Light is an important cue for orientation used by many animals and also influences their diurnal rhythms.
While focusing on conventional forms of pollution, we tend to pay less attention to the night time light pollution and how that affects nature. Turns out, this kind of pollution affects bats during their seasonal migration.
In the latest study, scientists tested the response of European bats to red and white light sources during their seasonal migration and according to the research, Soprano pipistrelles and Nathusius' pipistrelles (types of bats) were recorded more frequently near red LED light, indicating that the animals might be attracted to red light during their migration. In contrast, the scientists did not observe such behaviour near-white LED lights.
The wavelength of the experimental red LED lights was similar to that of red safety lights used for indicating the presence of wind turbines or tall buildings to aircraft pilots.
Warning lights such as these lure migrating bats precisely towards the danger which the lights help people to avoid, leading to bat collisions and casualties. Scientists suggest that deploying on-demand lighting, which only turns on if an aeroplane approaches, would most likely reduce the problem.
Each year, light pollution increases by around six per cent worldwide. In particular, energy efficient and cheap LEDs are used more and more. Light is an important cue for orientation used by many animals and also influences their diurnal rhythms and behaviour. It is well established that bats are sensitive to light while hunting at night.
During late summer, thousands of bats migrate along the coastline of the Baltic Sea in Latvia, through Pape Nature Reserve. Nights are starlit and largely devoid of light pollution as there are only a few human settlements in the area. Here, the scientists installed an eight-meter-high pole near the shoreline. A plastic board fixed to the pole was lit-up in 10-minute intervals alternating with darkness.
The LED lights illuminating the board switched between red or white LED light. By using ultrasonic microphones the scientists recorded the echolocation calls of bats coming close in order to identify both the species and the number of bats passing by the unlit or lit experimental site.
Soprano pipistrelles and Nathusius' pipistrelles were recorded more frequently at the experimental site during the red light phase than during darkness.
Oliver Lindecke, co-author of the study explained, "Bats are at a higher collision risk at wind power stations during their autumn migration. Our study indicates that the use of red light signals could have fatal consequences for them as this appears to attract them to operating wind turbines"
Oliver also suggested that existing light signals could easily be replaced by bat-friendly alternatives or context-dependent illumination could be deployed which is only activated if planes or helicopters are approaching a wind power plant.
Exactly why bats are attracted to red light sources is unclear. According to the researchers, bats have excellent eyesight and can even detect wavelengths invisible to us. Some red light sources might potentially blind and disorient them. "Many bat species already struggle in our current anthropogenic landscapes characterised by intensive agriculture and high densities of wind turbines. Light pollution is likely to increase pressure on them even further," explained Christian Voigt, one of the researchers.
The full findings are present in the journal- Ecology and Evolution.