Insect pollinator communities are known to change from bee-dominated communities at low elevations to fly-dominated communities at high elevations. Along an elevation gradient in northern Arizona we examined changes in both species richness and abundance for bees and flies, characterizing specific taxonomic changes that occurred throughout the “bee-to-fly” transition zone. We also assessed whether the bee-to-fly transition could be due to an increase in tree canopy cover that would reduce bee habitat and favour flies. Insects were sampled using coloured cup traps and we restricted the study to encompass the elevation range where we presumed the transition from bees to flies would occur.
Four key patterns emerged. 1) Bees and flies were equally abundant at the, mixed conifer life zone; which emerged as our transition area. 2) While bee communities’ changes at each elevation, Tachinid flies were the primary flower visiting flies. 3) Although communities were different among life zones, abundance trajectories were primarily due to changes in the abundance of a few common species, whereas species richness changed were more unique to each life zone. 4) Bee community composition was increasingly divergent between forest and meadow habitats with increasing elevation. In contrast, fly community composition was distinct among the three elevation zones but not between forest and meadow habitats. 5) In area of no canopy cover the bee to fly transition was not shown, bees persisted as the dominate pollinator into the highest life zones. In conclusion, this is one of the first study to document a bee-fly transition for both species richness and abundance and more specifically show that the transition was due to both common species changing abundances across the transition and species replacement. We also documented that forest cover was correlated with this transition, suggesting increased canopy cover is at least in part a factor mediating the bee-to-fly transition.