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Simons, A.L.; Martin, K.L.M., and Longcore, T., 2022. Determining the effects of artificial light at night on the distributions of Western Snowy Plovers (Charadrius nivosus nivosus) and California Grunion (Leuresthes tenuis) in southern California. Journal of Coastal Research, 38(2), 302–309. Coconut Creek (Florida), ISSN 0749-0208. This study covers the role of exposure to artificial light at night (ALAN) in shaping the spatial distributions of two species of conservation concern, roosting sites of the Western Snowy Plover and locations of California Grunion spawning runs, along the coast of southern California. Observational data on plover and grunions, derived from community science sources, were obtained along with remotely sensed environmental measurements along the coast of southern California. The study area comprises a 1.5 km wide coastal strip, bounded by the mean low-tide line, and stretching from 10 km north of the northern Ventura County line to 10 km south of the southern Orange County line. These data were used as inputs within three species distribution models: a generalized linear model, Maxent, and random forest. Exposure to ALAN was based on a ground-verified model of night sky illuminance. In the highest performing models, which used random forest modeling, exposure to ALAN was the most important environmental factor influencing distribution of grunion runs and second-most important factor for plover roosts. Significant declines were found in the likelihood of plovers roosting in locations where exposure to ALAN exceeded illuminance levels equivalent to that produced by approximately one half a full moon and for grunion spawning at one full moon. Disruption of behaviors related to reproduction, roosting, and spawning associated with elevated levels of ALAN are likely a result of increased predation risk in illuminated coastal areas. With evidence of ALAN providing significant ecological disturbances to these two managed species, it is therefore recommended that control of nighttime illumination be used, even at naturalistic intensities, to mitigate disturbances to critical reproductive coastal habitats and potentially other environments. [ABSTRACT FROM AUTHOR]