There is considerable interest in the fragmentation and loss of natural and semi-natural habitats, but few studies have examined the dynamics and mechanisms of change. A temporal analysis of landscape change on the South Downs in Sussex, UK, provides a clear description of the process of change over the 20 years 1971–1991. Transition probabilities were calculated from digital interpretations of an aerial photography time series of West Sussex. The analysis enabled quantitative comparison of landscape mosaics within different landscape ecoregions and under different management regimes to be made. Past changes in land use have produced a fragmented downland landscape. The key land conversion sequences identified show a substantial transition towards arable production, often at the expense of the internationally and nationally important unimproved grassland systems. A geographical information system facilitated greater understanding of the environmental and topographical characteristics of land converted to arable and other uses, and highlighted areas for protection and potential restoration. The patterns of land-use conversion observed in the study provide a landscape-scale planning tool for assessing the potential impact of agri-environmental policies, plans, and programmes in semi-natural grassland habitats.