Lantern bearers and beavers: ecosystem engineers foster abundance of fireflies along European rivers

Abstract Insect declines are among the most pressing biodiversity issues of our time, with consequences for ecosystem functioning and human well-being. Fireflies (Coleoptera: Lampyridae) are emblematic insects, whose populations are declining worldwide due to habitat alteration, light pollution, and hydrological disturbance. In Europe, ecological factors shaping firefly abundance in riparian ecosystems are poorly known. The Eurasian beaver Castor fiber , a key ecosystem engineer, profoundly alters stream morphology, vegetation, and hydrology, potentially generating habitat conditions favorable to moisture-dependent insects such as fireflies. In our work, we tested whether beaver activity influences firefly abundances in two European river systems: the Merse river in central Italy and the Danube River in Slovakia. We conducted standardised nocturnal transect surveys comparing sites with active beaver presence to nearby control sites lacking recent beaver activity. The Italian surveys focused on adult Luciola pedemontana , an Italian endemic species, whereas the Slovakian surveys targeted larvae and adult Lampyris noctiluca . We detected a marked effect of beaver presence on fireflies, with significantly higher numbers found at active beaver sites in comparison to controls. This difference was mainly driven by male abundances, since females and larvae were found in very low numbers, or were not directly targeted by our methodological approach in the case of Luciola . Among environmental variables such as temperature, canopy cover, and local humidity, only the occurrence of beavers proved as a significant driver of firefly abundances at the local scale. These findings provide the first empirical evidence linking beaver ecosystem engineering to firefly occurrence in Europe. By promoting wetland heterogeneity, moisture retention, and vegetative structure, beavers may indirectly facilitate firefly populations. Our results, albeit limited to a small spatial scale, pave the way to further research tackling the ecological relationships between fireflies and ecosystem engineers in Europe and worldwide.