Shorter seasonal snow cover poses a risk to solitary bee populations in a mountainous ecosystem

Climate change poses a threat to organisms globally, but its effects may be particularly pronounced in multi-trophic systems. For example, pollinators are vulnerable to changes in climate, as are flowering plants, but the extent of direct versus floral-mediated indirect effects of climate on pollinators has not been studied for most solitary bee species. We use a long-term dataset (9 years) encompassing a diverse assemblage of bees (13 species) and their flowering plants from a region experiencing rapid effects of climate change (Rocky Mountains, USA). Utilizing piecewise structural equation models, we assess direct and flower-mediated indirect effects of climate variables (snow cover duration and growing degree day accumulation) on each bee species's abundance. For 62% of species, longer snow cover duration had a direct, positive effect on bee abundance the following year and on flower abundance and duration. However, flower abundance and duration in the preceding year did not affect bee abundance the following year. Snow cover provides temperature stabilization for energy-efficient diapausing among bees and frost protection for flowering plants. Given the shift towards earlier snowmelt in this system, our findings raise concern for regional bee community stability and underscore the importance of seasonal snow cover for bees in other systems.