underlying the tragedy of the commons is this: an individual who uses their own resources for a service gets the benefit of that resource, but at a cost - they have had to use up resources that they personally own and consequently, cannot use them again. However, an individual who uses a shared resource for a service still gets that all of the benefits that this entails, but they are not using up their own resources. Thus, the individual gets all of the benefits from a service, but at no personal cost. So using common resources seems like a good idea, right? And yes, it is for an individual at their own personal level. The problem is however, that many individuals will have worked out that they can use the resource at no cost to themselves and as such, the common ground will be over used and will be exploited until it is completely depleted and becomes useless. This fact led Hardin to coin the phenomenon of the 'tragedy of the commons': multiple individuals that are acting rationally and in their own personal best interests (as they usually are) will ultimately deplete a shared resource, even when it's clear that this is not always in the best interests for the resource for them to do so.
The classic example for the tragedy of the commons and the one used by Hardin to first describe it, is medieval land tenure by European herders. A group of herders all have their own land on which to graze their animals, which by using, they will benefit by having their livestock fed and healthy. However, by feeding their animals off their land they are using up their own grass and without grass, the land's soil is washed away and it may eventually become useless. Knowing this, herders take their livestock to the village or town common instead - land which none of them own and allow their livestock to graze here. Therefore, their animals are still fed but they are not degrading the quality of their own land so that the herders, on a personal level, are in a 'win-win' situation. But, although this is good for a single herder, there is a major problem with this as you've no doubt realised. The problem is that while a single herder may have for example, 10 sheep, which would take a very long time to use up and deplete a field, multiple herders are using the field at the same time. Hence, if 10 herders use it who all have 10 sheep, then 100 sheep are all feeding off the same field - meaning that its resources will quickly be depleted. This rapidly degrades the quality of the common, meaning that soon it will not be able to support livestock and, with poor muddy ground, cannot really be used for recreation either (the purpose for which it was made available to everyone).
The tragedy of the commons can be applied to all common resources and modern-day examples include overfishing in the world's oceans and industrial logging in the world's rainforests: such resources are either not privately owned or are too big for that ownership to be enforced effectively, so that they are overexploited by multiple groups and as a result, are being rapidly degraded.
The tragedy of the commons is frequently used in arguments that support sustainable development programmes, allowing such programmes to satisfy both conservation and economic entities by encompassing economic growth and environmental protection. Furthermore, it is regularly used as a warning against the implementation of policies that restrict the use of private property or espouse the expansion of public land - factors that may drive individuals into exploiting shared resources! The tragedy of the commons is also used to argue for the privatisation of resources in order to protect them and although desirable, this is often impractical. How, for example, can you privatise the ozone layer or honeybees? To complicate things even further some areas, such as the Amazon Rainforest, are just too big - even it was completely privately owned it is unlikely that it could be protected in its entirety and therefore it is unlikely that illegal logging could be effectively prevented!
Therefore the tragedy of the commons is an issue of logic, where any rationally acting individual will first exploit a shared resource before using their own because this is in their best interests. This means that it is very hard to prevent and in order to combat it, policies must be put into place to control overexploitation and where possible, encourage individuals to use their own resources preferentially to shared ones.