Patterns and mechanisms of bird community assembly along an Afrotropical elevational gradient in Kenya

Elucidating the underlying factors driving the rich biodiversity in mountainous regions remains a central yet unresolved question in ecology, especially for tropical regions. The classical community assembly theory posits that environmental filtering and interspecific competition result in spatial variation in local communities. In this study, we characterized the taxonomic, phylogenetic, and functional diversity of birds, and examined the potential role of deterministic processes in shaping community assembly along elevational gradients in Mt. Kenya, a typical Afromontane ecosystem. Furthermore, we employed a novel partial least squares regression (PLSR) methodology to investigate the potential ecological mechanisms that may account for bird distribution patterns. The gradual unimodal or monotonic decline in richness metrics (species richness, phylogenetic diversity, and functional diversity) and/or phylogenetic and functional mean pairwise distances with increasing elevation suggested a progressive loss of bird species across clades at higher elevations. Conversely, in the productivity-scarce highlands, heightened competition led to reduced functional affinity among community members, resulting in increased functional mean nearest taxon distance. Moreover, the shift from overdispersion to clustering in terms of phylogenetic community structure with elevation indicated a potential transformation from limiting similarity to abiotic filtering, while the lack of clear patterns or weak increased tendencies of the functional assemblage structures suggested complex interactions between ecological and evolutionary processes. Our investigation highlighted several potential explanations for the observed diversity gradients, including factors related to primary productivity (plant species richness and enhanced vegetation index), water-energy availability (annual mean temperature and potential evapotranspiration), and climatic stability (temperature annual range). Our findings highlight the need for a comparative approach incorporating multidimensional measures to study biodiversity spatial distribution patterns, community assembly processes, and sustainable conservation of tropical Afromontane biodiversity.