The Dark Side of Night Lighting

Written by Ashley Wilson

When it becomes dark at night, we don’t think twice to turn the light on. Our homes, neighborhoods, and communities are all lit up by lamps, car lights, and street posts to increase visibility and ensure our sense of safety. Yet have you stopped to ask yourself: how does the light I produce affect the wildlife around me?

Artificial nightlight is defined as any excessive and obtrusive amount of undirected light. We can compare ambient night conditions to artificial nightlight in Figure 1, where the left side of the figure shows a dark neighborhood with clear skies filled with stars, yet the right side of the figure has a street light that shines light in all directions, affecting all organisms in a close proximity and obstructing the view of the night sky.

Figure 1: Ambient night conditions compared to artificial nightlight

Natural light is crucial for all organisms, as it determines the time of day an animal is active, navigation for migratory species, provides environmental cues for reproduction timing, when and where an organism forages for food, and other critical behaviors. However, within the last 100 years, we have drastically changed the spatial, temporal, and spectral components of the habitats organisms have adapted to, as well as disrupt biological cycles and patterns across a wide range of taxonomic groups (Gaston et al. 2013). Today we will focus on how artificial nightlight affects the physiology of animals, specifically amphibians.

Amphibians include three classes of animals: salamanders, frogs and toads, and caecilians. They are an effective group of model animals for studying artificial nightlight for three main reasons (Wise 2007).

         1)  Most of these species are nocturnal (active during the night), which means they will be highly impacted by the timing and length of exposure to artificial nightlight. This is denoted as changes in the photoperiod.

      2)   Amphibians have widespread distributions and occupy important roles in both terrestrial and aquatic systems.

      3)   These animals are sensitive to environmental pressures and are therefore a good indicator species, as they are typically the first taxonomic group to show declines in degrading habitats.

We will look at studies that have investigated two components of animal physiology: developmental growth and melatonin production.

Development

            Development focuses on how an organism transitions through different stages over time, and in amphibians, this change is seen in metamorphism. This change is most easily recognized in tadpoles transforming into adult frogs. In an unpublished study summarized by Wise (2007), a team of researchers exposed African clawed frog tadpoles (Xenopus laevis) of the same age to a 12 L (12 hours of light): 12 D (12 hours of darkness) period with varying levels of light intensity and recorded their progression in metamorphosis. They found the tadpoles exposed to the darkest treatments were more fully transformed than tadpoles in greater light intensity treatments (see Figure 2). What’s important to note is even small amounts of artificial light may delay metamorphosis, which is especially harmful to species that depend on temporarily existing pools and are exposed to drying out and predation if they cannot transform into adults on time.

       


     On the other hand, a study conducted by Eichler and Gray (1976) exposed Northern Leopard tadpoles (Rana pipiens) to treatments of constant lighting, diurnal lighting, and constant darkness, and found tadpoles accelerated in development in the constant lighting treatment. If amphibians are metamorphosing too quickly, they may miss important habitat signals, such as prey emergence and seasonal environmental cues, which may conversely have negative effects on their well-being.

Melatonin production

            Melatonin is a hormone that is involved with photoperiodic behavior and physiology, as well as the synchronization in circadian and seasonal rhythms (Vanecek 1998). Additionally, a reduction in melatonin reduces an animal’s tolerance to high temperatures as well as their ability to lower their body temperature (Vanecek 1998; Perry et al. 2008). For all species, melatonin production is increased at night/ longer dark periods, regardless if they are active during the day or night (Vanecek 1998). The change in melatonin production over time can be compared during the summer and winter months, as summer has longer photoperiods and melatonin production is short, while winter has shorter photoperiods and melatonin production is long (Figure 3). An experimental study by Rawding and Hutchinson (1992) were able to show this pattern in the common mudpuppy (Necturus maculosus), as adults exposed to a 12L:12D photoperiod produced a greater amount of melatonin during the dark treatment than the light treatment for both regular light hours (8 AM- 8 PM) and reversed light hours (6 PM- 6 AM).

Figure 3. Comparisons of melatonin production during the summer (16L: 8D photoperiod denoted by circles) and winter (8L: 16D photoperiod denoted by squares). 

Furthermore, a study by Whiteford and Hutchinson (1965) looked at different rates of respiration in the spotted salamander (Ambystoma maculatum) and were able to show animals in the longer photoperiod (16L:8D) had significantly higher rates of oxygen consumption (Figure 4). Therefore, it is reasonable to predict that a decreased amount of melatonin production and an increased amount of oxygen consumption in a longer photoperiod will contribute to a higher metabolic rate (Perry et al. 2008). This is problematic for animals experiencing situations of low food availability or periods of high energy demand, such as egg production or drought, as these animals are physiologically working harder— and will need more energy assimilation— to survive.

Figure 4. Comparisons of different types oxygen consumption of spotted salamanders. Note that the range of consumption for longer photoperiods (16L) are greater than oxygen consumption in shorter photoperiods (8L). 

Implications of artificial nightlight

Longer photoperiods created by artificial nightlight affects many more aspects of animal physiology besides the two mentioned here, such as reduced sperm production, reduction of gene expressions that regulate physiological processes, become out of sync with seasonal changes, and may be unable to adapt to climate change (Perry et al. 2008; Gaston et al. 2013). The amount of research completed on amphibians is severely lacking, as most studies focus on habitat loss, water and air pollution, and pay no regard to light pollution (Perry et al. 2008). Future long-term studies are needed to understand the chronic impact of artificial nightlight on amphibians in a field setting, which will provide insight into how land managers and city planners can alleviate their stress on wildlife.

In the meantime, there are ways you can contribute to helping decrease the amount of light you produce. The easiest way to decrease light pollution is to turn off any lights that are not necessary, which is also an energy saving tactic. Additionally, you could replace outside lightbulbs with red or yellow light, as the spectra these colors emit are less invasive for wildlife species. By being conscious of how we interact with the environment, we can make a positive difference for the amazing little critters who also call this world “home.”

References:

Eichler, V.B. and L.S. Gray. 1976. The influence of environmental lighting on the growth and prometamorphic development of larval Rana pipiens. Development, Growth & Differentiation 18(2):177-182.

Gaston, K.J., Bennie, J., Davies, T.W. and J. Hopkins. 2013. The ecological impacts of nighttime light pollution: a mechanistic appraisal. Biological reviews 88(4): 912-927.

Perry, G., Buchanan, B.W., Fisher, R.N., Salmon, M. and S.E. Wise. 2008. Effects of artificial night lighting on urban reptiles and amphibians. Urban Herpetology. Herpetological Conservation, 3.

Rawding, R.S. and Hutchison, V.H., 1992. Influence of temperature and photoperiod on plasma melatonin in the mudpuppy, Necturus maculosus. General and comparative endocrinology 88(3): 364-374.

Vanecek, J., 1998. Cellular mechanisms of melatonin action. Physiological reviews 78(3): 687-721.

Whitford, W.G. and V.H. Hutchison. 1965. Effect of photoperiod on pulmonary and cutaneous respiration in the spotted salamander, Ambystoma maculatum. Copeia (1): 53-58.

Wise, S. 2007. Studying the ecological impacts of light pollution on wildlife: amphibians as models. StarLight: a Common Heritage, C. Marın and J. Jafari, eds. (Canary Islands, Spain: StarLight Initiative La Palma Biosphere Reserve, Instituto De Astrofısica De Canarias, Government of The Canary Islands, Spanish Ministry of The Environment, UNESCO-MaB.), pp.107-116.

Image sources:

https://sites.psu.edu/natercl/2016/01/25/125/