Anthropogenic Effects, Not Climatic Factors, Are Correlated With Holocene Population Expansion of an Insular Bee Fauna

Abstract

Aim Remote island systems with small landmasses and reliable estimates of human occupancy can be ideal models to disentangle the roles of global climatic changes and local human occupation on biota. Here, we explored this with demographic reconstructions of native, ground‐nesting bees with non‐specialised floral visitation habits to infer changes in effective population size ( N e ). Location Viti Levu, Fiji, South West Pacific. Taxon Lasioglossum ( Homalictus ) bees (Hymenoptera: Halictidae). Methods We applied mitochondrial and genomic datasets from five species distributed across the elevational gradient for our analyses. These data were used for validation of population assignments and testing for structure, and to investigate broader demographic trends. Results All lowland species and populations showed strong signals of increasing N e that correspond to the timing of human occupation of Fiji, but not Holocene climatic changes. Highland populations, with greater isolation and less affected by anthropogenic impacts, did not show evidence of recent rapid increases in N e . Population expansion rates across the elevational gradient differed between taxa, with earlier and steeper increases in predominantly lowland species. This is consistent with the movement of people inland and into montane elevations of the island, and corresponding landscape changes that benefit the ecology of these bees. Main Conclusions Our correlative findings, together with published direct ecological observations, are consistent with the hypothesis that anthropogenic forest clearing and floristic changes (including invasive plants) can increase nesting opportunities for Fijian bees, especially at lower altitudes. This contrasts with recent evidence that human occupation of Fiji has resulted in decreased ant biodiversity and raises the paradoxical possibility that human‐mediated environmental changes may benefit some native montane tropical insect faunas.

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