Grassland biodiversity reflects the dynamic balance resulting from the vegetation and the utilization to which it is subjected, and constitutes an index of its ecological and conservational value ; it does also indicate how grasslands work and how sustainable their utilization can be. In mountains, the relationships between the utilization of space and biodiversity are strongly affected by the spatial dimension. The characteristics of space utilization by the farms are presented here thanks to results from various studies made for 20 years in the central Pyrenees. The spatial particularities of the farm lands affected the pastoral practices as well as the dynamics of swards and of landscapes. A reference model has been set up for the diagnosis of how the vegetation is controlled and utilized. The effect of pastoral practices on the number of species is illustrated by a number of actual instances. A poorly controlled management led at first to a larger diversity within fields and to a smaller one among fields, but later on to a rapid decline in both, as a consequence of the appearance of woody plants. How biodiversity became homogenized at the level of the landscapes is shown in an example involving the territory of a village. A rational organization of pasture utilization must be done at larger levels (a whole territory, the totality of farmers utilizing it) if the biodiversity in mountain landscapes is to be preserved.