Dispersal constrains the biotic connectivity of mountain assemblages

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.