How the response of light-averse bats varies with light spectrum is not well known. This difference can most likely be attributed to the scarcity of insects around LPS lights. The response of bats is also reported to be dependent on the light source spectrum: the activity of some non-light-shy bat species around low-pressure sodium (LPS) lamps, which produce monochromatic orange light, is much lower compared to light sources that contain shorter wavelengths. Spectrum-dependent responses are known for example in insects, birds, reptiles, toads and mice. How species react to light often varies with light colour. When flying at an illuminated location, bats are observed to fly faster than in the dark. In addition, a significant negative correlation has been shown between flight speed and emergence time, suggesting that slow-flying bats are more wary of predation. Heavier bats, which are less manoeuvrable, emerge later than lighter bats of the same species, and delayed emergence has likewise been reported for young bats that are still improving their flight skills. Bats that do not need to forage early in the evening on crepuscular insect species tend to emerge later in the night when the light intensity has dropped further. The most widely accepted hypothesis explaining why these bat species avoid light is the fear of predators capable of hunting bats by visual cues. For these species, avoidance of experimental light sources has been shown for bats flying along commuting routes. For example, slow-flying Myotis and Plecotus species are likely deterred by light. However, not all bat species are recorded around light sources at night in high densities. , although other studies show that these species tend to avoid light in absence of tree cover, suggesting that the attraction to light is context dependent. This cascading effect of light was recently confirmed in an experimental set-up by Minnaar et al. The increase in density of insects, but also the impairment of defence mechanisms of moth species, facilitate bats' foraging. This does not appear to be a direct effect, but the result of the accumulation of insects that are first attracted to the light sources. A conspicuous and long known effect on some bat species is the attraction to light. The disappearance of the natural night-time darkness affects many species groups among these, bats are well represented. The use of artificial light will continue to grow and sky brightness increases with an estimated 6% per year. Hence, in order to limit the negative impact of light at night on bats, white and green light should be avoided in or close to natural habitat, but red lights may be used if illumination is needed.Īrtificial light at night has shown a dramatic increase over the past few decades. Forest-dwelling Myotis and Plecotus species and more synanthropic Pipistrellus species are thus least disturbed by red light. The agile, opportunistically feeding Pipistrellus species were significantly more abundant around white and green light, most likely because of accumulation of insects, but equally abundant in red illuminated transects compared to dark control. Plecotus and Myotis species avoided white and green light, but were equally abundant in red light and darkness. The results reveal a strong and spectrum-dependent response for the slow-flying Myotis and Plecotus and more agile Pipistrellus species, but not for Nyctalus and Eptesicus species. We measured activity of three bat species groups around transects with light posts emitting white, green and red light with an intensity commonly used to illuminate countryside roads. We used a unique set-up of eight field sites to study the response of bats to three different experimental light spectra in an otherwise dark and undisturbed natural habitat. Different spectra may therefore be applied to reduce negative impacts. Among these, bats are strongly affected, and how bat species react to light is likely to vary with light colour. Effects are reported for many species groups, and include changes in presence, behaviour, physiology and life-history traits. Artificial light at night has shown a remarkable increase over the past decades.
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