and Dayton-Johnson (Chapter 3). However, studies at the international level, especially studies of international peace and provision of international public goods, suggest that heterogeneity induces cooperation (Martin, 1993, 1995). Although most scholars agree that heterogeneity of resource users makes a difference, considerably more work is needed to clarify this concept and its effects.17

It has become increasingly clear that global and local common-pool resources are not only analytically similar, but interrelated. The use of resources at the local level affects international and global resources, and vice versa. Thus devising the rules for using international and global resources requires a careful examination of local characteristics of resource use. For example, to devise a workable international regime for the use of global atmosphere as a sink for greenhouse gases, it is important to understand that different resource users emit various greenhouse gases for various reasons, that these uses cannot all be measured with the same degree of reliability, and that different resource users have drastically different capabilities to reduce their resource use. Many of these issues of linkage and interplay among institutions at different scales are discussed more fully in Chapters 8 and 9.

Cost of Measurement

To devise effective institutions that limit the use of common-pool resources so that they do not suffer congestion, overuse, or destruction, one needs to be able to measure the quantity and location of resource units. Common-pool resources vary substantially from one another in the reliability and cost of measuring current stocks and flows and predicting future conditions. Schlager and colleagues (1994) identify two physical attributes of resources that have a strong impact on the ease of measurement: the capability for storage and the mobility of resources. Storage (for example, a dam on a water distribution system) allows managers and users to measure the stock of a resource and to allocate its use over time in light of good information about what is currently available. Mobile resources, such as wildlife and undammed river water, are much harder to measure and account for than stable resources, such as forests and pasture lands. Again, the mobility of the resource makes measurement, and thus management, of wildlife much more difficult than stable resources.

The Search for Effective Institutions

Devising better ways of governing resource systems will continue to be a major issue in the new century. Climate change, loss of biodiversity, ozone depletion, the widespread dispersal of persistent pollutants, and most other environmental problems involve the commons. Practitioners at international, national, regional, and local levels will continue to seek solutions and to debate the appropriate roles for government, private, and community ownership of natural re-

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