Catching insects while recording bats: impacts of light trapping on acoustic sampling

Collecting information on bat prey availability usually involves the use of light traps to capture moths and flies that constitute the main prey items of most insectivorous bats. However, despite the recent awareness on the adverse effects of light on bats, little is known regarding the potential impacts of light trapping on the bat sampling outcomes when passive acoustic sampling and light trap- ping are implemented simultaneously. Using a before–after experimental design that involved the installation of a 6 W actinic light trap 1 m away from the bat detector, we tested the predictions that (1) slow-flying bat species will be less active when the light trap is present, while the opposite will be true for fast-fly- ing species; and (2) bat species richness will be lower at lit conditions compared to dark ones. Our results suggest that the use of light traps in combination with bat detectors may considerably influence the outcomes of acoustic sampling. Although the activity of fast-flying bat species did not differ between the two treatments, we found that the activity of slow-flying ones such as Rhinolophus ferrumequinum and Rhinolophus hipposideros decreased significantly at lit condi- tions. Furthermore, we recorded fewer bat species when the light trap was deployed. To overcome this issue, we strongly recommend either (1) placing light traps at a considerable distance from bat detectors; or (2) using light traps during the night that follows the bat sampling if sampling needs to be at the same position; or (3) deploying non-attractant insect traps such as Malaise traps if Lepidoptera is not the main order targeted.