Wildfire severity influences offspring sex ratio in a native solitary bee

Although ecological disturbances can have a strong influence on pollinators through changes in habitat, virtually no studies have quantified how characteristics of wildfire influence the demography of essential pollinators. Nevertheless, evaluating this topic is critical for understanding how wildfire is linked to pollinator population dynamics, particularly given recent changes in wildfire frequency and severity in many regions of the world. In this study, we measured the demographic response of the blue orchard bee ( Osmia lignaria ) across a natural gradient of wildfire severity to assess how variation in wildfire characteristics influenced reproductive output, offspring sex ratio, and offspring mass. We placed nest blocks with a standardized number and sex ratio of pre-emergent adult bees across the wildfire gradient, finding some evidence for a positive but highly variable relationship between reproductive output and fire severity surrounding the nest site at both local (100 m) and landscape (750 m) scales. In addition, the production of female offspring was > 10% greater at nest sites experiencing the greatest landscape-scale fire severity relative to the lowest-severity areas. The finding that blue orchard bees biased offspring production towards the more expensive offspring sex with increasing fire severity shows a functional response to changes in habitat quality through increased density of flowering plants. Our findings indicate that burned mixed-conifer forest provides forage for the blue orchard bee across a severity gradient, and that the increase in floral resources that follows high-severity fire leads females to shift resource allocation to the more costly sex when nesting.