Conservation in post-industrial cities: How does vacant land management and landscape configuration influence urban bees?

1. Rich pollinator assemblages are documented in some cities despite habitat fragmentation and degradation, suggesting that urban areas have potential as pollinator refuges. To inform urban bee conservation, we assessed local- and landscape-scale drivers of bee community composition and foraging within vacant lots of Cleveland, Ohio, USA. Cleveland is a shrinking city, a type of urban area that has an over-abundance of vacated greenspaces as a result of population loss and subsequent demolition of abandoned infrastructure. As such, Cleveland represents over 350 post-industrial cities worldwide that are all promising locations for bee conservation. 2. Across a network of 56 residential vacant lots (each similar to 30 m x 12 m), we established seven unique habitats, including seeded native prairies, to investigate how vegetation management and landscape context at a 1,500 m radius influenced urban bee communities. We assessed the distribution of several bee functional traits, diversity and abundance with pan and malaise traps. Foraging frequency was determined with plant-pollinator interaction networks derived from vacuum collections of bees at flowers. 3. We observed higher bee richness and increased abundance of smaller sized bees as the size of surrounding greenspace patches increased within a 1,500 m radius landscape buffer. Within habitats, seeded treatments had no effect on bees but greater plant biomass and shorter vegetation were correlated with increased bee richness and abundance. Plant-pollinator interaction networks were dominated by spontaneous non-native vegetation, illustrating that this forage supports urban bees. 4. Synthesis and applications. Our study indicates that proximity to larger greenspaces within an urban landscape promotes overall bee richness and increased occurrence of smaller bee species within residential vacant lots. While we did not observe our seeded native plants enhancing the bee community, native wildflowers were still establishing during the study and may have a greater influence when blooming at higher densities. Importantly, spontaneous non-native vegetation provided the majority of urban bee's forage. Thus, vacant land that is minimally managed and vegetated with what many consider undesirable 'weeds' provides valuable habitat for bee conservation in cities.

Automated summary

Rich pollinator assemblages can be found in urban areas with larger surrounding greenspaces, where vegetation management and landscape context influence bee communities. Non-native vegetation plays a significant role in providing forage for urban bees in vacant lots.