Trait‐mediated filtering drives contrasting patterns of species richness and functional diversity across montane bird assemblages

Abstract

AbstractAimMost studies of diversity changes along elevational gradients show a synchronous change in taxonomic, functional and phylogenetic diversity, such that all decline with elevation or show a mid‐elevation peak. However, some studies show asynchronous changes, which challenge us to explain their assembly. Here we used functional trait variation and niche spacing to test the roles of environmental filtering and competition‐driven niche separation in assembly and biodiversity patterns across an elevational gradient.LocationNanling Mountains, southern China.TaxonBirds (Timaliidae, Pycnonotidae and Sylviidae)MethodsBreeding birds were surveyed in 2012–2017 from 300 to 1900 m ASL. We tested bird diversity pattern and community structure relevant to environmental filtering and limiting similarity by integrating functional traits and evolutionary distances, as well as vegetation heterogeneity. A null modelling approach tested specific trait variation and niche spacing in relation to habitat associations and niche segregation.ResultsContrary to expectation, bird richness peaked at just below the highest site, and functional diversity declined significantly with elevation. Functional–phylogenetic distance metrics converged with increasing elevation, corresponding to strong environmental filtering and habitat change. For the spatial arrangement of specific traits, restricted dispersal ability, elevational range and clutch sizes were related to filtering effects, as indicated by trait convergence in clustered highland bird assemblages. Conversely, overdispersed lowland assemblages have evenly spaced trait values predicted by limiting similarity, including bill length, vertical stratum‐use and flocking propensity. In addition, bird functional diversity was highly correlated with both vegetation productivity and tree height, which reinforces the role of filtering in structuring local bird assemblages.Main conclusionsThis study highlights how strong filtering can produce a counter‐intuitive, contrasting pattern of changes between species richness and functional diversity. Fundamental assembly processes and species roles in ecosystems are predictable through examining key traits of life‐history and foraging characteristics, and their associations with environmental variables.