Do past and present abiotic conditions explain variation in the nutritional quality of wildflower pollens for bees?

Abstract

Floral traits such as color, scent, and nectar often vary substantially within plant species. However, when it comes to pollen chemistry, the scale of intraspecific variation is largely unknown, as are its potential abiotic drivers. Bees collect pollen as their primary source of protein and lipids, and interspecific variation in pollen quality influences bee foraging preferences. Understanding the scale of intraspecific spatiotemporal variation in pollen macronutrient content could further uncover the nutritional basis of many plant-pollinator interactions influenced by geographic and climatic factors. Here, we sampled pollen from 35 bee-visited wildflower species across multiple sites in Great Basin/Eastern Sierra sagebrush steppe habitat (Nevada/California, USA) and analyzed their protein and lipid concentrations. Then, using Bayesian sparse regression, we explored the relationship between 44 site-specific climate variables and variation in pollen nutritional content. In some plant species, we discovered variation in protein or lipid concentrations across sites at a scale likely meaningful to bee performance. Further, this variation was weakly but significantly related to both current season below-ground (climatic water deficit) and previous season above-ground (dewpoint) conditions, uncovering the potential for community interactions mediated by floral nutrition to be altered via multiple plant ecophysiological pathways. Identifying the causes and consequences of variation in pollen nutrition is an effort critical to understanding how climate change impacts plant fitness via interactions with pollinators as well as the health of managed and wild bees.