Insect Decline-Evaluation of Potential Drivers of a Complex Phenomenon

The decline of insects is a global concern, yet identifying the factors behind it remains challenging due to the complexity of potential drivers and underlying processes, and the lack of quantitative historical data on insect populations. This study assesses 92 potential drivers of insect decline in West Germany, where significant declines have been observed. Using data from federal statistical offices and market surveys, the study traces changes in landscape structure and agricultural practices over 33 years. Over the years, the region underwent major landscape changes, including reduced cropland and grassland and increased urbanization and forest areas. Potential detected drivers of insect decline include: (1) urban expansion, reducing insect habitats as urban areas increased by 25%; (2) intensified grassland management; (3) shifts in arable land use towards bioenergy and feed crop cultivation, particularly corn, driven by dairy farming intensification and renewable energy policies. While the toxic load of pesticide application has decreased, land-use changes, most likely driven by market demands and shifts in national and EU policies, have reduced habitat availability and suitability for insects. This study highlights how these landscape and land management changes over the past 33 years align with the observed decline in insect biomass in the region.