Tropical Bee Assemblage Diversity Decreases With Elevation While Body Size Increases

Elevation gradients are powerful drivers of changes in species composition, richness, and functional traits. We studied bee communities at 14 sites across a 3000 m elevational gradient in the Colombian Andes. We measured morphological traits on all individuals (body size, relative leg length, relative wing length) and used assemblage data to measure composition, richness, evenness, Simpson's diversity, and trait diversity. We found that assemblage composition turned over rapidly with sites greater than 1000 m in elevation apart, sharing less than 1% of species. Bee richness decreased with elevation, with low elevation sites (< 2000 m) having an average of 26 species and high elevation sites (> 3000 m) averaging 3.4 species. Simpson's diversity and trait diversity showed similar patterns, while evenness did not change with elevation. Assemblage trait means also changed with elevation, as high elevation communities had larger body sizes and shorter wings and legs. These results fit with predictions from both Bergmann's and Allen's rules, suggesting thermoregulation may be a factor driving changes at high elevation. This is the first study of its kind in the Andes and adds to a large number of bee assemblage studies that also show decreases in bee diversity with elevation, indicating that it is a near‐universal pattern. However, results from the small number of existing studies on bee morphological trait changes with elevation are highly variable, suggesting these changes have a variety of mechanisms driving them and are likely context dependent.