Different influences of phylogenetically conserved and independent floral traits on plant functional specialization and pollination network structure

Abstract

Plant specialization and pollination network structure play important roles in community assembly. Floral traits can mediate plant-pollinator interactions and thus have important impacts on nestedness and modularity of pollination network. When such traits are phylogenetically conserved, therefore, phylogeny and traits should predict network structure to similar degrees. Moreover, conserved network structures were also found attributed to pollination syndrome or pollination system. However, we still know little about the relation between pollination syndrome and pollination network, especially under a phylogenetic framework. Herein, we established a phylogenetic framework including five floral traits (flower density, floral size, floral shape, floral symmetry, and floral color) and five species-level metrics (species strength, weighted closeness, specialization d', nestedness contribution, and modularity contribution) to test how floral traits could directly or indirectly influence species' specialization and network structure in central China. Phylogenetic signals were found in all floral traits except flower density. Structural equation model and phylogenetic structural equation model results showed that both floral size and floral density affected plant specialization and its contribution to network modularity indirectly. However, compared with phylogenetic independent flower density, phylogenetic conserved floral size had much more complexed influences, having a direct influence both on species' specialization and on modularity contribution. In this nested and modular network, abundant species with larger flowers tend to be more central and had larger values of z. Floral shape, symmetry, and color could act as co-flowering filters in pollination sharing and help to shape network modularity. Our results emphasize that phylogenetically conserved traits partially represent pollination syndrome and are important drivers for modular structure of local pollination network. This study may improve the understanding how the evolutionary history and ecological process drive local network structure and dynamics.

Keywords: floral size; flower density; phylogenetically conserved floral traits; phylogenetically independent floral traits; plant specialization; pollination network.

Figure 1

Network modular structure (A) with plant species’ roles in the network and floral traits as shown in (B). (A) Four modules delineated by red boxes, which was calculated using computeModules functions in the R package “bipartite”. The color intensity indicates the interaction frequency between partners. Species are sorted according to their modular affinity; plants as rows and pollinators as columns. (B) Role in network of each species, and their flower density, floral size, floral shape were shown as different color. The heatmap of flower density and floral size was drawn using the package “complexHeatmap” (Gu et al., 2016) in R.

Figure 2

The SEMs showing how floral traits influence network metrics in the transect. (A) shows that the pattern of floral size, with phylogenetic constraints, has influences on the plant–pollinator network by PGLS, and (B) shows that flower density, with phylogenetic independence, has effects on the plant–pollinator network via GLMs. Paths among variables included in the model are shown. The solid arrows indicate a positive effect of a variable on another. Standardized path coefficient was given on each arrow.