Global populations of bees are in decline, threatening the stability of crop production and disrupting ecological communities. Unexpectedly, cities can harbor rich assemblages and sometimes rare species of bees. Thus, urban areas are increasingly recognized as potential refuge habitats for declining pollinators and as important targets for future bee conservation. Current urban bee populations are thought to be supported by residential gardens, parks, vacant lots and urban farms, which can contain a high floral abundance and therefore foraging resources for bees. Yet, there is no clear consensus about what drives the abundance and distribution of wild bees across cities. We also have little knowledge about how urbanization affects bee fitness, including their reproduction and health. For cities to achieve their conservation potential, we need to better understand what constitutes a high-quality urban habitat and how management can optimize urban greenspace for bee foraging and nesting.
Herein, I examine patterns of bee biodiversity, nesting success, and resource capture within Cleveland, Ohio, USA as a case study for urban pollinator conservation. Cleveland is one of 350 legacy cities worldwide which are promising candidates for future bee conservation due to their high abundance of vacant urban land. For example, following protracted economic decline, Cleveland, Ohio now contains over 27,000 vacant properties, representing 1,600 ha of vacant land. While extensive vacant land can be perceived as blight, it has also provided a transformative opportunity for the city of Cleveland to invest in urban greening initiatives, including more than 235 urban farms/community gardens and the large-scale urban field experiment which I conducted this research in.
To contextualize current knowledge of urban pollinators, I first review how greenspace design, management, and landscape context alter urban habitat’s value for bees (Chapter 1). In addition, I assert that pollinator conservation cannot succeed long-term without (1) transdisciplinary collaborations, and (2) accounting for city residents’ aesthetic opinions, safety concerns, and community values regarding urban greenspace. From the pollinator perspective, effective urban conservation also depends on a better basic understanding of bee community patterns in cities. So, in Chapter 2, I identify habitat and landscape factors that influence bee diversity, abundance, and foraging in urban greenspaces of Cleveland, Ohio. I discovered that large patches of greenspace in the urban landscape were associated with greater bee richness and were critical refuges for smaller bees, possibly because these habitats provide undisturbed nesting habitat. From observational data, I also determined that bees foraged predominately on urban spontaneous vegetation, or non-native plants that colonize urban land without intentional cultivation.
However, bee community trends do not illustrate population fitness and persistence in cities long-term. Thus, in Chapter 3, I evaluate urban habitat’s potential to enhance insect reproductive success and larval health. For this chapter, I focused on a select group of cavity nesting bees and wasps, which are important crop pollinators and frequently abundant in cities. Once again, I identified that large greenspace habitats were critical, and native bees and wasps exhibited greater larvae abundance when their nesting site was located within 1500-m of a >6–ha greenspace patch. Further, installations of native wildflowers were also positively correlated with native larvae abundance, indicating that native wildflower habitats conferred a fitness advantage to native cavity nesting bees. To better assess the relative importance of seeded native wildflowers for bee population growth, I then used pollen metabarcoding to analyze pollen provisions from bee’s nests (Chapter 4). This next generation DNA sequencing technique enabled me to quantify which plant species bees foraged on throughout the process of nest provisioning. Pollen metabarcoding revealed that urban spontaneous vegetation from the Fabacaeae family was frequently foraged on, but foraging preferences were mediated by bee’s taxonomic identity and native status. Moreover, we identified that greater landscape connectivity and presence of a large greenspace patch were associated with bee diet diversification and enhanced native bee reproductive success in trap nests.
Collectively, my dissertation’s findings will help transform urban bee conservation efforts into conservation outcomes, thereby addressing global bee decline and securing future pollination services. I consistently identified that greenspace configuration influences urban bee success and that presence of a large greenspace patch is critical for bee richness, nesting productivity, and access to diverse foraging opportunities. This suggests that legacy cities are prime urban conservation targets due to their high abundance of vacant land which could accommodate such large greenspace patches. Finally, since native and alien plant species were both valuable forage for urban pollinators, future conservation endeavors should consider creating new native wildflower habitat and adopting management practices which tolerate flowering weeds. It is my hope that these evidence-based recommendations and my appeal for increased partnership with city residents will facilitate more effective, socially responsible urban pollinator conservation.