Turn off the lights! Light pollution and its impacts on behavior

by Alexander Louis Birk, Pitzer College [edited by Lars Schmitz as part of BIOL 167 “Sensory Evolution”, an upper division class at the W.M. Keck Science Department. Written for educational purposes only].

As more and more scientific research is conducted, we continue to find additional anthropogenic effects on the natural environment and the ecosystems, especially surrounding large metropolitan areas. Besides well-documented anthropogenic processes that affect climate through pollution, emerging data reveal effects on the sensory biology of animals. For example, there is evidence that pollution is hindering animals’ sight and smell. There is evidence that industry and motor vehicle use affect the hearing of animals. But one factor that is just now being researched more is the effect of light pollution on different animals. Specifically, light pollution and other changes to the visual environment can potentially have adverse effects on animal behaviors like camouflage and mate choice.

Urban development has spread widely across almost any land that we can inhabit, and, in a nutshell, artificial light has been an incredible breakthrough for humans. Light has become such a centerpiece for our societies; we see bright and flashy lights prominent wherever humans are present. We have so many lights in such a concentrated area that we are able to see the lights from space (Figures 1 and 2).

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Figure 1. Global view of city lights (from NASA’s Earth Observatory).

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Figure 2. Maps of light intensities throughout the United States. Light is one of the most prominent ways how humans alter their environment. [From Swaddle et al. 2015]

Having lights with such intensity is bound to have some effects on animals, and light pollution due to human development is expected to impact the behavior of many animals. According to a recent and very timely essay (Delhey and Peters 2016), human changes to the visual environment can potentially increase predation risk, lead to maladaptive patterns of mate choice, and disrupt mutualistic interactions between pollinators and plants. I want to take this opportunity to highlight some of the important consequences of light pollution on behavior.

Camouflage is an adaptation which relies on animals blending in with their background to remain undetected by potential predators or prey, depending on whether the strategy is not to be seen to avoid being eaten, or not to be seen to strike an ambush attack.

The amount of light in terrestrial environments will alter the way colors are perceived. If a naturally dark environment is lit due to anthropogenic lighting, the camouflage that the organisms use in that environment may become unviable (Swaddle et al. 2015). This will potentially result in an increase in predation on these organisms, exposing them to increased extinction risk. An important issue here is that the change in light intensity through urban development is so sudden that there is not a chance for the organisms to adapt their camouflage on an evolutionary scale before being predated to extinction.

Implications for camouflage may also arise from physically changing the visual environment. Similar to changes in light, changing the environment (the “backdrop”) will make camouflage less viable. As referenced before, camouflage relies on the organism having the specific color and/or pattern to match its background. However, if there is a substantial habitat alteration, with substantial removal of the most commonly found background the animal is trying to blend into, camouflage is essentially useless (Delhey and Peters 2016). While this is not a direct link to light pollution, most environmental degradation happens in or around the metropolis where light pollution is pervasive.

Quite the opposite to camouflage, some organisms have conspicuous, colorful traits which they rely on to communicate with each other and transfer important signals. In tests on cichlid fish it was found that changes in irradiance (ambient light) and transmission properties of water due to increased turbidity had a direct effect on mating behaviors of females (Delhey and Peters 2016, Figure 3). This is potentially problematic because coloration is one way that these fish can differentiate males of their own species from other cichlid species. If female fish attempt to mate with males of another species they will likely not have viable offspring, and may not even be able to mate successfully at all. This could have drastic effects on the fitness of the species as a whole.

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Figure 3. Changes of ambient light and the transmission properties of water may change the color perception of fishes, with detrimental effects on mate choice. [From Delhey et al. 2016]

The artificial environment that humans have created is also potentially problematic for the visual perception of organisms. The colors that are used in buildings and even advertisements commonly found in cities have been shown to alter animal behavior (Delhey et al. 2016). For example, pollinators will often seem distracted by and/ or attracted to vibrant colors, which results in a waste of their time and energy. In addition, it is common for people in cities to use their own plants and flowers to decorate their homes and businesses. These flowers draw the attention of pollinators which can result in the native plants having to compete with introduced and invasive species. This can result in lower seed production and thus lower the fitness of the native plant species (Delhey et al. 2016).

Whether it is due to increased lighting or due to changes in environment, changing the visual aspects of an organism’s environment will undoubtedly have an effect on the population. It is likely that potential impacts will include decreases in both population viability and mating success. This is due to the fact that mates will have a hard time recognizing the colors used for reproductive signaling. There will also be the threat of over predation on organisms that typically use camouflage to avoid their predators. The long term population consequences from weakening sexual selection and increasing predation may cause an overall decrease in the fitness of the population.

In order to change this, we must be more aware of the anthropogenic effects on the environment, especially considering light pollution and changes in visual stimulation. More extensive research is needed to proceed with mitigating the effects of anthropogenic light pollution. I think that it would be beneficial to test how animals react to different colors and intensities of light. This would help us to understand possible changes in behavior. In general, with more research we will have a better understanding of how changes in light affect animal behaviors in and around metropolitan areas, leading to better strategies on preventing, or at least decreasing, the impact of light on the environment.



Delhey, K. & Peters, A. 2016 Conservation implications of anthropogenic impacts on visual communication and camouflage. Conservation Biology 31, 30-39.. DOI: 10.1111/cobi.12834

Swaddle, J.P., Francis, C.D., Barber, J.R., Cooper, C.B., Cyba, C.M., Dominoni, D.M., Shannon, G., Aschehoug, E., Goodwin, S.E., Kawahara, A.Y. 2015 A framework to assess evolutionary responses to anthropogenic light and sound. Trends in Ecology and Evolution 30, 550-560. DOI: 10.1016/j.tree.2015.06.009

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