P. Laiolo

Aim Climate warming is shifting the bioclimatic optima of species towards mountaintops, but the ability of organisms to track these changes also depends on their dispersal skills. Here, we assessed the role of dispersal over niche‐driven processes in connecting assemblages along mountain slopes and between mountain massifs. Location Cantabrian Mountains, Spain. Taxon Birds (Animalia; Aves) and Lichens (Fungi; Ascomycota, Basidiomycota). Methods We examined the change with elevation of community‐level traits that are dispersal proxies (wing shape in birds and type of dispersing propagule in lichens) and ecological niche proxies (micro‐habitat, substrate, and foraging features). We then permutate species composition within sites and massifs to create models of species distribution constrained by dispersal and niche processes. These models were compared with observed species distribution to disclose the relative contribution of dispersal and niche‐based processes in the biotic interchange along mountain slopes (vertical connectivity) and between isolated summits (horizontal connectivity). Results Both bird and lichen communities were formed by species with traits that enhance dispersal at high elevations. These groups also showed similarities in the elevational patterns of niche diversity, which dropped at high elevations. Dispersal was by far the dominant assembly mechanism in both taxa. Pairwise community comparisons among elevation belts showed weak vertical connectivity, with predominant dispersal limitations but also niche barriers between the extremes of the gradient. Among massifs, horizontal connectivity was higher among high mountain assemblages than those from lower elevations. Main Conclusion Dispersal was found to be the dominant assembly mechanism in mountain systems, even in taxa with high dispersal potential. Highland assemblages had low functional diversity but their species had high mobility. This permits biotic interchange between isolated summits and, potentially, colonization of other summits as climate warms. Our framework combining traits and occurrence‐permutation models improve the understanding of community assembly mechanisms along elevation gradients and points to dispersal limitations, especially at low‐middle elevations.

Aim The 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. Location Cantabrian Mountains, Spain. Taxon Grasshoppers (Orthoptera, Acrididae) and bumblebees (Hymenoptera, Apidae, Bombinae). Methods We 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. Results Grasshoppers 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 Conclusions These 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.