NIGHT SKY AND DARK ENVIRONMENTS: BEST MANAGEMENT PRACTICES FOR ARTIFICIAL LIGHT AT NIGHT ON BLM-MANAGED LANDS TECHNICAL NOTE 457 29 to flowering plants by nighttime insect pollinators when there are artificial light sources, resulting in reduced fruit set. The reduced plant reproduction was predicted to negatively impact daytime pollinators that rely on the plants for food. 3.5.1.2 Effects on Mammals Dark environments that are artificially lit can affect mammals that are nocturnal or active both day and night. Available laboratory and field studies on various mammal species (e.g., bats, rodents) indicate that artificial light affects dark repair and recovery from melatonin suppression, circadian rhythm, and vision (Gaston et al. 2013). Artificial light at night may influence mammal reproduction, foraging, predator-prey relationships, and potentially spatial orientation. A laboratory study shows that exposure to very low levels of light at night suppressed melatonin production and promoted tumor growth in rats (Dauchy et al. 1997). The illuminance used in the study was well below that expected for the ground underneath a streetlight. Even lower levels of light at night can affect circadian rhythm in bats (Erkert 2004). Melatonin suppression is shown to negatively affect rat reproduction and links to weight gain in mammals (Tan et al. 2011). Captured field mice subjected to extended periods of light produced significantly fewer young than those subjected to natural light/dark cycles (Baker and Ranson 1932). Compared to other animals, nocturnal animals such as bats may be more affected by artificial light. More than half of the 46 bat species known to occur in the U.S. are found on lands managed by the BLM (BLM 2018). While some species of bats hunt in the vicinity of lights because of the abundance of insect prey found there (Rydell and Baagøe 1996), other species of bats will avoid lighted areas (Stone et al. 2015) or delay their normal period of foraging (Boldogh et al. 2007; Stone et al. 2009). Juvenile bats that colonize on buildings with lighting have shown reduced growth rates and fitness, due to delayed and slower emergence of bats at dusk. Desert rats and mice in the Mojave Desert have shown reduced foraging opportunities (Brown et al. 1988; Kotler 1984). A study by Bengsen et al. (2010) shows that various rats and other small mammals reduce their foraging intensity. Similar results were found in studies on the Pacific pocket mouse and the endangered San Bernardino kangaroo rat (Shier et al. 2020). Another study in the Mojave Desert (Bouskila 1995) shows that certain rodent species engaged in predator avoidance behaviors under conditions of brighter moonlight to avoid owls, which rely on vision for hunting. Perry et al. (2008) suggest that the same predator avoidance might occur around artificial lights. Bliss-Ketchum et al. (2016) found that artificial light reduced deer mouse and opossum usage of a bridge under-road passage structure. Several studies have examined the effects of artificial light at night on larger nondomesticated mammals. A study of mountain lions in California suggests that when exploring new habitat, they avoided urban glow and navigated toward the darkest horizon (Beier 1995). Bliss-Ketchum et al. (2016) found that Columbian black-tailed deer used an under road passage structure less when lit. Robert et al. (2015) show that reproductive timing in a wild population of wallabies was delayed by melatonin suppression as a result of exposure to artificial light at night. Desert rats and mice in the Mojave Desert have fewer foraging opportunities in artificially lit areas.
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