Past occupancy changes in Central European insects and their drivers

Recent changes in climate and land-use are having substantial impacts on biological organization including population declines, range shifts, and changes in community composition. In contrast, conservation policy like the European Habitats Directive aims to protect biodiversity from negative impacts. However, few studies have managed to compare these influencing factors among multiple taxa, particularly because of a lack of standardized time series data over long periods. To make matters worse, existing datasets are typical of low resolution or poor coverage, thereby limiting what inferences can be made from their use. Here, we analyze occupancy changes in butterflies, grasshoppers, and dragonflies using an extensive dataset of highly heterogeneous observation data collected in the central European region of Bavaria over a 40-year period. Using occupancy models, we show that the occupancies (the proportion of sites occupied by a species in each year) of cold-adapted species across taxa have already declined in past decades, while those of warm-adapted species have increased. In butterflies, habitat specialists also decrease, while in the other taxa, there was no difference between habitat generalists and specialists. We show that although the Habitats Directive increased monitoring efforts toward species protected in its annexes, occupancy trends both improved and deteriorated after its implementation. Therefore, contrary to its main goal, the European Habitats Directive did not prevent a worsening of species' status. Our findings illustrate not only why butterflies, grasshoppers, and dragonflies showed differing trends in the past as well as the effectiveness of Europe’s most important nature protection instrument, but also how we might mitigate the detrimental effects of human development on species diversity in the future.