Turn off the lights – Bats avoid lit underpasses
Written By Manisha Bhardwaj
Postdoctoral Researcher, Swedish University of Agricultural Sciences
6th April 2021
Illuminating the issue
Wildlife underpasses provide safe passage for wildlife to cross under roads and railways and maintain connectivity in the landscape. These types of structures are widely used around the world by animals as big as elephants to as small as bats. Underpasses are also used extensively to enable people to safely cross roads via trails for walking, cycling and horse riding. In recent times, there has been a push to combine underpasses for both people and wildlife, potentially providing cost-benefits by having multi-purpose structures (Figure 1). However, does the combining of uses make them less suitable for wildlife?
Figure 1. An underpass combining a bicycle/walking path and rock gabion walls intended for reptiles. Credit: Rodney van der Ree, Netherlands 2018
Designing structures for human-use often means prioritizing human needs such as safety or accessibility. For example, it is common practice to light multi-purpose underpasses at night to improve the sense of safety and visibility for the public using the structure. However, lighting at night can have adverse impacts on nocturnal wildlife, such as increased risk of predation, interference to natural cues and circadian rhythms, and increased stress. Thus, nocturnal wildlife may avoid lit crossing structures and choose to cross the road by going over the road instead. This is problematic because the multi-purpose structure no longer functions as intended for wildlife.
We investigated if artificial night-time lighting influenced underpass use by a predominantly nocturnal group of species – bats. Bats have variable responses to lighting – some species avoid lit landscapes, and suffer from decreased fitness in lit landscapes, whereas others are attracted to light, and can even be advantaged by preying on the insects that are attracted to the light sources. Thus, bats could also have variable responses to lighting in underpasses – some species may avoid lighting, and some may be attracted to the light. For the species that avoid the light, they may cross above the road, flying with an increased risk of collision with vehicles or they may not cross at all, reducing their connectivity over the road. In contrast, bats that are attracted to the light may use the structure more and have increased activity within the structures. In this study, we experimentally lit highway underpasses and investigated how active bats were within and above the underpasses using a before-during-and after study design.
Investigating bat activity before-during-after artificial lighting
Using ultrasonic bat detectors, we recorded how often bats flew over a highway or through 11 different underpasses in south-eastern Australia. We compared bat activity before, during and after adding lighting to the underpasses (Figure 2). In the ‘before’ stage of the experiment, we determined the baseline levels of bat activity at each underpass. In the ‘during’ stage, we lit the underpasses using white LED lights and determined the immediate effect of lighting on bat activity at the underpasses. In the ‘after’ stage, we determined if there were any lag effects of the light on behaviour (i.e. did crossing rates return to pre-light conditions after the lights were turned off). For each lit structure, we also monitored one unlit structure as a control.
Figure 2. Top: Experimental set up at lit and unlit crossing structures during the three phases of the trials: before, during and after. Note: the figure depicts two structures; these were replicated resulting in four structures used during each trial. Bottom: Potential responses of bats to lighting in the underpasses: avoidance, attraction or no response. Arrow thickness indicates the change in crossing activity.
We found that bats actively use dark underpasses, but when underpasses are illuminated, bats tend to fly over the road rather than through the underpasses. The most likely outcomes of lighting underpasses were 1. an increase in crossing rate above the freeway and a decrease through the underpasses, or 2. a reduction in crossing rate both above freeways and through the underpasses, when structures were lit. Only one species out of 10 showed an attraction to the lit underpasses.
Darkness may provide brighter solutions
Current designs for multi-purpose underpasses may not be a successful solution for wildlife and humans. Since lighting underpasses reduces their effectiveness for bats, and potentially other nocturnal wildlife, we recommend that underpasses intended to facilitate the movement of wildlife across roads should not be lit. If underpasses must be lit, there are alternative solutions that may potentially work. For example, reflectors on the path may work well enough to light paths for cyclists who already have headlights on their bikes. If illumination is required, push-button, sensor or timer systems can reduce the amount of time lighting is on, and careful placement and shielding of the lighting can limit unnecessary spread. In addition, wider underpasses and restricting people and lighting to one side may effectively conserve ‘dark’ areas for wildlife. It is clear that more research and creative planning and design are urgently required to create crossing structures that both wildlife and humans can use.
Author information:
Manisha Bhardwaj, Manisha.bhardwaj@live.ca
Postdoctoral Researcher, Swedish University of Agricultural Sciences, SLU
Department of Ecology, Grimsö Wildlife Research Station, SE-730 91 Riddarhyttan, Sweden
Source:
Bhardwaj, M., Soanes, K., Lahoz-Monfort, J. J., Lumsden, L. F., & van der Ree, R. (2020). Artificial lighting reduces the effectiveness of wildlife-crossing structures for insectivorous bats. Journal of Environmental Management, 262, 110313. doi: 10.1016/j.jenvman.2020.110313
Editor:
Rodney van der Ree
Cite this summary:
Bhardwaj, M. 2021. Turn off the lights - Bats avoid lit underpasses. Edited by van der Ree, R. TransportEcology.info, Accessed at: https://transportecology.info/research/turn-off-the-lights-bats-underpasses [Date accessed].
