Influence of elevation, climate, snowmelt and floral abundance on body size of solitary bees

Abstract In mountain ecosystems, spring snowmelt triggers rapid plant growth and bee emergence from overwintering. These habitats are warming rapidly under climate change, leading to earlier snowmelt, flowering and bee emergence. Early snowmelt exposes new plant growth to late spring frosts and leaves the soil drier during the summer, reducing flower abundance peaks. Given that fewer flowers can impact intra‐specific bee body size in agricultural systems, we investigated possible knock‐on effects of a warming climate on intraspecific variation in bee body size using 10 years of solitary and facultatively eusocial bee specimens collected in Colorado, USA, along an elevation gradient. We asked: (1) How does body size vary with elevation? (2) Do snowmelt timing, summer precipitation or winter cold severity directly impact body size? (3) Do these climate variables affect body size indirectly through their effects on flower abundance? We found that specimens of the seven bee species we measured exhibited 5.3%–8.2% variation in body size. We were able to examine associations with elevation in four species, and in two of these we found significantly larger bees at lower elevations. Snowmelt timing the year prior was directly associated with body size of the offspring generation in only two of seven species, with smaller bees in earlier snowmelt years. Rainfall and cold severity had no significant direct relationships with body size. Although flower abundance was strongly dependent on snowmelt timing and summer precipitation, the body size of only one bee species had a significant (but weak) relationship with floral abundance. While these solitary bees varied intra‐specifically in size, the environmental variables we examined did not consistently explain this variation, suggesting that other unmeasured factors are more important in driving body size variation in our study species.