Date Published: June 12, 2019
Publisher: Public Library of Science
Author(s): Lindsie M. McCabe, Ella Colella, Paige Chesshire, Dave Smith, Neil S. Cobb, Manu E. Saunders.
Insect pollinator communities are thought to transition from bee-dominated communities at low elevations to fly-dominated communities at high elevations. We predicted that increased tree canopy cover and a subsequent decrease in meadows and flowering plants would limit bees but not flies at higher elevations. We tested and supported this prediction by examining changes in both abundance and species richness for 128 bee species and 96 fly species at key points along an elevational gradient in Northern Arizona represented by distinct vegetation life zones. In addition to an increase in fly species and abundance relative to bees with increasing elevation, there were changes in community structure). To better understand factors that might influence this transition we examined how tree canopy cover changed along the elevational gradient and how this influenced the change in insect pollinator communities. While bee communities were progressively divergent between forest and meadow habitats with increasing elevation and tree canopy cover, there was no significant pattern with flies between meadow and forest habitats. However, fly abundance did increase with increasing elevation relative to bees. Along a comparable elevational gradient on an adjacent mountain with no tree canopy cover (i.e., a fire burned mountain), the bee-to-fly transition did not occur; bees persisted as the dominant pollinator into the highest life zone. This suggests that tree canopy cover can in part explain the transition from bee-to fly-dominated communities. In conclusion, this is the first study in North America to document a bee-fly transition for both abundance and species richness and show that tree canopy cover may play a role in determining pollinator community composition, by restricting bees to open meadow habitats.
Bees and flies are the two dominant insect pollinators in almost every ecosystem ; however, they are thought to be differentially distributed along elevational gradients. It has been proposed that bees dominate pollinator communities at low elevations and are replaced by flies at higher elevations . This pattern has been consistently observed in studies spanning numerous regions across countries from New Zealand and Australia , Nepal , Chile , Switzerland [6, 7], New Hampshire, USA , and British Columbia, CA . However, most of these studies have only anecdotally observed this transition, and the few that have examined this transition  have addressed the phenomenon only in general terms (e.g., noting that flies are higher in abundance or richness than ‘normal’ but not directly comparing them to other taxa). No study to date has examined environmental factors beyond elevation that could explain this transition.
We conducted two complementary studies to examine changes in pollinator communities along elevational gradients resulting from: 1) elevation and tree cover effects in meadow and forest habitats of an unburned mountain, and 2) the effects of elevation in meadow habitats of a burned mountain.
Along elevational gradients bees have been shown to dominate pollinator communities at lower elevations. However, flies persist along elevational gradients and show an increase in abundance and species richness at the highest elevations. We propose that an important driver of this transition is an increase in canopy cover as elevation increases, likely reducing resources for bee species and increasing larval resources for fly species. Our results further support the notion that this bee-to-fly transition will shift up in elevation with the removal of trees.