Throughout the world, more and more people are living in urban areas. We’re called by the glimmering lights, the whirlwind of activity, the sights and sounds of the big city. Some of these same factors that draw people to cities are what drive other animals away–or, in many cases, cause harm to the animals who stay.
For bats, who are most active at night, urban light pollution can pose some major issues (Laforge et al., 2019; Langley, 2019; Seewagen and Adams, 2021). Artificial light can be disorienting for nocturnal animals, disrupting their circadian cycles and changing their typical activity levels throughout the day (Seewagen and Adams, 2021). Further, high levels of light leave species that are used to the cover of darkness vulnerable to predators (Cravens and Boyles, 2018; Laforge et al., 2019). Still, some bats–individuals or entire species–choose to spend more time foraging in lighter areas because UV lights attract their preferred food choice: insects (Cravens and Boyles, 2018; Langley, 2019; Seewagen and Adams, 2021).
So how does the presence of light impact the bats who choose to stay?
Seewagen and Adams (2021) found that several species of bats reacted to the presence of LED floodlights by greatly reducing their foraging activity. Migratory tree bats tended to not be impacted by light or even be attracted to it, while nonmigratory species tended to avoid light areas (Seewagen and Adams, 2021). Little brown bats (Myotis lucifugus) have been found to have difficulty avoiding obstacles in highly-illuminated areas, indicating that light may have some impact on their sensory systems, including systems involved in orientation (Seewagen and Adams, 2021).
Cravens and Boyles (2018) focused on the differences between levels of the blood metabolite beta-hydroxybutyrate in bats found in lit and unlit conditions. Beta-hydroxybutyrate is a fasting metabolite, and is used for energy in metabolic processes when triglyceride fat storage is low. Interestingly, this is not the exact case in bats: beta-hydroxybutyrate levels spike after feeding–so they are correlated with periods of intense exercise (Cravens and Boyles, 2018). For red bats (Lasiurus borealis), beta-hydroxybutyrate levels were highest just after sunset in highly-lit sites, while the opposite was true in dark sites (Cravens and Boyles, 2018). Cravens and Boyles (2018) suggest that red bats have altered their foraging activity to prey on insects at lit sites just after sunset, while other bats may forage throughout the night. Also, red bats captured at lights late at night had low levels of these blood metabolites, suggesting that they were able to gain more energy from well-lit areas with less work (Cravens and Boyles, 2018).
Despite this positive impact on foraging in some bat species, high levels of light pollution can negatively impact bat diversity and species abundance in urban areas (Laforge et al., 2019; Langley, 2019; Seewagen and Adams, 2021). Seewagen and Adams (2021) were only able to detect little brown bats on 14% of light nights, while they were able to hear little brown bats calling on 65% of dark nights. Laforge et al. (2019) had similar results, with artificial light being a significant predictor of bat presence and activity, as well as their ability to move through their landscapes.
While artificial nighttime light can provide light-tolerant bats with novel foraging opportunities, finding ways to mitigate the impacts of nighttime light can greatly improve bat biodiversity. The most obvious solution is to turn out the lights–reducing nighttime light can help bats expand their species range by providing opportunities to safely move between habitat patches (Laforge et al., 2019). With less light, bats will be better able to avoid predators, and their sensory systems and circadian cycles may fall back in line. However, reducing light is not enough to truly improve bats’ habitat quality (Laforge et al., 2019)–and it is not the only option. Increasing urban greenspace can provide bats with shelter from predators in the form of tree canopy cover (Langley, 2019). Bats may even be able to strike a balance–enjoying the benefits of insects attracted to UV lights while remaining safely covered (Langley, 2019).
Whatever the solution may be, one thing is for sure: providing safe habitats for bats is necessary for supporting pollination, controlling insect populations, and preserving biodiversity in a changing world.
Cravens, ZM & Boyles, JG (2019) Illuminating the physiological implications of artificial light on an insectivorous bat community. Oecologia 189:69-77. doi: 10.1007/s00442-018-4300-6
Laforge, A, Pauwels, K, Faure, B, Bas, Y, Kerbiriou, C, Fonderflick, J, & Besnard, A (2019) Reducing light pollution improves connectivity for bats in urban landscapes. Landsc Ecol 34:793-809. doi: 1o.1007/s10980-019-00803-0
Langley, L (2019, April 17) Light pollution hurts urban bats. Trees can help. National Geographic. https://www.nationalgeographic.com/animals/article/bats-appreciation-day-cities-urban-lights
Seewagen CL & Adams, AM (2021) Turning to the dark side: LED light at night alters the activity and species composition of a foraging bat assemblage in the northeastern United States. Ecol Evol 11(10):5635-5645
Image 1 source: https://pixnio.com/objects/electronics-devices/electric-lights-pictures/stadium-lights
Image 2 source: https://pixnio.com/fauna-animals/bats-pictures/myotis-lucifugus-little-brown-bat