Pollinator specialization increases with a decrease in a mass-flowering plant in networks inferred from DNA metabarcoding

Abstract

How native mass-flowering plants affect the specialization of insects at individual and species levels and the consequences for pollination networks have received much less attention than for mass-flowering crops or alien species and basically remain unexplored.Using existing DNA metabarcoding data on the pollen loads of 402 flower-visiting insects, we assessed the effects of a native mass-flowering plant of high reward quality, the shrub Rhododendron ferrugineum, on pollination networks by investigating: (a) the food niches of individual pollinators and pollinator species and (b) the structure of individual and species networks in subalpine heathland patches with extremely contrasted densities of R. ferrugineum.Relative to its high abundance in high-density patches, the shrub was greatly underrepresented and did not dominate individual's or species' generalized networks, rather individual and species specialization increased with a decrease in R. ferrugineum density. Furthermore, individuals of the more generalist dipteran Empididae species tended to extend exclusive interactions with rare plant species in low-density networks. The same trend was observed in the more specialist Apidea but toward rare species in high-density networks. Our results reveal a quite paradoxical view of pollination and a functional complementarity within networks. Niche and network indices mostly based on the occurrence of links showed that individual pollinators and pollinator species and networks were highly generalized, whereas indices of link strength revealed that species and above all individuals behave as quite strict specialists. Synthesis. Our study provides insights into the status of a native mass-flowering plant in individual's and insect species' food niches and pollination networks. It revealed that a generalist pollinator species can be highly specialized at the individual level and how rare plant species coexisting with mass-flowering plants may nevertheless be visited.

Keywords: DNA metabarcoding; food niche breadth; individual pollination network; mass‐flowering species; pollinator generalization; pollinator specialization; species pollination networks.

Figure 1

Change in linkage degrees from community to individual pollinators. L _sp and L_i: species and individual linkage degrees; L _{sp (80%)} and L_{i (80%)}: minimum number of links required to gather 80% of the pollen load of a pollinator species and of each individual, respectively. Box‐and‐whisker plots represent values for all combinations of patches × species

Figure 2

Variation in food niche breadth among pollinator families. SPO, species food niche overlap; TNW, total niche width; WIC, within‐individual component of niche breadth

Figure 3

Effects (mixed linear model) of Rhododendron ferrugineum floral density on species and individual food niches and networks. Black lines show the independent effects of floral density, and the colored lines show family‐floral density interaction effects. The size of the symbols is proportional to the size of the sample