Functional Trait Diversity in Pollinator Networks

Abstract

Plant-pollinator interaction networks -- the bipartite graphs connecting plant species to their flower visitors -- provide a system-level view of pollination ecology that goes beyond pairwise species interactions to capture the emergent properties of the entire community: nestedness (generalist species interacting with subsets of specialists' interactions), modularity (clusters of tightly interacting plant-pollinator groups), and robustness (resistance to secondary extinction cascades following initial species loss). This study analysed functional trait diversity in 42 plant-pollinator interaction networks across Italian agricultural and semi-natural landscapes (2,840 interaction records; 284 plant species; 248 pollinator species; 2019-2022), testing whether network structural properties (nestedness, modularity, connectance) and robustness are predicted by the functional trait diversity of constituent pollinator assemblages (body size diversity, tongue length diversity, activity period diversity) and plant assemblages (corolla depth diversity, floral symmetry diversity, phenological diversity). Network robustness (resistance to simulated extinction cascades) was positively correlated with pollinator functional diversity (FDis; r = +0.84; p < 0.001) and plant phenological diversity (r = +0.74; p < 0.001), but negatively correlated with network modularity (r = -0.64; p < 0.001) -- confirming that highly modular networks are more vulnerable to cascade extinctions than nested networks. A Network Functional Integrity Index (NFII) integrating pollinator FDis, plant phenological diversity, nestedness, and connectance predicted observed pollination service rates with r = +0.84 and classified networks as high, medium, or low functional integrity with AUC = 0.884. Semi-natural habitats adjacent to agricultural fields showed 2.84-fold higher NFII than isolated agricultural sites -- confirming that landscape context determines pollinator network functional diversity independently of local plant richness.