resources that can be stored. Shellfish and grazing lands are stationary but their degree of storage is limited, and conversely, irrigation canals with reservoirs have water resources that can be stored, but are mobile. Sheep flocks and cattle herds owned and/or managed as common property also would fall in this last category.

After examining the impact of these two physical characteristics of resources on externalities, Blomquist and colleagues conclude that these two factors have an impact on management because of their relationship to information. Greater mobility of resources and difficulties of storage make it more difficult for users to adhere to institutional solutions to common-pool resource dilemmas because of their impact on the reliability and costs of information needed for such solutions.30 This point also can be seen as a question about the extent to which resource availability is predictable, something noted by Naughton-Treves and Sanderson (1995) as well, and how unpredictability affects the abilities of users to allocate available resources or undertake activities that would augment supply (see also Wilson, this volume:Chapter 10).31

A second broad area to which the analyses by Wade, Ostrom, and Baland and Platteau pay only limited attention is the external social, institutional, and physical environment.32 Thus none of them explicitly remark on demographic issues in their conclusions, and they put equally little emphasis on market-related demands that may make local demand pressures relatively trivial. But variations in levels of population and changes in demographic pressures, whether as a result of local changes or through migration, are surely significant in influencing the ability of users to follow existing rules and norms for resource management. Indeed, there is an enormous literature that focuses on questions of population and market pressures on resource use and asserts the importance of these two complex factors.33

Writings on the role of population in resource management have a long history and an impressive theoretical pedigree (Ehrlich, 1968:15-16; Malthus, 1798, 1803, rpt. 1960). Much recent scholarship links environmental degradation in a relatively straightforward fashion with population growth (Abernathy, 1993; Durning, 1989; Fischer, 1993; Hardin, 1993; Low and Heinen, 1993; Pimental et al., 1994). On the whole it is clear that the debate is highly polarized. Some scholars assert that population pressures have an enormous effect (Ehrlich and Ehrlich, 1991; Myers, 1991; Wilson, 1992), and a smaller but vocal group suggests the impact to be far more limited (Lappé and Shurman, 1989; Leach and Mearns, 1996; Simon, 1990; Tiffen et al., 1994; Varughese and Ostrom, 1998).

The story is somewhat similar where markets are concerned, except that the terms of the debate are less polarized and there is wider agreement that increasing integration with markets usually has an adverse impact on the management of common-pool resources, especially when roads begin to integrate distant resource systems and their users with other users and markets (Chomitz, 1995; Young, 1994). As local economies become better connected to larger markets and common property systems confront cash exchanges, subsistence users are likely to



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