COS 42-3 - Climate and floral availability drive solitary bee sex ratios in a montane ecosystem

Abstract: The maintenance of biodiversity and ecosystem function under climate change will depend on how species respond to altered conditions. Among pathways of climate change response, sex ratio shifts could occur in many organisms, with important consequences for population persistence. In bees, the most important pollinators of many wild and agricultural plants, haplodiploid sex determination means that developing female larvae require a higher resource investment than males due to their larger genome and body size. Under environmental change, decreased floral abundance could thus lead to fewer female offspring and skewed sex ratios. But responses of bee sex ratios to climate change have been little-studied. We used long-term flowering phenology, bee abundance, and climate data collected near the Rocky Mountain Biological Laboratory (Gothic, CO, USA) to examine how solitary bee sex ratios varied with climate. We asked whether climate most strongly influenced sex ratios directly or indirectly via floral resources. To examine potential time lags, we also asked whether past or present year’s climate and floral resources were more predictive. In 2009-2018, all open flowers were counted in 29 plots every 2-3 days for the duration of the growing season, and bees were collected at 16 sites every two weeks using netting and passive traps. For 13 native solitary bee species, we used structural equation models to test the direct and indirect effects of the current and prior year’s climate variables (snowmelt date and summer precipitation) and floral variables (total flowers and number of days above a low flower threshold) on sex ratios. Across bee species, female/male ratios were higher following years with late snowmelt and more days above a low flower threshold (“floral days”). Floral days had a stronger positive effect on female/male ratios in years with late snowmelt relative to early snowmelt. Across species, the effects of summer precipitation and total flowers were generally weaker than the effects of snowmelt and floral days. But individual species varied in the strength of their sex ratio responses to different climate and floral variables, indicating greater susceptibility to climate-induced sex ratio shifts in certain taxa. Our results generally suggest that winters with increasingly early snowmelt could result in reproductive consequences for bee populations, with potential cascading effects on plant reproduction. Accounting for sex ratio shifts could thus improve predictions of climate change effects on bees and their pollination services.