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

Lindsie M Mccabe,Neil S Cobb,Bradley J Butterfield

doi:10.7717/peerj.7867

Abstract

BackgroundBees and flies are the two most dominant pollinator taxa in mountain environments of the Southwest USA. Communities of both taxa change dramatically along elevation gradients. We examined whether bee and fly traits would also change along elevation gradients and if so, do they change in a predictable way related to a decrease in temperature as elevation increases.MethodsWe used insect body size and darkness traits as proxies for energetic requirements and indicators of cold tolerance in order to assess patterns of bee and fly community trait differences along an elevation gradient. We examined 1,922 individuals of bees and flies sampled along an elevation gradient ranging from 2,400 meters to 3,200 meters and from 9.6 °C to 5.2 °C mean annual temperature. We examined bees and flies separately using community weighted means (site-level trait values weighted by species abundance) and estimates of environmental filtering (quantified as the inverse of the standardized range of trait values).ResultsBees and flies exhibited two somewhat distinct patterns; (1) Community weighted mean body volume and darkness of bees increased sharply at the highest elevation, and the intensity of environmental filtering also increased with elevation. This was due to both a change among bee populations within a species as well as species replacement at the highest elevation. (2) Community weighted mean body volume and darkness of flies also increased moderately with increasing elevation, but did not exhibit patterns of significant environmental filtering. In fact, the intensity of environmental filtering as indicated by the range of fly body volume weakened with elevation.ConclusionThe increase in filter intensity at high elevations exhibited by bees suggests a significant limitation on the breadth of viable functional strategies for coping with extreme cold, at least within this regional species pool. Flies, on the other hand, do not appear to be limited by high elevations, indicating that the shift from bee to fly dominance at high elevations may be due, at least in part, to greater environmental constraints on bee adaptation to colder environments.

Highlights

High-elevations generally represent restrictive environments in which only a narrow range of ecological adaptations are viable. Rahbek (1995) reviewed 163 ecological studies and confirmed that species richness decreases as elevation increases in almost every taxonomic group, including plants (Keller, Kienast & Beniston, 2000), animals (Terborgh, 1977), and even aquatic bacteria (Zeng et al, 2016)
We examined whether bee and fly traits would change along elevation gradients and if so, do they change in a predictable way related to a decrease in temperature as elevation increases
We examined bees and flies separately using community weighted means and estimates of environmental filtering

Summary

Introduction

High-elevations generally represent restrictive environments in which only a narrow range of ecological adaptations are viable. Rahbek (1995) reviewed 163 ecological studies and confirmed that species richness decreases as elevation increases in almost every taxonomic group, including plants (Keller, Kienast & Beniston, 2000), animals (Terborgh, 1977), and even aquatic bacteria (Zeng et al, 2016). Bees and flies exhibited two somewhat distinct patterns; (1) Community weighted mean body volume and darkness of bees increased sharply at the highest elevation, and the intensity of environmental filtering increased with elevation. This was due to both a change among bee populations within a species as well as species replacement at the highest elevation. The increase in filter intensity at high elevations exhibited by bees suggests a significant limitation on the breadth of viable functional strategies for coping with extreme cold, at least within this regional species pool. On the other hand, do not appear to be limited by high elevations, indicating that the shift from bee to fly dominance at high elevations may be due, at least in part, to greater environmental constraints on bee adaptation to colder environments

Methods

Results

Discussion

Conclusion