As the Sun sets in Chicago, the city begins to bustle with nightlife—and it’s not just from partying humans. Cats, coyotes, possums, raccoons, rats, and skunks all come out of their urban homes to hunt, mate, and roam the city. But a recent study shows that these behaviors can be altered by artificial lighting—street lamps, flood lights, and illumination around homes and businesses.

The study, led by researchers from Northeastern Illinois University, showed that the city’s nocturnal animals roamed less and were less active as nighttime light levels increased. Researchers started seeing significant changes in animal behavior in areas with lighting as dim as 6 lux, a unit of measurement that describes the amount of light falling on the surface. For reference, 6 lux is slightly dimmer than Earth’s surface at twilight; typical kitchen lighting is around 500 lux.

“If larger nocturnal animals are less active around the city at night, their movement patterns can be altered temporally and spatially,” said Aaron Schirmer, lead author of the paper and a biology professor at Northeastern Illinois University. “That would have the potential to affect the food web in ways that we might not fully understand yet.”

While other research has shown a significant increase in artificial lighting around natural and semi-natural ecosystems, Schirmer and colleagues sought to measure the actual effects on animal behavior through a series of lab and field observations. First, the team mapped light pollution across the Chicago metropolitan area. They traveled around the city measuring light intensity in various areas using handheld light meters. Then they combined and calibrated the ground measurements with photographs shot from the International Space Station (ISS) by astronauts. The image above shows a photo of Chicago shot from ISS on October 9, 2013.

Once the researchers measured the range of light levels in the city, they recreated the conditions in a series of lab experiments with mice. By slowly increasing the light exposure to correspond to the common nighttime light levels found in Chicago, they found that locomotor activity (in this case, a mouse running on a wheel) decreased as light levels increased. The light levels in the tests ranged from less than 0.01 lux (a quarter Moon) to 121 lux (a very cloudy day).

The team then took the collected data and ran an analysis to learn when animal behavior started to change. After running statistical models, researchers consistently observed behavior changes starting at 6 lux—brighter than a full Moon (0.108 lux) and dimmer than a twilight glow (10.8 lux). “As far as nighttime lights go, 6 lux is still fairly bright,” Schirmer said. “I would expect some animals would have behavior changes even below that threshold.”

Schirmer and colleagues compared their lab results with observations of wildlife around the city. They used camera trap data from the Lincoln Park Zoo, which has collected more than 1 million photos of Chicago-area wildlife over the past decade. As with the lab mice, the opossums, raccoons, skunks, and other animals of the city showed a decrease in movement when exposed to high levels of city light. Nocturnal species demonstrated 19.6 percent more activity in darker locations than in brighter areas. Again, the researchers observed behavioral changes starting in areas approaching 6 lux.

Using this information, the team mapped where electric light pollution in Chicago is likely to have the largest effect on wildlife. The image above shows the green spaces in Chicago and whether they are above or below light levels of 6 lux. Land cover data come from the Chicago Metropolitan Agency of Planning. To determine the lux levels, the researchers used the photos from the ISS, measuring the value of each pixel to determine which areas were above and below 6 lux.

The researchers found that about 36 percent of the green space around Chicago is regularly above 6 lux (shown on the map in purple). That reduction in nighttime darkness could significantly affect wildlife behavior. Schirmer also noted that city lights affect even the larger green areas, subdividing them into darker and brighter sections and shrinking the size of suitable habitats for animals.

“We want this study to raise awareness of the impact of electric light pollution on wildlife,” said Schirmer. “From an urban planning perspective, it is important to think about ways in which nighttime light impacts animals and to find creative solutions that work for people and the wildlife.”

NASA Earth Observatory image by Joshua Stevens, using data courtesy of Schirmer, A.E., et al. (2019). Astronaut photograph ISS037-E-8303 was acquired on October 9, 2013, with a Nikon D3S digital camera using a 50 millimeter lens and is provided by the ISS Crew Earth Observations Facility and the Earth Science and Remote Sensing Unit, Johnson Space Center. The image was taken by a member of the Expedition 37 crew. The image has been cropped and enhanced to improve contrast, and lens artifacts have been removed. Story by Kasha Patel.