Drivers of nocturnal and diurnal pollinating insect declines in urban landscapes.

Insect pollinators are essential for terrestrial ecosystems, delivering key ecosystem functions in the face of anthropogenic disturbance. Urbanization may be a key threat to pollinator communities. However, the scale of the threat remains unknown due to an overwhelming research emphasis on bees and a lack of comparative studies on hyper-diverse pollinating taxa such as nocturnal moths. As a result, it remains unclear which pollinator groups are most vulnerable to urbanization, and which habitat features are most critical for supporting them. We conducted a large-scale assessment of the effects of increasing urbanization on the diversity of bees, hoverflies and nocturnal moths in urban horticultural sites (allotments) across three cities. We report up to a 43% reduction in species richness along urbanization gradients, suggesting that a wide range of pollinators are under threat in urban landscapes. We show that these declines are driven by taxon-specific landscape drivers such as the reduction of tree canopy and semi-natural habitat, suggesting that urban insect conservation depends on the preservation or expansion of habitat features specific to different threatened taxa. We found that relative to bees, moths and hoverflies are particularly sensitive to urbanization, and we highlight the importance of including these frequently overlooked pollinator groups when assessing the biodiversity impacts of environmental change.