In densely populated northwestern Europe, native large mammals are confronted with a very fragmented landscape, and most of the areas they inhabit are island-like reserves threatened with total isolation from other reserves. The only way to counteract the threat of further decline in the numbers of large-mammal species is to restore their habitats. The appropriate size of future reserves could be estimated from the habitat requirements of wild ungulates that are considered key species for ecosystem functioning. The species selected to guide the design of large nature reserves in northwestern Europe is the red deer ( Cervus elaphus) because of its widespread distribution, key role in ecosystem functioning, and home-range size. We describe a network analysis of the Netherlands, Belgium, and adjacent parts of France and Germany, performed with the LARCH landscape ecology model, that was conducted in order to identify the structure of the ecological network for red deer and the spatial connectivity of the landscape. The resulting maps show areas that could support viable populations and indicate habitat areas that will support persistent populations only if they are in a network of linked habitats. The gaps and barriers that prevent connectivity in such networks guide the design of effective corridors to increase spatial connectivity. The results of our analysis can be used for policy decisions on nature conservation and spatial planning, and the method is applicable to other regions and species.