Flying insect biomass is negatively associated with urban cover in surrounding landscapes

Aim

In this study, we assessed the importance of local to landscape-scale effects of land cover and land use on flying insect biomass. Our main prediction was that insect biomass would be lower within more intensely used land, especially in urban areas and farmland. Location Denmark and parts of Germany.

Methods

We used rooftop-mounted car nets in a citizen science project ('InsectMobile') to allow for large-scale geographic sampling of flying insects. Citizen scientists sampled insects along 278 five km routes in urban, farmland and semi-natural (grassland, wetland and forest) landscapes in the summer of 2018. The bulk insect samples were dried overnight to obtain the sample dry weight/biomass. We extracted proportional land use variables in buffers between 50 and 1000 m along the routes and compiled them into land cover categories to examine the effect of each land cover, and specific land use types, on insect biomass.

Results

We found a negative association between urban cover and insect biomass at a landscape-scale (1000 m buffer) in both countries. In Denmark, we also found positive effects of all semi-natural land cover types, i.e. protected grassland (largest at the landscape-scale, 1000 m) and forests (largest at intermediate scales, 250 m). Protected grassland cover had a more positive effect on insect biomass than forest cover. The positive association between insect biomass and farmland was not clearly modified by any variable associated with farmland use intensity. The negative association between insect biomass and urban land cover appeared to be lessened by increased urban green space.

Main conclusions

Our results show that land cover has an impact on flying insect biomass with the magnitude of this effect varying across spatial scales. However, the vast expanse of grey space in urbanised areas has a direct negative impact on flying insect biomass across all spatial scales examined.