Complex temporal trends in biomass and abundance of Diptera communities driven by the impact of agricultural intensity

Insect biodiversity and abundance declines have been reported widely and are expected to alter ecosystem functions and processes. Land use change has been recognised as a major cause of such declines. However, variation in local environmental drivers and the scale of available monitoring data have left large knowledge gaps in which taxa are declining, where declines are the greatest, and how these declines will impact ecosystems. We used 11 years (2006–2016) of monitoring data on 40 farms distributed over ~10,000 km2 in southern Québec, Canada, to quantify the impact of agricultural intensity on temporal trends in abundance and biomass of Diptera (true flies). There was a large difference in temporal trends between farms, which we found to be driven by agricultural landcover. Contrary to expectation, increases in Diptera abundance over time were greater in areas with higher agricultural intensity, especially with an increase in cereal crops. In contrast, declines in biomass were steeper in areas of higher agricultural intensity, although only with greater maize and soy production rather than cereals such as wheat. Variation in forest cover around farms had the least effect on trends. We found steeper declines in biomass per total number of Diptera with increasing agricultural intensive cover, suggesting the presence of community turnover towards smaller bodied flies with lower individual biomass. Our results imply that land use may not only alter abundance and species composition of insect species assemblages but also the distribution of key functional traits such as body size.