Insect decline – Evaluation of potential drivers of a complex phenomenon

Insect decline is considered a complex phenomenon, resulting from a range of causal factors, which act at different spatial and temporal scales, with different impacts on different insect groups. There is a variety of data sets which describe changes in insect populations and biocenoses over time, however, only few long-term monitoring data are available which were generated according to a consistent scheme. In the absence of a broad data basis which could provide direct insights into factors causing insect decline, correlation analyses can help to identify potential factors which are associated with declines. Here, we are presenting a detailed analysis of the development of potential drivers of insect decline over the last decades at locations in Germany where a decline of insect biomass has been measured in previous studies, and of changes in the states of these potential drivers.
Potential factors investigated included landscape structure, land use change, grassland management, cropping practices, pesticide use, climate, and light pollution. Data about the developments of these parameters over the last decades were obtained from statistical data such as cropping statistics and market research, and long-term sequences of satellite imagery.
The results of the correlation analysis suggest that habitat loss caused by land use and land cover change, as well as intensification of land use are factors frequently correlating with declining tendencies in insects in Germany. This is to our knowledge the first study, where all of these factors got intensively investigated, from a retrospective point of view. With this extensive dataset we are able to correlate the often-mentioned habitat level drivers (landscape change, monotonicity of agricultural landscapes, loss of fragmented structures) with insect decline tendencies combined with other potential factors (pesticides, urbanization, night light).