Many studies document artificial lighting’s contribution to hatchling turtle and avian mortality due to disorientation 15, 16, 17, 18, 19, 20. For wildlife, light influences navigation, activity, and reproduction. It is associated with numerous biological and ecological impacts, potentially acting as a novel evolutionary selection pressure 14. Light is a powerful and consistent environmental signal that communicates important daily and seasonal changes in an organism’s environment 12, 13. Despite excess light becoming an important component of global environmental change 1, 2, 9, we are just beginning to understand photopollution’s effects 9. Urban wildlife faces numerous novel stressors, ranging from roads and traffic, to new competitors and predators, to direct interactions with humans 11, as well as altered lightscapes. Improved understanding of how urban wildlife reacts to modified light levels can support management and conservation.
Similarly, while urban wildlife has garnered increased attention 10, studies investigating the effects of altered lighting on wildlife in urban landscapes, particularly on terrestrial mammals, are relatively scarce. Population growth, economic development, and urbanization have increased the density and extent of artificial lights in natural, seminatural, and urban settings 1, 2, 3, leading to concerns about “ecological light pollution 4.” Though ecologists, astronomers, and other interested parties demonstrate that increases in artificial nighttime light represent a significant component of global change 5, 6, a general lack of public concern 7, 8 has resulted in limited research to inform antiphotopollution policies 9. Our findings provide evidence that artificial lighting patterns may influence wildlife behavior at a broad scale throughout urban areas, and should be considered in urban habitat planning. Based on this measure, we found that as much as 36% of the greenspace in the city is in areas illuminated at levels greater than or equal to those at which we observe behavioral differences in the field and in the laboratory. Using these results, we mapped areas across Chicago exposed to estimated illumination levels above the value associated with statistically significant behavioral changes. We then used camera trap and high-altitude photographic data to compare our field and laboratory observations, finding activity pattern changes in the field consistent with laboratory observations. A simplified sample of these levels was replicated in the laboratory to identify light levels at which C57BL/6J mice exhibited altered circadian activity patterns. We began by applying a Gaussian finite mixture model to 998 sampled illumination levels around Chicago to identify clusters of light levels. Combining laboratory and field techniques, we identified behaviorally relevant levels of nighttime light and mapped the extent of these light levels across the city of Chicago. Artificial nighttime lights have important behavioral and ecological effects on wildlife.