Narrow niches, narrow ranges: Linking floral specialization to geographic vulnerability in bees

Animal diet breadth is an integral life history trait, and can range from narrow (specialists) to broad (generalists). It has important implications for species conservation, as specialists are often considered more vulnerable to anthropogenic change across various taxa, including mammals, birds, and butterflies. One proposed explanation for this increased vulnerability is that specialists tend to have smaller geographic ranges. However, the relationship between dietary breadth and range size has not yet been evaluated in bees (Hymenoptera: Anthophila). To address this question, we examine multiple aspects of bee diet breadth, including criteria for classifying bees as specialists or generalists based on floral host use. We also generate bee species range estimates and analyze the relationship between range size and diet breadth in 717 species spanning six families. Our analysis reveals a positive correlation between diet breadth and range size, indicating that specialist bees tend to have smaller geographic ranges and may be more vulnerable to anthropogenic change than generalists. These findings suggest that diet breadth is a useful trait for informing conservation priorities. Furthermore, given that most bee species lack diet breadth data, additional research is needed to characterize the diets of unstudied taxa.