Habitat fragmentation is the principal threat on biodiversity in semi-natural habitats. The consequences of fragmentation for biodiversity may be studied within the framework of landscape ecology. In this context, three paradigms have generated hypotheses that need to be tested in field situations: the insular biogeography paradigm, the small population paradigm and the metapopulation paradigm. Examination of two case studies showed that Belgian calcareous grasslands have undergone a drastic fragmentation process during the last century. Size of patches is shown to be the most important determinant for species richness, species composition and probability of presence of characteristic species in a fragmented system of calcareous grasslands in Calestienne, Belgium.