Interannual differences in pollinator contributions to pollen transfer are mainly driven by changes in pollinator abundance

With the rising threat to insect pollinators and the upcoming pollinator crisis, it is important to know how pollinators contribute to pollen transfer. The contributions of individual pollinator taxa to pollen transfer depend both on their abundance and on how much pollen each individual can carry, with overall importance being a multiplication of these two values. Here, we quantified pollen load across a diverse spectrum of insect pollinator taxa and variation in their abundance over 11 years. We found that, while variation in pollen load was relatively small among pollinator taxa (compared to relatively high variability among individuals within each insect taxon), the visitation levels changed significantly over the years, resulting in a high degree of variation in pollinator contributions to pollen transfer of each insect taxon at the community level. Thus, we conclude that the overall importance of pollinator taxa for pollen transfer is determined further by their abundances than by their taxon-specific capability for carrying various pollen loads. As the insect abundances vary over time and may change dramatically from year to year, our results highlight the importance of diverse and species-rich pollinator communities, as the population decline of one pollinator can be buffered by an increase in another pollinator taxa.