Light pollution is a commonly occurring issue in urban areas and other areas of high human activity. An excess of artificial lighting, while it may be useful to us, has the potentially to negatively impact nocturnal or other light sensitive species. Bats make up the second largest order of mammals on earth but roughly a quarter of these species are threatened which may in no small part be due to the disruptive effects of light pollution (Stone et al 2015). However, some bat species have readily colonized city habitats so it is important to understand how artificial light may negatively impact these species
Exposure to artificial lights can significantly alter foraging and travel patterns in many bat species. Bats have been shown to alter their travel routes in order to avoid sources of artificial light (Kuijper et al 2008, Stone et al 2008). The avoidance of light sources by many bats effectively fragments their habitat. Additionally, alternate routes that bats may take in response to light pollution may be longer or impose a greater risk, such as predation, on bat species. Bats will also avoid foraging areas that are too well lit which can limit food availability and force bats to travel to farther or lower quality foraging sites (Polak et al 2011). However, not all bats avoid artificial light sources. Some species of insectivorous bat are actually drawn to artificial light sources as these sites often contain a higher density of insects (Schoeman 2015). These illuminated foraging sites are not without risk though as attraction to light sources put bats at a greater risk of being struck by vehicles and light levels have also been shown to interfere with a bat’s ability to avoid obstacles (McGuire and Fenton 2010, Stone et al 2015).
Bats can also experience light pollution at the roost. The presence of artificial light near bat roosts can artificially extend daylight conditions delaying the emergence of bats in the evening and reducing the time available for them to forage (Stone et al 2009). This may explain in part why light pollution around the roost sites has been linked to reduced growth rates in young bats (Stone et al 2015). Light pollution around roosts may also deter returning bats from re-entering the roost and in extreme cases may result in abandonment of the roost (Stone et al 2015).
While light pollution can have a number of impacts on bats the negative effects do not just stop there. Many bats provide essential ecosystem services which may be disrupted by artificial lighting. Fruit-eating bats in the tropics are seed dispersers and play an important role in the recolonization of abandoned farmland. However, the presence of artificial light near these abandon fields can cause fruit-eating bats to avoid the area which can delay the introduction of native plant life back into the area (Lewanzik and Voigt 2014). This is just one example of the cascading effects that light pollution can cause by disturbing bat species, but bats provide a range of important ecological services such as pollinating plants and regulating insect populations. In order to prevent the loss of these services and species that provide them measures need to be taken in order to reduce the impact of light pollution on bats. Some recent work has found that tree cover can help to mitigate the negative effects of light pollution so the expansion of green spaces within urban centers may be one viable method of conservation (Straka et al 2019). Other strategies such as limiting the number of streetlights near bat habitats or using lower intensity bulbs may also be viable, but more work still needs to be done to fully understand how light pollution impacts bat species and how it can be managed.
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McGuire LP, Fenton MB (2010) Hitting the Wall: Light Affects the Obstacle Avoidance Ability of Free-Flying Little Brown Bats (Myotis lucifugus). Acta Chiropterologica 12:247-250.
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Straka TM, Wolf M, Gras P, Buchholz S, Voigt CC (2019) Tree Cover Mediates the Effect of Artificial Light on Urban Bats. Frontiers in Ecology and Evolution https://doi.org/10.3389/fevo.2019.00091