Urban landscapes with more natural greenspace support higher pollinator diversity

Abstract

As cities around the world expand, we urgently need to better understand the drivers of urban biodiversity, especially for functionally important groups such as insect pollinators. In this study, we gathered hoverfly and bumble bee pollinator observations from natural history collections and community science initiatives from 462 urban landscapes across 85 US metropolitan areas. We tested whether urban greenspace functions as pollinator habitat by examining whether the total area of greenspace in an urban landscape predicted pollinator occurrence, that is, the presence or absence of species in a landscape. Our study was designed to determine whether there were differences between natural greenspace area (i.e., urban greenbelts, nature reserves and forest/grassland fragments) and developed greenspace area (i.e., managed parks, cemeteries and golf courses) in their ability to support a diversity of pollinator species. After accounting for sampling biases using an integrated occupancy modeling approach, we found a positive association between native hoverfly occurrence and natural greenspace area. This implies that urban landscapes with more natural greenspace support higher native hoverfly diversity. On average, bumble bee occurrence was not associated with natural greenspace area; however, the response varied among species, with several at-risk bumble bees showing a positive association. In contrast to natural greenspace area, we found no association between pollinator occurrence and the area of developed greenspace. In addition, we found that the proportion of racial minority households in an urban landscape was negatively associated with pollinator occurrence. This is consistent with the hypothesis that a history of systematic, unjust policies in neighborhoods with more racial minority households has lasting negative impacts on urban biodiversity. In conclusion, our results support the hypothesis that natural greenspace functions as vital habitat for urban pollinators. We recommend that cities preservation of remnant natural greenspace and improve developed greenspaces in order to promote urban pollinator conservation. These efforts should be prioritized in urban landscapes with a higher proportion of racial minority households to improve equal access to nature and pollinator ecosystem services.