A growing number of studies argue that forest transition should be enhanced by policymakers given its potential benefits, for instance in slowing climate change through carbon sequestration. Yet the effects of forest transition in landscape heterogeneity and biodiversity remain poorly understood. In this paper we explore the relationships between the forest transition and the landscape changes occurred in a Mediterranean mountain area. Historical land-use maps were built from cadastral cartography (1854; 1956; 2012). Metrics on land-cover change, landscape structure, and landscape functioning were calculated. Multiyear data on butterfly assemblages from two transects (1994 2012) was used as indicator of landuse change effects on biodiversity. Results show a forest expansion process in former cereal fields, vineyards and pasturelands along with rural outmigration and land abandonment. Such forest transition involved large changes in landscape structure and functioning. As peasant management of integrated agrosilvopastoral systems disappeared, landscape became less diverse. Even if forest area is now larger than in mid-nineteenth century, ecological connectivity among woodland did not substantially improve. Instead, ecological connectivity across open habitats has greatly decreased as cereal fields, vineyards, meadows and pasturelands have almost disappeared. Butterfly assemblages under changing land-uses highlights the importance of agro-forest mosaics not only for these species but for biodiversity at large in the last decades. Our work emphasizes that conservation of landscapes with a long history of human use needs to take into account the role of humans in shaping ecological features and biodiversity. Hence the suitability of forest transitions should be critically examined in relation to context and policy objectives.