Aim: To evaluate community assembly processes, we comprehensively analysed avian functional and phylogenetic diversity along the only complete elevational forest gradient in west-central Africa. We aimed to reveal the signals of environmental filtering and interspecific competition in community assembly and determine whether increases in species richness across elevations are associated with niche space packing or expansion.

Location: Mount Cameroon, west-central Africa.

Taxon: Birds.

Methods: We analysed data on elevational distributions of birds collected within 10 years from the lowland forest up to the treeline. We calculated functional richness, evenness, and nearest neighbour distance, as well as phylogenetic mean pairwise dissimilarity and mean nearest taxon distance. Community assembly processes were inferred from each metric, after controlling for species richness. To estimate the importance of niche packing versus niche expansion, we assessed whether species of species-richer assemblages occurred within or outside the niche space of species-poorer assemblages.

Results: Stronger signals of environmental filtering were observed at mid-elevations. Functional distances among species increased at high elevations, supporting the importance of the Fox’s guild assembly rule (Fox, 1987). Species richness increments were mainly associated with niche packing. We found random basal phylogenetic structure across elevations, whereas terminal phylogenetic clustering appeared in lowland and highland areas.

Main conclusions: Environmental filtering showed a rather weak effect towards higher altitudes over the highly productive and relatively short tropical forest elevation gradient, where niche packing was the dominant process behind the species richness increments. Functional distances among species increased at higher elevations, suggesting lower species density in ecological space. At the same time, mild environmental conditions and a high level of resources in the lowlands may sustain functionally redundant species, downplaying the role of competition. Phylogenetic clustering at low and high elevations further suggests the existence of distinct lowland and highland bird faunas.