Global patterns of interaction specialization in bird-flower networks

Abstract

AIM: Among the world's three major nectarâ feeding bird taxa, hummingbirds are the most phenotypically specialized for nectarivory, followed by sunbirds, while the honeyeaters are the least phenotypically specialized taxa. We tested whether this phenotypic specialization gradient is also found in the interaction patterns with their floral resources. LOCATION: Americas, Africa, Asia and Oceania/Australia. METHODS: We compiled interaction networks between birds and floral resources for 79 hummingbird, nine sunbird and 33 honeyeater communities. Interaction specialization was quantified through connectance (C), complementary specialization (Hâ â ²), binary (QB) and weighted modularity (Q), with both observed and nullâ model corrected values. We compared interaction specialization among the three types of birdâ flower communities, both independently and while controlling for potential confounding variables, such as plant species richness, asymmetry, latitude, insularity, topography, sampling methods and intensity. RESULTS: Hummingbirdâ flower networks were more specialized than honeyeaterâ flower networks. Specifically, hummingbirdâ flower networks had a lower proportion of realized interactions (lower C), decreased niche overlap (greater Hâ â ²) and greater modularity (greater QB). However, we found no significant differences between hummingbirdâ and sunbirdâ flower networks, nor between sunbirdâ and honeyeaterâ flower networks. MAIN CONCLUSIONS: As expected, hummingbirds and their floral resources have greater interaction specialization than honeyeaters, possibly because of greater phenotypic specialization and greater floral resource richness in the New World. Interaction specialization in sunbirdâ flower communities was similar to both hummingbirdâ flower and honeyeaterâ flower communities. This may either be due to the relatively small number of sunbirdâ flower networks available, or because sunbirdâ flower communities share features of both hummingbirdâ flower communities (specialized floral shapes) and honeyeaterâ flower communities (fewer floral resources). These results suggest a link between interaction specialization and both phenotypic specialization and floral resource richness within birdâ flower communities at a global scale.