Unlike the impact of light pollution on animals and plants which are generally negative, the impact that light pollution has on bats is one that is not entirely bad. With the right amount of tree cover that shields light from street lamps, certain species of bats can in fact thrive in urban conditions as Ultraviolet (UV) light attracts insects that bats feed on and a dense tree cover provides protection for bats (Straka, 2019).
Trees, lights and bats
As bats are active nocturnal creatures, they are adversely affected by the effects of artificial light at night (ALAN). For example, lighting that is shone on buildings where these bats roost negatively affects their behavior and reproduction. Just like other nocturnal animals, the rods in bat retinas are extremely sensitive in low light conditions with maximum wavelength absorbance of 497-501nm (Rowse, 2015).
However, the impact of light pollution on bats is much more complex because this relationship is species and context specific. This means that as stated earlier, there are bats who are relatively unaffected by ALAN. These bats tend to be fast flying bats with long range echolocation pulses (see figure below). When foraging near street lamps, they typically fly above the lamps and dive into the light cone to catch insects (Rowse, 2015).
With that said, bat activity still tends to be higher in dark and densely forested areas compared to forest edges (Haddock, 2015). To minimise the impact of light pollution on these bats, having dense tree cover helps to block some of ALAN by shielding areas from light scatter.
Why should we care?
While bats are often perceived as pests in our cities, more credit should be given to these creatures as they are in fact our natural pest controllers. Bats feed on many insects that are harmful to humans such as mosquitoes and midges. According to a study, a brown myotis, a species of bat, has the ability to catch 1,000 mosquitoes in an hour. Especially with dengue cases that are rising in Singapore, perhaps it is time to rethink the positive impacts that these bats have for us.
While there are varied responses of different species of bats to light pollution, the population at large still suffers from reduced foraging activities and reproduction rates. On top of reducing light trespass and ensuring that lights remained switched off when not necessary, here we see another alternative, to plant more trees in urban areas to mediate the effects of ALAN on urban bats.
Stay tuned for more!
Jean
References:
- Straka, T. M., Wolf, M., Gras, P., Buchholz, S. & Voigt, C. C. (2019). Tree Cover Mediates the Effect of Artificial Light on Urban Bats. Frontiers in Ecology and Evolution, 7(91), 1-11.
- Rowse E.G., Lewanzik, D., Stone, E.L., Harris S., Jones G. (2016) Dark Matters: The Effects of Artificial Lighting on Bats. In: Voigt C., Kingston T. (eds) Bats in the Anthropocene: Conservation of Bats in a Changing World. Springer, Cham.
- Image retrieved from https://askabiologist.asu.edu/echolocation#:~:text=Bats%20use%20echolocation%20to%20navigate,an%20object%20they%20produce%20echoes.&text=Bats%20aren’t%20blind%2C%20but,very%20quickly%20in%20total%20darkness.
- Haddock, J. K., Threlfall, C. G., Law, B., & Hochuli, D. (2019). Light pollution at the urban forest edge negatively impacts insectivorous bats. Biological Conservation. 236, 17-28.