By Leonie Hetzer –
“’Cause even when there is no star in sight, you’ll always be my only guiding light” sing Mumford & Sons. However good these words could describe the fatal end for turtles when the guiding light is, in fact, street lighting on coastal roadways. It disrupts the nocturnal orientation of turtle hatchlings as they crawl towards the sea.
Figure 1 Loggerhead nestling by Hillebrand, S., CC-BY
This phenomenon falls under the heading of light pollution. Everyone has probably come across air pollution and ocean pollution. But light pollution? Until a few weeks ago, this term didn’t ring a bell with me either, but light pollution caused by humans is a big issue, especially for animals. It describes the constant presence of artificial light, which leads to animals and plants being disturbed in their natural way of life and ability to orient themselves [1]. Loss of orientation in newly hatched turtles is just one example.
In the specific case of hatchlings losing their orientation due to street lighting, however, there is a possible solution: Lights embedded in the street. Leseley Bertolotti and Michael Salomon wondered whether this would really prevent the death of the turtles. They conducted some experiments at Spanish River Park, Boca Raton, Florida, USA to find out [2].
The primary direction in which the animals run was determined, first when the street lights were on, then when only the embedded lights were on, and finally when both the normal and embedded lights were off. These three aspects were tested both at full moon and at new moon.
The researchers were able to determine the following:
At full moon, the turtles had no problems finding the sea regardless of any lights being switched on. This is because the contrast between the lights and the background was comparatively small. It was different with a new moon. The contrast between the lights and the surroundings was so great that the hatchlings’ orientation was disturbed.[2]
In addition, the researchers found that under the condition of clear to slightly cloudy skies, the embedded lights had no effect on the orientation of the turtles, compared to the normal street light [1]. In the case of overcast, however, a deviation from the optimum was also recognisable in the experiment with the embedded lights. According to the researchers, this is most likely due to the so-called skyglow, which occurs when clouds reflect the light back to earth [3]. Unfortunately, no experiment was carried out without any light turned on in overcast, which could have confirmed a possible skyglow even without light from the experimental setup itself. It is, after all, not unlikely that light from surrounding cities is also reflected off the cloud cover and could contribute to the hatchlings’ disorientation [2].
It can therefore be said that embedded light is the better guiding light and a sensible alternative to conventional streetlamps since it does not have any effect on the animals’ orientation and therefore does not misguide them.
References and further reading
[1] Chepesiuk, R., 2009. Missing the dark: health effects of light pollution.
[2] Bertolotti, L. and Salmon, M., 2005. Do embedded roadway lights protect sea turtles?. Environmental Management, 36(5), pp.702-710.
[3] Witherington, B.E. and Martin, R.E., 2000. Understanding, assessing, and resolving light-pollution problems on sea turtle nesting beaches.
