How urbanization is driving pollinator diversity and pollination - A systematic review

Urban areas are growing worldwide and alter landscapes in a persistent fashion, thereby affecting biodiversity and ecosystem services such as pollination in a little understood way. Here we present a systematic review of the peer-reviewed literature to identify the drivers of urban pollinator populations and pollination. A total of 141 studies were reviewed and qualitatively analyzed. Pollinator responses to urbanization were contrasting. We contend that positive responses were often associated with urban sprawl, i.e. moderate levels of urbanization of rural, mostly agricultural land below 50% impervious surface, whereas high levels of densification with high percentages of sealed and built-up area (above 50%), largely led to pollinator declines and loss of pollination services. Further, urbanization generally reduced pollinator diversity when compared to natural or semi-natural areas, but enhanced it when compared to intensified agricultural landscapes. In addition, pollinator responses were commonly highly trait- and scale-specific. Cavity nesters and generalist species usually profited more from urbanization than ground nesters and specialists. Overall, urban pollinator communities still seem to provide sufficient pollination services to wild vegetation and crops. Pollinator diversity generally increased with the amount of urban green spaces at the landscape scale, and locally with availability of nesting resources and flowering plants. Positive effects of floral additions were largely independent of the plant's origin, whether native or non-native. Only a few studies included landscape configuration. Likewise, abiotic urban drivers, e.g. heat island effects and air and light pollution, remain little studied. Tropical and developing regions, most heavily impacted by current and future urbanization, are strongly underrepresented. We conclude that biodiversity-friendly urbanization can make a valuable contribution to pollinator conservation, in particular in face of the continued intensification of rural agriculture.