The transition from bee-to-fly dominated communities with increasing elevation and greater forest canopy cover

Abstract

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.

Asynchrony among insect pollinator groups and flowering plants with elevation

“…Flies on the other hand are known to be highly abundant at high elevations where other insect taxonomic groups are less represented 14 , 16 , 47 . In our study, there was no significant difference in the abundance and species richness of flies across elevation zones, this may be attributed to the limited range of our mountain (1,640 asl) compared to other studies that have shown distinct variation in the abundance of flies with elevation 14 , 47 . However here, among taxonomic groups, flies and bees were more abundant compared to beetles at the peak elevation.…”

Environmental filtering of body size and darker coloration in pollinator communities indicate thermal restrictions on bees, but not flies, at high elevations

“…However, it is likely that global warming could also reduce habitat suitability for key high elevation bee taxa (Wilson et al, 2005), such as Bombus and Megachilidae, and lead to replacement by other low elevation bee taxa. We have found that in this community along the elevation gradient bee communities have a higher turnover in species along the gradient than flies (McCabe et al, 2019).…”

“…This could drive the community patterns since Megachilidae tend to be larger bee species. In general, bee species richness often decreases dramatically with increasing elevation (Inouye & Pike, 1988; Kearns, 1992; Lefebvre et al, 2018; McCabe et al, 2019; McCall & Primack, 1992; Primack, 1978). It has been suggested that the shift from bee dominated pollinator communities to fly dominated communities is primarily due to a greater temperature limitation in bees versus flies (Stone & Willmer, 1989).…”

“…Specifically, we tested the hypotheses that (1) a larger body size and darker coloration of the bee’s and fly’s body will increase with elevation, and (2) the range of trait values within communities, relative to expected values based on a null model of community assembly, will become narrower with increasing elevation and we will see an environmental filtering effect at high elevation environments. We predicted that these patterns will be stronger for bees than for flies, given the greater species diversity, habitat affiliation (McCabe et al, 2019) and stronger ties to their host plants while flies tend to be more generalized with host plant selection (Lefebvre et al, 2018) Assessing trait patterns can provide insight into the constraints of temperature on the composition of high elevation pollinator communities.…”

Direct and Indirect Effects of Overstory Canopy and Sex-Biased Density Dependence on Reproduction in the Dioecious Shrub Shepherdia canadensis (Elaeagnaceae)

“…The dependence upon insect pollination, combined with short foraging distances of generalist fly pollinators, can lead to expectations of higher pollen limitation since the plants must depend on pollen transfer from relatively stochastic and density-dependent pollen vectors [7]. Buffaloberry flowers soon after snowmelt in cooler temperatures and pollen limitation may also be expected in early-blooming and alpine plant species as a result of lower overall pollinator activity in cooler temperatures [10,39,89]. The association between cool temperatures and reduced pollinator activity has led to the cautionary observation that if climate change leads to variations in spring-flowering phenology in early-flowering species, plants may flower at times of restricted pollinator activity, leading to low fruit set and reduction of seedling recruitment [10,90].…”