In this chapter, we focus on the ifficulties of public policies implementation whan they face ecological and population constraints. We rely on the use of original time series on landuse in the Causse de Sauveterre, combined with the Agent Based Simulation Model to describe and simulate several mode of implementing agricultural policy. While controling stocking rate, the timing of the implementation of such a policy instruments is crucial. Acting to late with regard to the ecological process becomes costly and innefficient. Moreover, in order to avoid the generalisation of the same type of landscape accross the region, it's necessary to organize landscape planing at both very local and very regional level.