Climate change can be a key driver of bee declines, which are a pressing ecological and economic concern. However, direct climate effects on bee fitness, mediated by physiological or behavioral traits, remain unstudied for most bee species. Body size, which can govern temperature and water regulation in insects, could determine bee susceptibility to shifts in both climate mean and variability, which few have examined. We combined body size measurements with long-term bee abundance data from the Sevilleta National Wildlife Refuge (NM, USA), where the climate is becoming drier and more interannually variable. We asked: (1) Population-scale: How does bee species abundance vary with aridity (drought) over time? Are relationships nonlinear, signaling sensitivity to climate variance in addition to mean, and does body size predict the shape and/or magnitude of these relationships? (2) Community-scale: How has community-level bee body size changed over time in concert with climate change? We sampled bees using passive funnel traps (March–October, 2002–2015), aggregated climate data from on-site meteorological stations, and focused our analyses on 20 species that comprise 85% of total abundance in the dataset and represent morphological and taxonomic diversity. We measured bees’ intertegular distance and used allometric equations to predict body mass.
Results/Conclusions
Bee species had abundance relationships with aridity that ranged from linear (positive and negative) to quadratic and cubic, suggesting that species differ considerably in their sensitivity to climate mean and variability. Across the 20 focal species, small-bodied bees tended to be more abundant under cooler, wetter conditions, while large-bodied bees were generally more abundant under hotter, drier conditions. We found strong evidence for trait-mediated bee community shifts over time: community-weighted mean body mass increased over the 14-year study period (r = 0.66, P < 0.0001), and bees were larger on average in hotter, drier years (r = -0.94, P < 0.0001), due to shifts in species’ relative abundances rather than within-species body size change. Our results suggest that increasing mean and variability in aridity in our system are favoring large-bodied bees, perhaps because they are less susceptible to desiccation or have greater thermoregulatory capacities relative to smaller bees. Larger bees may also be favored due to greater average foraging distances or competitive superiority. Our study demonstrates how trait-based approaches can galvanize predictions on bee responses to climate change and improve understanding of pollinator declines. Forthcoming work will examine suites of traits that may together govern bees’ climate responses.