NIGHT SKY AND DARK ENVIRONMENTS: BEST MANAGEMENT PRACTICES FOR ARTIFICIAL LIGHT AT NIGHT ON BLM-MANAGED LANDS TECHNICAL NOTE 457 25 Finally, the direct effects to living organisms may cause changes in population sizes and community composition (Sanders and Gaston 2018; Desouhant et al. 2019). These effects in turn may affect the nonliving parts of ecosystems by changing flows of energy, nutrients, and water. 3.4.1 Temporal Niche Partitioning in Animals Bright sunlight is approximately 100 million times brighter than a moonless night (Schroer and Hölker 2016). Animals have highly developed vision systems and behaviors to function in a range of lighting conditions, and some species are active only within a particular range or show a strong preference for a particular level of light. Animals in the same environment may be adapted to different lighting levels and will be active at different times as a result. This is an example of niche partitioning, the process by which competing species use the environment differently to help them coexist. Under natural lighting conditions, animals that are diurnal (active during the day) become inactive as darkness falls. Nocturnal animals become active only after darkness falls. This is temporal niche partitioning, the partitioning of various species’ activity periods over time based on varying levels of light. Changing the natural light levels can change interactions between species. Artificial light at night can cause diurnal animals or crepuscular animals (those active during twilight) to extend their period of activity into the nighttime hours (sometimes referred to as exploiting the “night light niche”) (Garber 1978) and may also affect the activities of nocturnal animals. These changes in activity periods can disrupt interactions between species. For example, these changes can result in competition between diurnal/crepuscular and nocturnal species that would not normally encounter one another. Diurnal animals that may extend their activity in the presence of artificial light at night include anoles, skinks, snakes, toads, tree frogs (Garber 1978; Perry et al. 2008), fish (Keenan et al. 2007), and birds (Amichai and Kronfeld-Schor 2019; Santos et al. 2009; Nordt and Klenke 2013). Artificial light at night may affect predator-prey relationships by causing some predators to hunt at times when it would otherwise be too dark. Visually oriented predators (those that rely primarily on vision to hunt) may be able to extend their hunting periods to times when they would normally be inactive. Prey species that forage only under specific lighting conditions may delay their activities or may avoid lit areas to avoid exposure to predators. As a result, artificial light may change predator-prey activities and reduce foraging opportunities, both of which can influence survival at the individual and population levels. For example, research shows that exposure to artificial light can reduce or delay foraging opportunities in small desert mammals (Brown et al. 1988; Kotler 1984; Shier et al. 2020) and certain bats (Boldogh et al. 2007; Stone et al. 2009). Certain species of fireflies are only active under particular light levels, which separates the activity periods among species to reduce competition. This partitioning may be disrupted in the presence of artificial light at night (Lloyd 2006). Other species, including certain insects and frogs, only mate during the darkest part of the night, presumably to avoid predators (Buchanan 2006). A lesser nighthawk hunting several hours after twilight to take advantage of insects drawn to bright lights.
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