Abstract
Elevational gradients offer valuable opportunities to investigate biodiversity patterns and the ecological and evolutionary processes that shape them. Although tropical mountains are recognized as biodiversity hotspots, the various dimensions of biodiversity in these systems, particularly in equatorial Africa, remain poorly understood. In this study, we examined the functional (FD) and phylogenetic diversity (PD) of bird assemblages along a primary forest elevational gradient in Cameroon, West-Central Africa, spanning from lowland forests to the treeline (~2300 m a.s.l.). We analyzed how FD and PD vary with elevation and tested the roles of abiotic filtering and biotic interactions, such as competition, in community assembly. Additionally, we assessed whether taxonomic diversity (TD) increases through niche packing or expansion, based on morphological and resource-use traits. Using null models and bird occurrence data, we inferred the drivers of FD and PD patterns and evaluated whether species in more diverse assemblages occupied novel functional space compared to less diverse assemblages. Our results showed that both functional richness and TD declined with elevation, whereas functional nearest neighbor distance, functional evenness, and mean nearest taxon distance increased. Traits related to resource use suggested that bird species at higher elevations were functionally less similar than expected by chance, partially supporting the influence of competition consistent with the guild assembly rule. Phylogenetic clustering observed at both low and high elevations pointed to independent species radiations, likely shaped by historical forest dynamics. In species-rich lowland assemblages, we found evidence of niche packing, suggesting increased specialization or niche overlap. In contrast, niche expansion appeared to contribute to higher TD at elevated sites. Overall, our findings indicate that while abiotic filters along forested elevational gradients and competition in lowland forests play roles in shaping avian diversity, they are not the sole or dominant mechanisms. Nonetheless, partial support for competition aligns with theoretical expectations under the guild assembly framework.