Functional trait space of forest passerine bird assemblages along a latitudinal gradient in China

Abstract

Quantifying the functional trait space of natural assemblages could help ecologists understand the underlying mechanisms that sustain species richness. In this study, we investigated the functional trait space patterns of six forest passerine bird assemblages, with species richness increasing to lower latitudes. Further, we examined the underlying ecological processes that maintain species coexistence along the latitudinal gradient. The functional trait volume and the mean nearest neighbor functional distance were calculated to quantify the functional trait space. In order to determine whether the observed value was different from a random expectation, we obtained the standardized effect size of the functional trait volume and the mean nearest neighbor functional distance using null models. We also explored the indicative function of environmental factors, specifically climate and vegetation structure, to functional trait space occupation patterns. The relationship between the two standardized effect sizes and explanatory environmental factors were modeled using a multiple linear regression. Our results revealed that species richness and the observed functional trait volume of the six assemblages increased from high to low latitude, whereas the observed mean nearest neighbor functional distance decreased. The observed functional trait volume of one assemblage was significantly lower than the null expectation along the latitude gradient, while the observed mean nearest neighbor functional distance of all assemblages showed higher than null expectation aside from the assemblage from the lowest latitude, two out of the five assemblages showed significant. Our study suggests that environmental filtering is important for passerine bird assemblages from this regional species pool. Simultaneously, the spacing of species within functional space tend to be constrained by negative biotic interactions across the latitudinal gradient. In addition, we found that both the climate factor and vegetation structure were important indicators of environmental filtering, with vegetation structure potentially acting as a key factor influencing the biotic interactions of forest passerine bird assemblages on a local scale.

Keywords

• Forest passerine bird
• Functional traits space
• Ecological process
• Environmental variable
• Latitudinal gradient