11 páginas, 5 figuras, 1 tabla. Although it is well established that butterfly richness is affected by climate and human factors (e.g. habitat disturbance and degradation) at different spatial scales, the drivers behind these changes vary greatly according to the geographical region and the ecology of the species concerned. It is essential that this variation be understood if trends in diversity are to be predicted with any degree of confidence under a scenario of global change. Here we examine patterns of butterfly species richness among groups differing in degree of habitat specialization, diet breadth and mobility in the north-west Mediterranean Basin, a European hotspot for this taxon. We analyze a large number of butterfly communities and take into consideration the main potential drivers, that include climatic, geographic and resource variables, landscape structure and human environmental impact at different spatial scales. Our study shows that both climatic and anthropogenic factors play an important role in determining butterfly species richness in the north-west Mediterranean Basin, but that their relative impact differs between specialist and generalist groups. At lower altitudes, water availability, a product of the interplay between temperature and rainfall, and negative effects of temperature appear as the most determinant factors. Maximum diversity was observed at mid-altitudes, which reveals the importance from a conservation point of view of Mediterranean mountain ranges. The results suggest serious population declines in specialist species restricted to mountain areas as a result of climate warming in combination with habitat loss caused by the abandonment of grazing and mowing. They also suggest negative trends for generalist species due to an increase in aridity in combination with an increase in intensification of human land use in lowland areas. Such synergies are expected to lead to rapid declines in Mediterranean butterfly populations in the coming years, thereby posing a severe threat for the conservation of European biodiversity. This research was supported by grants from the Spanish Government (CGL2006-04025/ BOS, CGL2009-08798/BOS and Consolider-Ingenio Montes CSD2008-00040) and the Catalan Government (SGR 2009- 458). J. Carnicer was funded by 2008-0200 MEC/Fulbright Postdoctoral research grant. The Butterfly Monitoring Scheme in Catalonia is funded by the Dept de Medi Ambient i Habitatge de la Generalitat de Catalunya. The Diputacio´ de Barcelona, Patronat Metropolita` Parc de Collserola and Fundacio´ Caixa de Catalunya have also given financial support to this project. The BMSAnd project is supported by the Centre d’Estudis de la Neu i la Muntanya d’Andorra (CENMA), from the Inst. d’Estudis Andorrans (IEA). Peer reviewed