Syrphids in the City: A 10-Year Citizen Science Program Sheds Light on How the Greenness and Quality of Green Spaces Impact Flower Flies

Abstract

Although Syrphidae is an essential family of flies because they provide key ecosystem services like pollination and pest control, their richness and abundance respond negatively to urbanisation, even when high-quality green spaces are present. These are the conclusions of many studies from the northern hemisphere; however, there are shortfalls in our knowledge of flower fly ecology in Neotropical megacities. We used a 10-year dataset obtained from a long-term citizen science program devoted to flies to test whether the richness, total number of records (as a proxy of abundance) and diversity of flower flies could be influenced by green space predictors such as total surface and percentage of green areas, the quality of public squares and parks indices and greenness index (NDVI) across 34 municipalities in Greater Santiago. Furthermore, we described for the first time an interaction network exclusively for flower flies within the city. We analysed 1084 records and 607 interactions of 29 flower fly species. We found that the north-east municipalities have more flower fly species. Based on the richness and abundance, the two-way Jaccard analysis grouped the municipalities into three major clades: (a) those municipalities with more than 10 flower fly species, (b) those between five and 10 species and (c) those with fewer than five species. Generalised linear mixed models associated positively and significantly the flower fly richness, abundance and diversity, mainly with the quality of public squares and the mean NDVI value. The structure of our interaction network between flower flies and plant species showed low values for connectance and specialisation, but a significant value for interaction diversity. The quality of green spaces and NDVI information could help local decision-makers improve the management of the municipality's green spaces, enhance the flower fly biodiversity inside the cities and provide potential ecosystem services such as pollination or pest control.