NIGHT SKY AND DARK ENVIRONMENTS: BEST MANAGEMENT PRACTICES FOR ARTIFICIAL LIGHT AT NIGHT ON BLM-MANAGED LANDS TECHNICAL NOTE 457 26 3.4.2 Role of Darkness in Biological Repair and Recovery Mammals require darkness for melatonin production. Melatonin affects sleep and wakefulness in animals, regulates circadian rhythms, and regulates biological repair mechanisms in both plants and animals. Many animal species need uninterrupted darkness to produce melatonin. Evidence shows that exposure to relatively low levels of artificial light can interfere with melatonin production, and this can negatively affect animals in several ways. For example, there are links between melatonin suppression and increased growth of tumors in rats (Blask et al. 2005). The reduction in melatonin production is greatest from blue light (Brainard et al. 2001). Exposure to artificial light can suppress immune system response in quail (Moore and Siopes 2000), cockerels (Kirby and Froman 1991), and rats (Oishi et al. 2006). Research also shows that exposure to artificial light can inhibit recovery of clover from ozone damage (Vollsnes et al. 2009). 3.4.3 Photoperiodism and Circadian Rhythms in Animals and Plants Light affects the timing of biochemical, physiological, and behavioral processes in plants and animals. Organisms that live in illuminated environments respond to the cycle of day and night, seasonal changes in day length, and the monthly lunar cycle. Photoperiodism is the physiological reaction of organisms to the length of day or night. An organism’s response to the 24-hour day/night cycle is referred to as its circadian rhythm. For many organisms, daylight of a certain length is an essential trigger for important events or behaviors. Day (or night) length often interacts with temperature to signal or initiate photoperiodic events. For example, leaf drop in certain plants is triggered by shortening day length and falling temperatures in autumn, while bud set in some plants is triggered by longer day length and warmer temperatures in the spring. In certain animals, photoperiod and temperature changes result in changes to fur color (e.g., in snowshoe hares) and feathers in birds. Other behaviors subject to photoperiod include hibernation, migration, and mating behavior in various animal species (Gaston et al. 2013). Even very low levels of light can affect organisms’ photoperiodic triggers and circadian rhythms. These can include changes to immune system response, metabolism, and stress levels in some animals (Bedrosian et al. 2011; Fonken et al. 2010; Zubidat et al. 2010); changes in reproductive behaviors with photoperiodic triggers, such as the timing of dawn chorus in some birds (Kempenaers et al. 2010); reproductive timing in mammals (Robert et al. 2015); and negative effects on seed set and leaf drop in plants (Bennie et al. 2016). Artificial light can affect egg laying in chickens (Burger 1949) and mating receptivity in juncos (Rowan 1925). Artificially lighting otherwise dark environments can influence seasonal niche partitioning. For example, if artificial light illuminates environments that would otherwise be in darkness, it could delay the departure of a particular species during seasonal fall migration (Tabor et al. 2004), and it could facilitate hunting by visually oriented predators of prey that are seasonally available during darker times of year (Jetz et al. 2003). 3.4.4 Visual Perception in Animals Artificially illuminating otherwise dark environments makes it possible for some animals to see better in lowlight conditions, and these animals may benefit more than others from the increased light levels, including predators that may have increased success at hunting (Santos et al. 2010). Predators tend to benefit over prey species in the presence of artificial light (Longcore and Rich 2016), though some prey species that cluster, such as flocking birds and schooling fish, may benefit because the extra light facilitates communal vigilance (Nightingale et al. 2006).
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