Urban landscapes with more natural greenspace support higher pollinator diversity

AbstractAs 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 U.S. 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, i.e., 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 prioritize the 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. MethodsIn this study, we used publicly available data on urban land cover and urban socioeconomics and estimated their associations with urban pollinator occurrence rate. The publicly available urban land coverand urban socioeconomic data are large (continental scale) spatial files. The orginial sources for the spatial land cover and socioeconomic data are cited in the manuscript. We provide the site-specific land cover and socioeconomic predictor values that were derived from the publicly accessible data in a table - \"site_data.csv\". We focused on two groups of pollinator species - hoverflies (family Syrphidae) and bumble bees (family Apidae, genus Bombus). Pollinator detections were obtained from GBIF for hoverflies, and from the Bumble Bees of North America Database for bumble bees. The GBIF data request is cited in the README and in the manuscript text. The full Bumble Bees of North America Database data set is available upon request from the original provider, also cited in the README and in the manuscript text. We used integrated community occupancy models to quantify the associations between urban landscape predictors and urban pollinator occurrence. This model form estimates and accounts for detection biases in the data while simultaneously estimating associations with whether or not species occur. We looked at whether four focal predictrs were associated with occurrence: natural greenspace, developed greenspace, household income and neighborhood racial composition. Natural greenspace versus developed greenspacewere synthetic categories that we constructed based on the categorical land cover raster values provided by the original source. The integrated community occupancy models estimates community average associations with each of the four focal predictors. The effect of natural greenspace was our primary interest so we also estimated species-specific effects of the natural greenspace area on occurrence. We did not estimate species-specific effects for all four predictors because those more complex models failed to converge given the amount of data in our study.摘要 随着全球城市持续扩张，我们亟需更系统地厘清城市生物多样性的驱动机制，尤其是传粉昆虫这类功能重要的类群。本研究整合了来自美国85个大都市区462处城市景观的食蚜蝇与熊蜂传粉者观测记录，数据来源涵盖自然历史标本馆藏与公民科学项目。我们通过检验城市景观内绿地总面积能否预测传粉者出现情况（即某一景观中物种的存在与否），以此验证城市绿地是否可作为传粉者的适宜栖息地。本研究旨在对比两类绿地在支撑多样传粉物种种群能力上的差异：一类为自然绿地（包括城市绿带、自然保护区与森林/草原片段），另一类为人工绿地（包括管理型公园、公墓与高尔夫球场）。 借助集成占有模型方法（integrated occupancy modeling approach）校正采样偏差后，我们发现本土食蚜蝇的出现概率与自然绿地面积呈显著正相关，这表明自然绿地占比更高的城市景观，其本土食蚜蝇多样性水平也更高。整体而言，熊蜂的出现概率与自然绿地面积并无显著关联，但不同物种的响应存在分化：数种处于濒危风险的熊蜂呈现出正相关关系。与自然绿地的结果形成鲜明对比的是，传粉者出现概率与人工绿地面积未呈现显著关联。此外，我们发现城市景观内少数族裔家庭占比与传粉者出现概率呈显著负相关，这与下述假说相符：少数族裔聚居社区长期遭受的系统性不公正政策，会对城市生物多样性产生持久的负面影响。 综上，本研究结果证实了“自然绿地是城市传粉者的关键栖息地”这一假说。我们建议城市优先保护残存自然绿地，并优化人工绿地建设，以推动城市传粉者保护工作。此类保护举措应优先在少数族裔家庭占比较高的城市景观中推进，以提升民众获取自然环境与传粉者生态系统服务的公平性。 方法 本研究采用公开可得的城市土地覆盖与城市社会经济数据，估算其与城市传粉者出现率的关联。上述公开数据为大陆级大尺度空间数据集，原始来源已在论文中注明引用。我们从公开数据源中提取了样点专属的土地覆盖与社会经济预测变量值，并整理为表格文件"site_data.csv"。 本研究聚焦于两类传粉物种类群：食蚜蝇（食蚜蝇科Syrphidae）与熊蜂（蜜蜂科Apidae，熊蜂属Bombus）。传粉者观测数据分别来自全球生物多样性信息设施（Global Biodiversity Information Facility, GBIF）的食蚜蝇数据集，以及北美熊蜂数据库（Bumble Bees of North America Database）的熊蜂数据集。GBIF的数据获取请求已在README文件与论文正文中标注引用。完整的北美熊蜂数据库数据集可向原始提供者申请获取，其引用信息同样标注于README文件与论文正文。 我们采用集成群落占有模型，量化城市景观预测变量与传粉者出现情况之间的关联。该模型可在估算物种出现关联的同时，校正数据中的检测偏差。我们考察了四类核心预测变量与传粉者出现情况的关联：自然绿地、人工绿地、家庭收入与社区种族构成。自然绿地与人工绿地为我们根据原始数据源提供的分类土地覆盖栅格值构建的合成类别。集成群落占有模型可估算群落与四类核心预测变量各自的平均关联。自然绿地的效应为本次研究的核心关注点，因此我们同时估算了自然绿地面积对物种出现情况的物种特异性效应。由于更复杂的多变量模型无法基于本研究的数据量实现收敛，我们未对全部四类预测变量估算物种特异性效应。