How to really help wild bees: Nutrient demand and supply in a changing environment

Wild bees often cannot access the optimally balanced diet required for their survival because local and global changes affect which pollen varieties are available to them in the environment. Understanding how the nutritional quality of various pollens affects bees could help us determine whether and how floral diversity and composition shape bee communities and populations. Therefore, to study how the nutritional quality of pollen influences wild bee fitness-related traits (survival, cocoon development and body and cocoon sizes), we asked questions about bee functioning in environments offering various nutrient diversities and compositions via available pollen species. Utilizing a feeding experiment, we show that the nutritional quality of the pollen diet eaten by bee larvae is shaped not by pollen diversity per se but by a specific pollen species composition affecting bee fitness-related traits via diet nutritional (un)balancing. Suboptimal concentrations of certain nutrients in pollen produced by specific plant species resulted in increased mortality, decreased size and underdeveloped cocoons, and the strength of the observed negative effect was sex dependent and alleviated if scarce nutrients were added to the pollen diet. Accordingly, we propose that the functioning of bee populations and communities may depend indirectly on the floral diversity of the local habitat, which directly determines whether a nutritionally balanced pollen diet obtained from certain species can be provided to bee larvae. This approach could reveal interactions between bees and other parts of the food web, and lead to new ways of conserving bees and the critical roles they play in ecosystems.