AbstractAimThe future of biodiversity in increasingly warmer mountains may be poorly predicted by climate variation if dispersal affects ecological change. We assessed the influence of dispersal limitations in the assembly of mountaintop communities, focusing on the relationship between proxies of flight abilities and species diversity in insects.LocationCantabrian Mountains, Spain.TaxonGrasshoppers (Orthoptera, Acrididae) and bumblebees (Hymenoptera, Apidae, Bombinae).MethodsWe analysed the magnitude of variation in the relative wing length of individuals, species and communities along elevation by means of phylogenetic multilevel and generalized least square models, to assess the environmental fit of this morphological trait. Then we analysed whether wing length variation among assemblages affected species diversity and the biotic interchange between foothills and mountaintops, and between nearby mountaintops, by means of linear models and metrics quantifying dispersal.ResultsGrasshoppers and bumblebees converged in the evolution of shorter wings at higher elevations. The effects of this adaptation scaled to the community level and affected diversity patterns. Mountaintop assemblages were richer (grasshoppers) or shared more species with lowlands (bumblebees) when the average wingspan of their member species was larger. The species composition of mountaintops was significantly affected by dispersal processes and their species richness was more strongly correlated with that of their foothills than that of nearby mountains.Main ConclusionsThese results show a wingspan reduction in upland insects, the role of dispersal in improving species richness and reducing beta diversity, and the dependence of mountaintop diversity from the species pools of foothills. In these settings, we can envisage that upward movements of long‐winged species will be favoured and increase the species richness and nestedness of upland biotas as climate warms. However, the fate of upland inhabitants will depend on how they tackle novel biotic and abiotic pressures, given the constraints to peak‐to‐peak displacement.