Natura 2000 : debated expertise - The effect of the directive is to make the decision from a scientific point of view reversible because it is always possible to turn back, and, also, to give a frame to certain decisions and to stop the proliferation of debates because it deals with the very elements of nature consolidated by various normative processes. Instead of the fragile balance of any social négociation, the process of identification of sites prescribes a "cognitive frame "..