Disruption of a belowground mutualism alters plant-pollinator interactions

Plant fitness is influenced by diverse and intimate interactions with unrelated taxa.  Aboveground, floral visitors provide pollination services in exchange for floral rewards.  Belowground, arbuscular mycorrhizal fungi (AMF) enhance nutrient capture while feeding upon photosynthate.  As these above- and belowground process occur within, or on, the same individual plants, there exists the potential for antagonistic or synergistic interactions between these fundamental ecological processes.  However, there has been a historical separation in the study of above- and belowground processes, and thus the actual degree of interdependence between these mutualisms is unclear.  To test this, we experimentally suppressed AMF in a native grassland for three years.  We measured flowering plant abundance and composition, as well as behavioural observations of floral visitors over the course of an entire flowering season.  After three years, the pollinator community shifted from large-bodied social bees to small-bodied solitary bees and flies.  Additionally, the total number of floral visits by potential pollinators per flowering stem was reduced 67% across all 23 flowering species found in the plots.  In contrast, the absolute number of floral visits did not vary with AMF suppression summed across all species.  Instead, there was a significant species x treatment interaction indicating AMF suppressing increase visitation for some species, while decreasing it for others.  These species-specific effects were mediated through indirect interactions within the community, rather than through direct effects on floral morphology.  Our findings indicate that AMF are an important driver of pollinator community structure and pollination services.