Artificial light at night (ALAN) is one of the most significant environmental changes of the past century.1 In 2020, there were approximately 361 million streetlights globally,2 and ALAN continues to increase at an alarming rate of 7%-10% per year.3 But how does this rapid expansion affect wildlife, particularly nocturnal creatures like bats? Research suggests that ALAN poses considerable challenges for bats, whose nocturnal behaviors are finaly tuned to natural light cycles. For example, bats are known to be less active during a full moon compared to a new moon.5 Considering that ALAN can be up to 200 times brighter than a full moon,5 it is expected to have even more profound impacts on bat activity. Reduced activity could have drastic ecosystem impacts, as bats play critical roles as pest control, pollinators and seed dispersers.8 However, understanding the nuances of ALAN’s effects on bats requires a closer look.
ALAN’s Challenges for Bats
There are several ways that ALAN is a disadvantage for bats. For some species, activity does decrease in the presence of ALAN. For example, the tricolored bat is four times more active at sites with no pervasive light at night.5 Illuminated areas often lead to habitat fragmentation, delaying bat flights and forcing them to take suboptimal travel routes.7 These alternative paths can increase energy costs and predation risks.1 Additionally, bats that attempt to avoid ALAN often experience reduced dispersal and limited habitat access, further stressing populations.
The responses of bats to ALAN vary by species and foraging guild.3 Narrow-space foraging bats avoid ALAN under all circumstances, open-space foraging bats avoid ALAN after the first two hours of feeding and edge-space foraging bats avoid ALAN around midnight.3 Furthermore, species richness significantly decreases in brightly lit areas, such as construction sites, reflecting the barrier that ALAN poses to movements and habitat connectivity.1
Potential Benefits of ALAN
While it may appear so far that ALAN only causes complications for bats, this is not always the case. Certain wavelengths of artificial light attract insects,5 providing increased foraging opportunities for insectivorous bats.4 This phenomenon, known as the “vacuum effect,” can benefit light-tolerant species. However, this advantage comes with trade-offs. Increased competition among bats in light-polluted areas8 and unsustainable predation rates on local insect populations disrupt trophic structures1 and lead to long-term negative consequences for ecosystems.
Some species may even develop novel foraging behaviors or diet preferences4 in response to ALAN, but these responses are not without risks. Over time, the reliance on pollution-mediated food sources can destabilize bat populations and ecosystems,1 particularly as insects are increasingly predated in light-polluted areas.8
Mitigating the Effects of ALAN on Bats
Although ALAN’s effects on bats are concerning, several mitigation strategies offer hope for reducing its impact. Research suggests that the color and intensity of artificial light variably influences bat activity.7 White, green, and yellow lights have consistently negative effects on bats, while red and blue lights are less disruptive.6 Red light, in particular, reduces ALAN’s impact on both bats and insects, offering a potential avenue for mitigation through the use of targeted light composition.4
Other practical measures include implementing motion-triggered lighting in low-traffic areas,3 which minimizes light pollution without compromising human safety.1 Shielding corridors that connect bat roosts to foraging habitats can also reduce habitat fragmentation caused by ALAN.6 These measures, combined with continued research and conservation efforts, could mitigate the ecological disruption caused by light pollution.
Conclusion
Current research overwhelmingly suggests that ALAN has a net negative impact on bats, despite isolated benefits for certain species. For most bats, the disadvantages of ALAN–such as reduced activity, habitat fragmentation, and increased energy costs–far outweigh any foraging opportunities it might provide.
Moving forward, targeted research should continue to investigate the generalizability of these findings across bat species and ecological contexts. Simultaneously, implementing mitigation strategies such as color-modified lighting and habitat shielding can help reduce the ecological consequences of ALAN, allowing bats to continue their essential roles in pest control, pollination, and seed dispersal.
References
[1] Gili, F., Fassone, C., Rolando, A., & Bertolino, S. (2024). In the Spotlight: Bat Activity Shifts in Response to Intense Lighting of a Large Railway Construction Site. Sustainability, 16(6), 2337. https://doi.org/10.3390/su16062337 [2] Andrews, J. (2021, January 19). A quarter of streetlights could be smart by 2030. Cities Today. https://cities-today.com/a-quarter-of-streetlights-could-be-smart-by-2030/ [3] Heim, O., Chávez, F., Courtiol, A., Paul, F., & Voigt, C. C. (2024). Guild‐specific response of bats to motion‐triggered LED lighting of bicycle trails. Conservation Science and Practice, 6(4). https://doi.org/10.1111/csp2.13116 [4] Hermans, C., Litovska, I., De Pastors, M., Visser, M. E., & Spoelstra, K. (2024). Artificial light at night drives diel activity patterns of synanthropic pipistrelle bats and their prey. Science of the Total Environment, 173699. https://doi.org/10.1016/j.scitotenv.2024.173699 [5] Li, H., Allen, P., Boris, S., Lagrama, S., Lyons, J., Mills, C., Moussi, P., Nichols, C., Tacosik, C., Tsaousis, M., Wilson, N. L., Grider, J. F., Parker, K. A., & Kalcounis-Rueppell, M. C. (2024). Artificial light at night (ALAN) pollution alters bat lunar chronobiology: insights from broad-scale long-term acoustic monitoring. Ecological Processes, 13(1).https://doi.org/10.1186/s13717-024-00491-y [6] Voigt, C. C., Dekker, J., Fritze, M., Gazaryan, S., Hölker, F., Jones, G., Lewanzik, D., Limpens, H. J. G. A., Mathews, F., Rydell, J., Spoelstra, K., & Zagmajster, M. (2021). The impact of light pollution on bats varies according to foraging guild and habitat context. BioScience/Bioscience, 71(10), 1103–1109. https://doi.org/10.1093/biosci/biab087 [7] Walsh, C., Hüppop, O., Karwinkel, T., Liedvogel, M., Lindecke, O., McLaren, J. D., Schmaljohann, H., & Siebenhüner, B. (2024). Light pollution at sea: Implications and potential hazards of human activity for offshore bird and bat movements in the Greater North Sea. https://ecoevorxiv.org/repository/view/7173/ [8] Zhou, D., Deng, Y., Wei, X., Li, T., Li, Z., Wang, S., Jiang, Y., Liu, W., Luo, B., & Feng, J. (2024). Behavioral responses of cave-roosting bats to artificial light of different spectra and intensities: Implications for lighting management strategy. Science of the Total Environment, 916, 170339. https://doi.org/10.1016/j.scitotenv.2024.170339
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