This issue is raised without further details in Article 10c of the CBD (Sustainable Use of Components of Biological Diversity): "protect and encourage customary use of resources in accordance with traditional cultural practices that are compatible with conservation or sustainable use requirements." This Article is now in legal tribunals by native populations experiencing difficulties with norms they feel are being imposed on them (Goldman, 2004). Such rules may lead to confusion about ownership or accountability of "resources" that have meaning and values at the local as well as at the global levels or aggravate the situation of those who are marginalized by the negotiated rules (Allier, 1997, 2002).

     Studies on local management systems can contribute to designing new systems that better fit to evolving and dynamic conditions. Conceptual analyses has greatly benefited from scientific research since the late 1980s (Schlager and Ostrom, 1992; Sandberg, 1994; Le Roy, 1996; Chauveau, 1998; Lavigne Delville, 1998; Karsenty, 2003). Taking into account the different forms of knowledge involved, e.g., "explicit" and "incorporated," can lead to a more complex view of what is at stake in a range of situations (Box 7-1).

      The principle of legal plurality facilitates operational understanding of two coexisting legal worlds These normative productions were defined as "droits de la pratique", i.e., rights based on practice, as a "plural set based on different ages and particular stakes, actors and formalisms", specifying what is commonly designated as the "law of the land".

     A piece of land may be viewed as a "good" while the resources may be seen as "things" free of access or as an "having" (as defined above) open to harvesting by people other than the owner with his/her authorization. All these management practices may be subject to seasonal variation depending on the types of resources to be taken (grass, crops, berries, mushrooms, game, fish, etc.). The "right to hand over" (Chauveau, 1998) between the right of exclusion and of alienation, as hybrid forms of access to land, such as buying land for migrants, which gives them the right to pass it on to their heirs, but not the right to sell it. This traditional order may evolve with time. This system commonly falls within the more general social norms, and follows an intrinsic evolution as a result of overall change in the customary order, and of interactions with the positive law implemented by the modern state. The trend towards commoditization of land and resources will challenge the authority of these different modes and is likely to lead to individual property and ownership as understood by capitalist economy and modern law.

     However when the excess capacity of common goods is limited, congestion may turn the consumption of the good as rival, i.e., when an additional unit of the good consumed by one member negatively affects other members' satisfaction of the public good. An example of this situation is the fish-stock in oceans. Overfishing depletes the world's fishstock and threatens endangered species with extinction (Wouters and de Meester, 2003). Thus a complex set of laws and agreements have completed the UN Convention on the Law of the Sea (1982), which introduced two fundamental principles: (1) the territorial sea, providing a coastal state with the right to control a narrow band of sea as an exten-