Synchronous long-term trends in abundance and compositional variability of butterflies in Central Europe

Annual insect abundances are determined by short-term population fluctuations and long-term trends in community composition. Any assessment of this temporal variability needs long-term quantitative data on abundances. Here we use museum data (1940-2017) of butterflies and burnet moths from southwestern Germany and central Austria comprising 220,758 records from 155 species, and 55,641 records from 170 species, respectively. We ask whether population fluctuations and long-term population trends are spatially synchronized across larger spatial distances (>300 km) according to the Moran effect, and whether common trends in community composition and stability exist? Population fluctuations of 85.5% of species jointly occurring in both study regions were positively correlated (mean r = 0.30 +/- 0.03). Comparing different time windows, we found different sets of species being in spatial synchrony reflecting temporal changes in butterfly community composition. Mesophilic, habitat generalist, and dispersive species were significantly more synchronized than xerothermophilic, habitat specialist, and sedentary species. Long-distance travelers were particularly synchronized. In both study regions, species rank orders became less even with time. Stability in community composition increased with time with an ongoing trend to faunal homogenization, but not to species loss. Our study points to global and not to local drivers behind trends in butterfly population fluctuations and compositional stability, in the present case to altered land use and urbanization as major drivers toward synchrony and homogenization. Finally, our data have implications for conservation strategies as they strongly indicate that natural landscape structures are associated with higher faunal compositional variability in comparison to agricultural and urban landscapes.