Beyond Dispersal Limitation: Drivers of Fine-Scale Population Structure in Two Common Solitary Wild Bees

In landscapes shaped by intense agriculture, even common bee species may face limits to their dispersal capacity. We explored how spatial isolation and land-use types influence the genetic population structure of two generalist mining bees — Andrena haemorrhoa and A. nigroaenea —which differ in body size and putative dispersal potential, yet sharing similar ecological niches. Using a network of isolated wetland patches as a model for fragmented habitats, we hypothesized that body size, spatial isolation, and landscape features, such as intensive crop production, affect genetic structure. We expected the larger-bodied A. nigroaenea to show less genetic differentiation due to higher dispersal potential, while gene flow in the smaller A. haemorrhoa would be constrained by landscape resistance and isolation. Using nine microsatellite markers per species, we found low genetic differentiation, with no consistent link between body size and genetic structure. Genetic clusters did not align with spatial clustering, suggesting that factors beyond geographic isolation — like mating systems, male-biased dispersal, and female philopatry — may shape genetic structure. Landscape resistance, i.e. species-specific habitat permeability, showed a weak influence on gene flow, more evident in A. haemorrhoa , indicating some, albeit limited landscape impact on dispersal. Despite evidence for inbreeding, both species maintained high allelic richness. Our results highlight how species life-histories, ecological factors, and landscape features interact to shape population structure. Despite considerable landscape fragmentation, generalist bees showed little spatial genetic structure, emphasizing the value of high-quality habitat patches and corridors for supporting gene-flow, especially in smaller-bodied species.