Consequences of artificial lighting on bats and ecological systems

Artificial Lighting -- Impacts on Bats

When Thomas Edison introduced the incandescent light bulb in 1879, it was seen as an enormously helpful technology. Little did Edison know that by 2013 environmentalists, naturalists, and medical professionals would consider "…light pollution to be one of the fastest growing and most pervasive forms of environmental pollution" (Chepesiuk, 2009). Light pollution is known to affect flora and fauna, along with frogs (whose mating calls are inhibited when they are "…exposed to excessive light at night"), turtles, migratory birds and bats. Indeed, the "…feeding behavior of bats is altered by artificial light" (Chepesiuk, 24).

In this paper documented, empirical research reveals that bats are certainly affected by light pollution. Bats are found around the world in various habitats, and there are several species within the bat community, but virtually all bats are "…exclusively nocturnal" (Duverge, et al., 2000, 32). Why are bats exclusively nocturnal? Duverge and colleagues write in the peer-reviewed journal Ecography that bats are nocturnal for a couple reasons: one, they avoid predators by flying in the dark; and two, their ability to use ultrasonic echolocation (high-frequency sound) to locate food is more effective after dark. This paper delves into the reasons why bats emerge to forage in the air for insects at different times of the evening, and in specifics, why bat feeding habits are "…constrained at bright light conditions" -- in other words, when there is a great deal of light pollution bats' "predation risk" -- from raptorial birds -- and their normal feeding habits are shaped and affected by light pollution. The paper also delves into other reasons why bats chose certain times to emerge into the nighttime environment.

Literature review on the Consequences of Artificial Night Lighting on Bats

Significance of emerging timing in bats

Lighting certainly has an impact on the feeding habits of bats, and in fact this paper reports studies on Northern bats and greater horseshoe bats that reflect their nocturnal habits based on the light conditions and other variables. The Northern Bats were studied in southern Sweden and the Greater horseshoe bats were observed in Pembrokeshire, UK. The authors spend a good share of their narrative investigating the flight times and flight performances of lactating and pregnant bats -- which is not related to the light issues -- but they also report that both species of bats emerged "…earlier at protected exits than they did at exposed ones" (Duverge, 38). One salient point that was made by Duverge and colleagues: since different entrances to caves allow varying amounts of light, the bats' perceptions of darkness outside tend to be based on how much light is entering through the cave's portal.

The Greater horseshoe bats came out earlier from roosts that were located in "…deciduous woodland" -- where presumably it is darker earlier -- than they did from sites that were more exposed to light, and this is logical given the hypotheses by the researchers. The bottom line of this scholarship is that bat conservation can be enhanced by "protective tree cover"; that is, at those protected sites bats can emerge earlier and "…take advantage of the dusk peak in insect activity, and also extend their foraging time" (39). Moreover, with trees as protection, the chances of a bat being attacked by a raptor are reduced.

Street lighting disturbs commuting bats

A scholarly article in Current Biology (Stone, et al., 2009) takes the discussion of bats and light further than Duverge and colleagues; the authors note at the outset that "Anthropogenic disturbance is a major cause of worldwide declines in biodiversity," and for biologists, one of those anthropogenic disturbances that has a particularly negative impact on bats is anthropogenic light pollution (Stone, 1123). The fact that bats are strictly nocturnal makes them prime subjects for evaluating the effects of light pollution on wildlife, Stone explains.

Using a different bat species than Duverge used -- lesser horseshoe bats -- the researchers investigated this species because the global populations of lesser horseshoe bats is dwindling to a dangerous level, and in many industrialized countries in central Europe. In England, about 11% of bats' mortality results from "predatory birds," hence, the issue of when bats emerge after sunset is a key in terms of trying to understand the decline of this species (Stone, 1123). To wit, if bats emerge too early hawks, falcons and other predators are still in the air and can snatch a bat out of the air because there is plenty of light for the predators.

The researchers installed high-pressure sodium lights -- that "mimic the intensity and light spectra of streetlights" -- along the commuting route of horseshoe bats in southern England (Stone, 1123). What they discovered was that "bat activity was reduced dramatically" and indeed the emergence of these bats (from hedgerows) was delayed due to the light that bats perceived as daylight (1123).

Normally bats emerge from hedgerows in England around 29.9 minutes after sunset; but on nights when the high-pressure sodium lights were switched on (using a generator that made noise and had an effect on the bats' timing as well), the average time the bats left the hedgerows was 78.6 minutes (Stone, 1124). What this research clearly shows is that light pollution does have "…significant conservation consequences" for bat species, particular threatened bat species; what was learned was not just that bats emerge from the hedgerows later when there is light introduced into their environments, but also that bats alter their normal "flight routes" (Stone, 1125). The research reflects that light pollution may well force bats "…to use suboptimal flight routes, potentially…" forcing them to be isolated from their "preferred foraging sites" (Stone, 1125).

Conserving energy at a cost to biodiversity?

Another article that features Emma Stone, Gareth Jones and Stephen Harris -- albeit this one was published more recently, in 2012 -- suggests that not only does artificial lighting threaten biodiversity, but it also produces 1,900 million tons of CO2 emissions globally (Stone, 2012, 2458). The amount of CO2 that is produced is over three times what aviation produces, Stone argues. Why is this global warming data important to bats and biodiversity? Because energy-efficient lighting is now being installed to lower the amount of CO2 -- high-brightness, light-emitting diodes (LEDs) are replacing the previous lighting technologies -- and that change is causing "ecological impacts that have not been tested" (Stone, 2458).

What happens when LED lights are introduced is similar to what the previous article reported: these energy-saving lights "…can potentially fragment commuting routes for bats" and this dynamic is associated with what Stone calls "…negative conservation consequences" (2458). Interestingly, under European law ("European Habitats Directive / Conservation of Natural Habitats of Wild Flora and Fauna 1992/42/EEC"), it is unlawful to disturb a bat "deliberately" -- and that means doing anything that impairs a bat's "ability to survive, breed or reproduce" is against the law. Moreover, to negatively affect "significantly" the local distribution or abundance of a species is a violation of European wildlife statues.

Stone raises that point because it would appear that the use of LED artificial lighting "may potentially constitute an offence" under European law; hence, the authors insist that during environmental impact studies, lighting must be taken into account. The bottom line of this research piece is that the authors proved empirically through their research that LED street lights have a negative impact on the lesser horseshoe bat species. Indeed, there was a "significant reduction in activity" of bats using the "…high and medium light treatments" of LED lighting (Stone, 2461).

Bats and lighting in the UK

Alison Fure writes in the London Naturalist that while many biologists studying bats have focused on light levels vis-a-vis predator avoidance, there is "…actually ample evidence" that indicates bats' existing eye physiology helps to guide them as they "…exploit low light levels and avoid disorienting bright light" (Fure, 2006, 1). In other words, Fure believes the issue isn't so much that predators are attacking bats because of light pollution, but in fact the issue is that in England wildlife advocates are at a disadvantage as to bat mortality and bat conservation. To wit, when new lighting proposals are made -- and there has been a "surge in lighting applications in the London region" -- because protocols are not in place, few experts are prepared to vouch for the need to protect bats from light pollution. In particular, when there are new lighting developments (soccer fields or other sporting facilities) in areas that are dark corridors or "…foraging areas known to be of interest to bats," protocols are needed but are not yet in place (2). In conclusion, Fure references the same European laws that were mentioned earlier in this paper; she believes "…adequate mitigation measures" should be put in place prior to new outdoor lighting installations for any purpose, sports or other activities (11).

Bat Attacks and moth defensive behaviour

In the Canadian Journal of Zoology the issue of lights and bats is presented in a different context albeit one that is important to the survival of bat species. The authors explain that "Large-scale habitat loss and fragmentation…" that results from urban sprawl is a major cause of the lack of biodiversity within the insect species (Acharya, 1999, 27). Even the building of a new road, or street lights, in places where previously there were no roads or lights, what the authors call "undisturbed areas," has an impact on insect biodiversity, Acharya explains. Meanwhile, moths, which are known to be drawn to light, have trigger mechanisms that detect the echolocation signals of bats; and on the other hand bats feed "…heavily" on moths, Acharya continues; in fact many bat species use moths as their "main food item" (Acharya, 27).

The point of that information (and of this study) in this peer-reviewed piece is that if "…eared moths" exhibit behaviors that allow them to avoid bat attacks, they would not be caught as often by bats and hence this would have an effect on bat feeding (28). The authors "deafened" some moths as an experiment to determine of the moths would still be evasive to the echolocation abilities of bats. The authors released 33 "deafened" moths and 80 "eared" moths and none of the deafened moths exhibited evasive behaviors when bats attacked while 47.5% of the moths that could "hear" bats' echolocation did attempt evasive behaviors. And so, while lighting changes bat behaviors and causes bats to alter their normal paths, lighting also interferes with "…a moth's ability to respond to echolocation calls" and hence moths "…suffer a selective disadvantage around lights" -- which ironically is to the advantage of bats (Acharya, 32).

Light dependent shift in the anti-predator response of a pyralid moth

Another research paper dealing with moths and bats is published in the journal Oikos; in this case the authors demonstrated that moths are actually able to "…switch between defensive strategies from insectivorous birds [in the daytime] to bats in the evening" (Svensson, et al., 2003, 239). Moreover, this research places emphasis on what Acharya was explaining in the previous research article: that is, a moth's ability to dive away from a bat -- "a spiral flight towards the ground" which is set in motion by the detection of a bat's echolocation signal -- is "inhibited when the moth flies close to a mercury-vapour streetlamp" (Svensson, 239). This in turn, can explain why bats (in this Swedish study they observed the pyralid moth's behaviors) "are successful when hunting moths around streetlights" (Svensson, 239-240).

Again, the irony in these research articles is that while light pollution has a negative impact on bat mortality, a streetlight can also be the bats' friend because it draws in moths. That said, the ultimate goal of Svensson's paper is to prove that moths seem to adapt to avian attacks differently in the daytime than they do at night. In daylight moths seem to understand that birds are their enemies, and hence moths perform "erratic flight patterns" to defence against birds. But at night, moths seem to know their predators are bats, and in response moths land on water or in vegetation "…then remain motionless for a few seconds" (until the threat is gone) (Svensson, 245).

Contradiction to the previously referenced upside of streetlights for bats

According to the book, Fighting Light Pollution: Smart Lighting Solutions for Individuals and Communities, written by the International Dark-Sky Association (IDSA), long-eared bats (including horseshoe and mouse-eared bats) are "…repelled by light," which makes it problematic for bats to "…procure [food] when insects swarm around lights" (IDSA, 2012, 29). That contradicts Svensson's research, which was conducted in Sweden. But on the other hand the IDSA's research echoing what Stone's research paper suggested: the IDSA explains that recent studies in the UK of the "…endangered lesser horseshoe bat show that the species will take a longer route to feeding grounds in order to avoid lighted areas" (29).

In turn, the longer commute for bats (that are reportedly repelled by streetlights) means the bat "…expends more energy, is exposed to predators longer, and has a shorter time to feed" (IDSA, 29). Hence, because moths are a "favorite food of lesser horseshoe bats," when moths are drawn to "…the very light sources these bats avoid" moths are therefore not a "viable pray" for the food-focused mammals (IDSA, 29). And so, according to the authors, the decreasing source of food (moths) for bats, along with a combination of loss of habitat and "life-cycle disruption," can be considered contributing factors in the "overall decline in many bat populations" (29).

Still another research article referencing moths and bats

Kelvin Conrad and colleagues write in the peer-reviewed journal Biological Conservation that of the myriad insect populations -- which comprise "more than two-thirds of terrestrial species" -- marco-moths in England are in serious decline (Conrad, et al., 2006, 279). Of the 337 species of moths, the authors explain that 71 (including marco-moths) of those are threatened. That, in turn, offers "worrying implications" for bats, Conrad continues.

Using sophisticated traps (RIS light-traps), the authors have been counting marco-moths over the years from 100 traps; they have discovered "…alarming declines in the overall abundance of wide-spread marco moths. In the thirty-five years of trapping and sampling, marco-moths have declined by 31% (Conrad, 281). The greatest decline in the marco-moth population has occurred in southern England, which has also been the site of the decline of various butterfly populations.

What does it mean to the larger ecosystem when a species of moths suffers serious declines? Conrad asserts that this decline in moth populations signals "…strong impacts on the wider ecosystem and at higher trophic levels such as predacious insects, insectivorous spiders, birds and bats" (284). This deterioration in the population of moths in England signals a "…biodiversity crisis for Britain" and is an indicator that insects (and hence, bats) may be facing great losses in other countries and temperate-zone industrialized countries in particular (Conrad, 284).