The impact of plant diversity and vegetation composition on bumblebee colony fitness

The current decline in pollinators may disrupt ecosystems and ecosystem services with potentially harmful effects on nature and human society. While the importance of habitat loss and fragmentation, pollution and increased disease risk in driving pollinator decline has been clearly demonstrated, the impact of resource diversity is less well understood. In this study, we investigated the effect of pollen diversity and composition on reproductive success and fitness of Bombus terrestris colonies. We asked the question whether a higher plant diversity results in a more diverse diet, lower pathogen incidence and a higher colony fitness. To answer these questions, colonies of lab-reared bumblebees were placed in species-poor heathlands and species-rich semi-natural grasslands that strongly differed in plant community composition and diversity. We examined pollen loads on the bodies of foragers and identified the plant taxa present in the realized diet via DNA metabarcoding of the ITS2 marker. Liquid chromatography-mass spectrometry (LC-MS) was used to compare peptide composition of pollen samples from both habitats. Colony fitness was assessed by counting the number of sexuals produced by the colony at the end of its cycle. At the same time, colonies were examined for parasite incidence. Pollen composition and diversity on pollinators' bodies differed significantly between bees foraging in grasslands and heathlands. Concomitantly, peptide composition differed significantly between pollen samples from grasslands and heathlands. Contrary to our prediction, colonies developed significantly better in heathland sites than in grasslands. In addition, the relationship between colony fitness and pollen diversity was weak and varied between the two habitats. Pathogen incidence was very low and not affected by habitat. Overall, our results indicate that plant diversity is not necessarily a good predictor of colony fitness, and that vegetation composition and associated differences in both the quantity and quality of pollen are more important than pollen diversity per se.