Dimming streetlights in urban areas reduces insect attraction but pipistrelle bats forage more in darkness

Artificial light at night (ALAN) disrupts the activity patterns of a wide range of organisms. Nocturnal animals are particularly vulnerable to this disruption, which can be especially severe in strongly illuminated urban environments. Reducing light intensity has been proposed as a mitigation measure, but the effectiveness of this strategy remains unclear. Here we studied how dimming urban streetlights affects pipistrelle bats and their prey, flying insects. We measured bat and insect activity around dimmable streetlights, with night-to-night alternating high and low light intensity, and at a dark control site nearby. We applied interval photography to assess insect arrival on sticky sheets with a high temporal resolution, and continuously recorded bat activity. Our results show that dimming streetlights did not affect pipistrelle bat activity, but reduced insect attraction. The presence of light, either dimmed or undimmed, did alter the predator-prey relationship in unexpected ways. Even though insects clustered around the lights, urban pipistrelle bats were more active and displayed higher relative foraging activity (‘buzz ratio’) in the relatively dark sites nearby. Nightly activity patterns of pipistrelles were not affected by dimming or light in general, whereas insect activity in the second half of the night was earlier around streetlights compared to in darkness. Limiting insect attraction by dimming urban streetlights will likely benefit insect populations, which in turn is beneficial for bats in the long term. Our findings show that darkness is not only essential for bats and insects in natural areas, but in urban habitats as well.