Bat responses to artificial light at night are complex, varying by species, foraging strategy, and light type and intensity. While some bats appear drawn to lights, this is often an indirect result of insects gathering there, creating a concentrated food supply. This interaction significantly impacts bat ecology and conservation.
How Bats Perceive Their World
Bats navigate their nocturnal environment using both echolocation and vision. Echolocation involves emitting high-frequency sound waves and interpreting the echoes that return from objects. This allows bats to construct a detailed “sound map” of their environment, determining an object’s distance, size, shape, and movement in darkness. Different bat species utilize specific frequency ranges for their calls, adapting to their habitats and prey.
While echolocation is a primary tool, bats are not blind. Their eyes are highly sensitive to low-light conditions, with more rod cells for enhanced night vision. Many bat species also possess some form of color vision, often including sensitivity to ultraviolet (UV) light, which can be beneficial for detecting prey or navigating in dim light. However, their visual acuity is generally less developed than human vision, though some fruit-eating species have excellent color vision.
Diverse Bat Responses to Light
Bat responses to artificial light are largely dictated by their foraging strategies and flight characteristics. Fast-flying, open-space foragers, such as species from the genera Pipistrellus, Nyctalus, and Eptesicus, may appear attracted to artificial lights. This attraction is primarily an opportunistic response to insects accumulating around light sources, providing a concentrated food supply. These agile bats exploit this readily available prey, sometimes flying between streetlights to hunt.
In contrast, slower-flying, clutter-adapted bats, including many Myotis and Rhinolophus species, consistently avoid illuminated areas. These species are more vulnerable to visually-hunting predators, such as owls, in lit environments. Artificial light increases their perceived predation risk, leading them to shun lit commuting routes and foraging grounds. This light aversion forces them to use darker, potentially less optimal, areas for foraging and travel, which can reduce their feeding efficiency. White and green light are generally avoided by light-averse species, while red light has less impact.
Broader Impacts of Artificial Light
Widespread artificial light at night (ALAN) has significant ecological consequences for bat populations. Light pollution can disrupt the natural timing of bat emergence from roosts, causing delays and shortening their foraging time. This impacts their nutrition and the growth rates of their young, as they may miss the peak abundance of insect prey, which often occurs just after dusk.
Artificial lighting can also fragment bat habitats by creating barriers along their commuting routes. This forces light-averse species to take longer, more energetically costly detours or abandon routes entirely, leading to reduced access to foraging grounds and water sources. Increased visibility in lit areas raises the risk of predation for bats, particularly slower-flying species. Overall, ALAN can alter species composition in bat communities, favoring light-tolerant species while negatively impacting those that rely on dark conditions, thereby threatening bat diversity.