Urban design: rain gardens support biodiversity and ecosystem services?

Combined sewer systems are prone to overflows of sewage and other pollutants into water bodies, leading to serious environmental concerns. Various strategies are in place to decrease these overflow events, including green infrastructure (e.g., rain gardens) which can capture stormwater before it enters the system. The impact of green infrastructure on conservation, specifically the beneficial insect community warrants investigation. Rain gardens were installed on vacant lots in the Slavic Village neighborhood of Cleveland, Ohio. We measured how this land use change influenced the abundance and richness of pollinators and natural enemies using bee bowls and yellow sticky cards. Arthropods were sampled during June, July, and August in 2014 within 6 rain gardens and 8 vacant lots. A total of 1,016 bees were collected with bee bowls and 6,779 natural enemies and 368 bees on sticky traps. The majority of collected bees were in the family Halictidae (n=1,001) with Lasioglossum spp. being most common (n=725). Bee richness and community were similar between sites. However, bee abundance was lower within rain gardens based on average number of bees collected per bowl trap. The total bloom area was significantly lower at the rain gardens, but did not correlate with bee abundance. Another potential factor influencing the decrease in bee abundance at the rain gardens was disruption of nesting resources by replacement of soil to improve infiltration. Understanding if rain garden construction methods result in short-term versus long-term declines in urban pollinators and natural enemies will inform future implementation strategies for green infrastructure.