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

Abstract

Aim: Most 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.Location: Nanling Mountains, southern China. Taxon: Birds (Timaliidae, Pycnonotidae and Sylviidae)Methods: Breeding 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.Results: Contrary 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 conclusions:This study highlights how strong filtering can produce a counterintuitive, 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.

Trait–environment relationships differ between mixed‐species flocking and nonflocking bird assemblages

“…In functional diversity studies, dietary guild and vertical stratum have commonly been related to resource requirements and species' sensitivity to habitat change (Petchey et al 2007). Wing chord and body mass have been used to calculate dispersal ability (Zhang et al 2020). Breeding phenology and fecundity responses to environmental stress conditions include a combination of fecundity-survival schedules and behavioral strategies that yield the highest fitness at higher elevations (Badyaev and Ghalambor 2001).…”

“…Breeding phenology and fecundity responses to environmental stress conditions include a combination of fecundity-survival schedules and behavioral strategies that yield the highest fitness at higher elevations (Badyaev and Ghalambor 2001). In our study, trait data for most species were collected from individuals captured by mist-netting at the study site (Zhang et al 2020), with missing values for species measured on museum specimens from localities as close as possible to the study site, and from published literature about the birds of southern China (Zhao 2001; Appendix S1: Table S3).…”

Diversity of Bird Communities in Tea (Camellia sinensis) Plantations in Fujian Province, South-Eastern China

“…In Yunnan, Yang et al [47] found 43 species, and surveys in Anhui Province report 14-31 species [22,48]. An extensive survey in a protected forest area at similar geographical latitude in the Nanling Mountains, Guanzhou Province, reported 80 species [18], and a year-long survey found 51 species in Dinghushan Nature Reserve, in the neighbouring Guangdong Province [49]. Unfortunately, a more quantitative comparison is not possible, because most of the pre-existing literature contains occurrence lists and descriptive notes only.…”

“…China, with its 1300+ species, is a country with a high diversity of birds, especially in the south [16]. Previous studies of forest birds in the south of China detected >110 species in traditionally managed forests [17] or 80 species in protected ones [18]. Agroforestry [19] and shade-grown coffee [20] or cocoa [21] can support several forest-based bird species in Asia, and Central-South America.…”

Pitfalls of ignoring trait resolution when drawing conclusions about ecological processes

“…We emphasize, though, that the magnitude and direction of the effects of trait resolution on perceived community structure are currently poorly known, and this bias may not be limited to the narrow conditions presented here. Moreover, our conceptual predictions assume a single strong process, but non‐random structure is more difficult to detect when multiple interacting processes are acting upon a community (Aiba et al., 2013; Götzenberger et al., 2016; Kraft & Ackerly, 2010), a more realistic expectation (Spasojevic & Suding, 2012; Zhang et al., 2020). It is thus likely that, in real communities, inflated convergence resulting from low trait resolution may lead to incorrectly concluding environmental filtering to be a driving factor when the true pattern is random (Type I error).…”