New York City’s nickname, the “city that never sleeps,” refers to the constant light show throughout the city. Usually we think of this as representative of the ever bustling human activity through all hours, but local animals are forced to follow this motto as well. Artificial light at night (also known as ALAN) is an understudied form of human pollution that is ever-present in highly populated areas. A large number of animals rely on natural sunlight and the light of stars to inform their behavior and migration. When we introduce artificial light into their habitats, we can confuse and alter their light dependent behaviors in very harmful ways. Marine animals are particularly dependent on light and stars as informative landmarks because there are few other physical ones to be referred to in the ocean depths.
The pattern of high intensity sunlight and lower intensity celestial light informs a lot of animals’ natural daily patterns in behavior. These patterns follow a 24 hour cycle and repeat with each rotation of the earth. Humans, for example, sleep at night and are more active during the day on a 24 hour rotation. This pattern is called a circadian rhythm and can be found in a wide variety of plants, animals, and even bacteria. Each organisms’ circadian clock adjusts their behavior to match available light levels. When exposed to ALAN, the animals’ circadian clocks can register this as “daylight” and will adjust circadian rhythms to be in sync with this alternate light pattern.
A research team led by Duarte et al. (2019) wanted to study how ALAN affects the behavior and circadian rhythms of sandy beach isopods. These isopods look like sea roly polies that live in shallow tidal areas. Isopods bury themselves into the sand during the day and emerge at night to forage for food. Other studies of water invertebrates have shown that changes to light intensity inform their emergence behavior. This means that as luminosity decreases with the setting sun and just the celestial light from stars is left, they travel towards the dimmer light to emerge from the sand and later re-bury themselves as light levels increase at sunrise. The same applies to sandy beach isopods which emerge at night because of the ideal conditions that follow, such as higher humidity levels and lower temperatures than those found during the day.
The research team started by examining changes to this light dependent activity cycle in the field. The research team found a location with no artificial lighting and placed a number of traps in the sand to catch emerging isopods. Groups were split into control with only the natural celestial light and the ALAN group which was exposed to an LED on the shoreline (light intensity was determined based on average local luminosity of light pollution). Traps were then placed at different distances from the light and isopods were collected on an hourly basis from the traps throughout a 24-hour period. The traps showed that ALAN exposed areas did not have any isopod emergence unless at a much farther distance away from the light source. In contrast, hundreds more isopods were collected and emerged in the non-lit areas and had similar isopod emergence levels at the farther distances. These results indicated that isopod emergence and activity were very significantly decreased when exposed to artificial light at night.
This phenomenon was tested in the lab after Durante et al. decided to study how the physical locomotive activity of these isopods was altered in the presence of ALAN. Isopods were placed in boxes with infrared lights passing through to record their emergence and movement throughout a 24 hour period. Half of the boxes were exposed to natural light patterns while the other half was exposed to ALAN. The research team observed that isopods with natural lighting had normal circadian rhythms and were highly active during dark hours (night) and decreased activity leading up to daylight hours. The LED light exposed group, on the other hand, were much less active during dark hours and sometimes even became active during the daylight hours. This quickly disrupted exposed isopods’ original circadian rhythms and reduced their overall activity. These lab results can help explain the relative lack of active isopods found in the field portion of the experiment. With their locomotion slowed down or stopped completely in response to ALAN, few of them would have emerged and they would not get caught in the sand traps. This is a major alteration to their natural behavior, and can really harm their ability to survive.
The destructive effects ALAN had in this experiment is not limited to isopods. Durante et al. demonstrated how untimely artificial light can throw off an entire biological system by confusing their circadian clocks. The variety of organisms reliant on these circadian systems may show similar responses to artificial light as the isopods, a scary thought for the health of organisms globally. As human development expands, especially along the shoreline, artificial light is becoming a bigger pollutant in organisms’ habitats. We need to better understand the implications of this forgotten form of pollution in order to properly protect the health of local wildlife.
References:
Duarte et al., 2019 Cristian Duarte, Diego Quintanilla-Ahumada, Cristobal Anguita, Patricio H. Manríquez, Stephen Widdicombe, José Pulgar, Eduardo A. Silva-Rodríguez, Cristian Miranda, Karen Manríquez, Pedro A. Quijón, Artificial light pollution at night (ALAN) disrupts the distribution and circadian rhythm of a sandy beach isopod, Environmental Pollution, Volume 248 (2019) 565-573, ISSN 0269-7491, https://doi.org/10.1016/j.envpol.2019.02.037.
Navarro-Barranco et al., 2015 Carlos Navarro-Barranco & Lauren Hughes, Effects of light pollution on the emergent fauna of shallow marine ecosystems: Amphipods as a case study, Marine Pollution Bulletin, Volume 94, Issues 1-2 (2015), Pages 235-240, ISSN 0025-326X, https://doi.org/10.1016/j.marpolbul.2015.02.023.
