What’s Right with Lights at Night?

Primary Source: Owens ACS, Lewis SM. 2022 Artificial light impacts the mate success of female fireflies. R. Soc. Open Sci. 9: 220468. https://doi.org/10.1098/rsos.220468

Artificial lighting is such an extensive part of our society that it is often hard to think about just how recent an invention it is. Despite occupying a short section of total human history, human society has adapted heavily to make use of the extra hours of productivity provided by artificial lighting. While human adaptation to artificial lighting has been swift, this is not the case for many species of animals, with the most affected being nocturnal animals. Light pollution has been shown to endanger animals such as moths, seabirds, deer, and even sea turtles. Historically these issues have been ignored under the pretext that if human light pollution seriously endangered these species, they could move to areas without artificial lights. However, as land development has progressed, it has become increasingly clear to conservation scientists that this is an increasingly unlikely option for animals affected by light pollution. As such, research into which species are affected by light pollution, and to what extent, has become increasingly prevalent. One such study from Tufts University sought to quantify the effects of light pollution on mating success in different North American species of the genus Photinus, a group of bioluminescent beetles known more commonly as fireflies.

Artificial light, like that from streetlamps, impacts local wildlife adapted to the routine darkness of night. In this example, the intense light exposure has caused the nearby chestnut tree to delay the fall of dead leaves. Credit: “Effect of light pollution on urban trees and dead leaves 03” by Lamiot, licensed under CC BY-SA 4.0
Doing Some Light Research

Fireflies use patterned flashes of their bioluminescence to attract mates. Males send out a flash to indicate their intent to reproduce, and willing females respond with a response of light flashes unique to the specific species. However, anthropogenic light pollution can overpower the bioluminescence of fireflies, obscuring the message to other members of the species. To test the extent to which light intensity affected this mating issue, researchers from Tufts University set up various scenarios in which pairs of fireflies from three different species of the Photinus genus were observed in attempts to mate under different lighting conditions. These tests were run both in lab as well as in the field, with artificial light being matched to the female’s response pattern as a control, and test cases being done under consistent artificial lighting of various intensities.

The researchers found that, overall, male fireflies were significantly less likely to approach a simulated female response pattern under light pollution of mild intensity (5 lux) than a female response pattern under no light pollution (0 lux), when given the choice between the two. Additionally, the species Photinus obscurellus was shown to be particularly sensitive to changes in light intensity compared to the other species in the study, with significant decrease to mating success between light intensity of 3 lux and 30 lux. While the species Photinus marginellus also showed a decrease in mating success between light intensities of less than 2 lux and greater than 20 lux, the difference was not statistically significant. On the other hand, the final species tested, Photinus pyralis, was not noticeably affected by changes in light intensity.

The common eastern firefly, Photinus pryalis, is active around dusk and twilight, and is thus adapted to the presence of low-intensity light. As expected, this species is less affected by artificial light pollution than nocturnal species of the same genus. Credit: “Photinus pyralis Firefly 3” by Terry Priest, licensed under CC BY-SA 2.0
Shedding Light on the Situation

Interestingly, the species which did not show significant decreases in mating success under different light intensities are both crepuscular, meaning they are most active at dusk and twilight. Conversely, the species which showed significantly decreased success in mating under intensifying light conditions, Photinus obscurellus, is nocturnal. This difference in lifestyle is likely the reason for the differing effects of light pollution on the various fireflies. While artificial light of a low intensity may be inconsequential to a species that is adapted for the presence of low intensity light, it may be a blinding interference to a species adapted for the absence of light. This research highlights that similar species may respond differently to the same environmental hazard as a result of occupying different niches. Additionally, it is part of a growing area of conservation research concerning pollution of the environment in ways not typically thought about, such as anthropogenic light pollution. Hopefully research such as this helps conservation scientists focus their efforts to the species that truly require them.

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Cypress Novick

Cypress Novick

I am a recent graduate of Occidental College in Los Angeles, California, where I studied for my Bachelor's in Biology. My main research interests are wetlands ecology, mycology, estuary ecosystem interactions, and plant-based trophic interactions. I have always been passionate about making science more available and understandable, and am always trying to improve my writing so I may help myself and others be better understood.

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