Anthropogenic effects, not climatic, shaped Holocene population expansion of an insular bee fauna

Remote island systems with small landmasses and reliable estimates of human occupancy are ideal model systems to disentangle the roles of global climatic changes and local human occupation on biota. Here, we used mitochondrial and nuclear genomic data from five endemic Fijian Lasioglossum (Homalictus) bee species to infer changes in effective population size (Ne). These ground-nesting bees are native, with non-specialised floral visitation habits, and distributed across the elevational gradient. All lowland species and populations showed strong signals of increasing Ne that correspond to the timing of human occupation of Fiji, but not Holocene climatic change. Highland populations, with greater isolation and present in regions less affected by anthropogenic impacts, did not show evidence of recent rapid increases in Ne. Population expansion rates across the elevational gradient differed between taxa, with significantly earlier and larger increases in predominantly lowland species than those with more restricted ranges in the highlands. This is consistent with the movement of people inland from coastal regions and into montane elevations of the island, and corresponding landscape changes that benefit the ecology of these bees. Specific life history traits of these bees, combined with substantive clearing of forest cover and floristic changes at lower elevations, has likely increased nesting opportunities and abundance of invasive floral resources. Our findings contrast with recent evidence that human occupation of Fiji has resulted in decreased ant biodiversity and raise the paradoxical possibility that human-mediated environmental changes may benefit some native montane tropical insect faunas.