Bombus-plant interactions defined by bipartite network analysis in an underexplored Mediterranean island (Sicily)

Bumblebees (Apidae: Bombus) are abundant pollinators in Mediterranean habitats; however, interaction networks with plants have not yet been investigated in large areas of Italy, including its islands. Here, we analyzed plant-bumblebee networks through seasonal transects in a suburban park and a nature reserve of Eastern Sicily. After molecular and morphological identification of bumblebees, we built bipartite networks extracting complementarity, niche overlap, modularity and nestedness. In total, we sampled 10 of the 15 species of Bombus known from Sicily. Richness was close to that predicted by accumulation curves, and no differences in community diversity were found between the two sites. At both sites, the networks were not nested, highly specialized and modular, with high complementarity and low niche overlap, again with no differences between sites. In both sites, species-level specialization varied considerably and was independent from sample size. The network topologies suggest a possible role of local interspecific competition and/or of interspecific morphological variation among generalist bumblebees. Comparison of previously published data revealed that networks in Mediterranean areas are more specialized than those with a continental climate. This suggests the possibility of increased competition and/or phenotypic variation under harsh Mediterranean conditions. The vulnerability of the Mediterranean basin to droughts caused by climate change calls for detailed studies on pollinator-plant networks for conservation plans.