Butterflies and bumblebees in green structures in Malmö – An evaluation of new established green roofs, green walls, courtyards and park plantings within the BiodiverCity project

In this study, 26 green structures were surveyed for butterflies and bumblebees in three areas in Malmö (Västra hamnen, Augustenborg and Hyllie). The investigated green structures were situated in parts of the city which are relatively densely built up. Of the 26 investigated green structures 9 have been established within the BiodiverCity project (green roofs and walls, courtyards, park section). The aim of the BiodiverCity project was to develop green structures which support and enhance biodiversity and thus contribute to a vision of a green, attractive and healthy city. The developed green-structures are aimed to fulfil multiple ecosystem services as water regulation, enhancing pollinator numbers and increasing aesthetic values. They are typical examples for green structures developed according to nature-based-solutions (Eggermont et al. 2015). The BiodiverCity project is a co-operation between the municipality of Malmö, the Regional administration of Scania, research institutes and universities and building companies and entrepreneurs in the landscape architecture sector.
The aim of this study here was to analyse and evaluate the species composition for butterflies and bumblebees in green structures developed within the BiodiverCity project. This was done to analyse to which extent the objective to support biodiversity and specifically pollinators was reached. Green structures which had not been established within the project were investigated for comparison (nearby flowerbeds, road verges, grasslands, sown meadow, green roofs). Most green structures were surveyed five times from June to August 2016. Two different methods were used: transect surveys and 10-min observations of squares (4m2). In total 156 butterflies were observed and 705 bumblebees. 40% of all butterflies were whites (Pieris spp.). 98% of all bumblebees were buff-/white-tailed bumblebees (Bombus terrestris/B. lucorum) and red-tailed bumblebee (B. lapidarius).The number of butterfly species was positive correlated to the number of flowering plants. The number of bumblebee species and abundances were positive related to the number of flowering plant species and percentage cover of flowering plants. Both insect groups had different preferences regarding their favourite food plants.
The number of butterflies and bumblebees found in green structures established within the BiodiverCity project was comparatively low with the exception of green roofs. From the results of the study it can be concluded that it is possible to establish green structures–also in much urbanised areas – that can attract the most common bumblebees. Important is the choice of plants and sufficient abundance of attractive plant species. For butterflies on the other hand it seems more difficult to create attractive habitats in the type of green structures investigated, as these are small and are often lacking extensively managed grassland patches - an important butterfly habitat - and larval habitats.