Light Pollution's Impact On Bats: Understanding The Threat

Light pollution, or artificial light at night (ALAN), is a growing environmental concern, affecting 50% of the United States and 90% of Europe. It has far-reaching effects on nocturnal wildlife, especially bats, by disrupting their navigation, roosting, and feeding. Bats are nocturnal and have evolved to be active in the dark, so artificial light can be extremely disturbing to them. The impact of light pollution on bats includes delayed emergence from roosts, changes in feeding behaviour, avoidance of lit areas, and exposure to predators. The lighting near their travel routes can also increase their flight time and energy expenditure. Certain types of lights, such as high-intensity and white lights, are more harmful to bats, while red lights are less likely to affect them. Preserving dark habitats and corridors is crucial for bats, and even small amounts of lighting can significantly impact their behaviour and habitat availability.

Light pollution disrupts bat navigation, roosting and feeding

Light pollution, or artificial light at night (ALAN), is a significant and growing threat to bats, disrupting their navigation, roosting, and feeding behaviours.

Bats are nocturnal animals that have evolved to be active at night, in part to avoid predators such as birds of prey. As such, artificial lighting can be extremely disturbing to bats, impacting their natural behaviours and survival. Lighting near bat roosts, access points, and foraging pathways can cause a range of problems for bats. For example, artificial light can delay or prevent bats from leaving their roosts at sunset, reducing their foraging time and causing them to miss the peak time of insect abundance. This can have serious consequences for their survival and the growth rates of their young, as all bats in the UK feed on insects.

In some cases, artificial lighting can lead to bats abandoning their roosts or becoming entombed within them, as they are less inclined to leave when the entrance is lit. This can be considered a breach of legislation, as bats and their roosts are protected by law in the UK. Lighting can also affect the feeding behaviour of bats, with slower-flying species avoiding illuminated areas and losing out on foraging grounds to faster-flying species. These slower-flying species are then forced to use poorer quality foraging sites, further impacting their survival.

Additionally, lighting near bat travel routes can increase their flight time and energy expenditure by cutting them off from food and water sources. Artificial lights can also attract the insects that bats hunt, exposing them to predators when leaving their roosts. Some bats avoid lights altogether, preventing them from crossing or feeding at sites with artificial lighting.

High-intensity, elevated, and upward-facing lights have the strongest effect on bats, and white lighting is harmful to a wide range of bat species. To minimise the impact on bats, lighting near their habitats should be avoided, particularly at roosts, feeding sites, water sources, and connecting corridors. If lighting is necessary, low-intensity red or amber lighting installed at least 50 meters from bat habitats is recommended.

Artificial light can cause bats to abandon their roosts

Bats emerge at sunset to feed on insects. Artificial light near their roosts can delay or prevent them from leaving, reducing their foraging time and causing them to miss the peak time of insect abundance. This can have serious consequences for their survival and the growth rates of their young.

In some cases, bats may abandon their roosts altogether if the entrance is illuminated. They become less inclined to leave their roosts when lights are present, and this can lead to entombment. Disturbance of bat roosts is also likely to be considered a breach of legislation, as bats and their roosts are protected by law in many places.

Artificial light can also affect the feeding behaviour of bats. Slower-flying species tend to avoid illuminated areas, forcing them to use poorer quality foraging sites and losing out on prey that are attracted to the surrounding lit areas. This "vacuum effect" results in reduced prey availability for these slower-flying bats.

Additionally, lighting near bat travel routes can increase their flight time and energy expenditure by disrupting their commuting and foraging patterns. Bats may be cut off from their food and water sources, further impacting their survival and reproductive success.

To mitigate these issues, it is essential to minimise artificial lighting near bat habitats. Preserving dark roosting and feeding sites, connected by dark corridors, is crucial for the well-being of bat populations.

Lighting near bat travel routes can increase their flight time and energy use

Lighting near bat travel routes can have a significant impact on bats, increasing their flight time and energy use. Bats are nocturnal animals that have evolved to be active at night, partly to avoid predators during the day. Artificial lighting near their travel routes can be extremely disturbing, altering their natural behaviours and exposing them to danger.

Bats rely on darkness for navigation and hunting, using echolocation to navigate and hunt insects. When their travel routes are illuminated, bats can become disoriented and confused. The light can also attract insects, drawing bats towards busy roads or exposing them to predators. This can lead to increased flight times as they deviate from their usual routes to avoid the light.

The presence of artificial lighting near their travel corridors can also cause bats to abandon their roosts or become entombed within them. This behaviour can have serious consequences for their survival and the growth rates of their young. It can also affect their feeding behaviour, with slower-flying species avoiding illuminated areas and losing access to their usual foraging grounds.

The impact of lighting on bat travel routes can vary depending on the species. For example, a study on North American bat species found that Little Brown Bats were significantly impacted by lighting at a distance of 75 meters, with a reduction in their foraging activity to only 43% of their usual levels. On the other hand, Big Brown Bats were less affected, showing no difference in presence or absence at any distance.

To reduce the impact of lighting on bat travel routes, it is important to minimise artificial lighting near known bat habitats and travel corridors. Preserving dark corridors and natural roosting sites is vital for bats. Where lighting is necessary, the use of low-intensity red or amber lighting, installed at a safe distance from bat habitats, can minimise the disturbance to these creatures of the night.

Some lights are more harmful to bats than others

Light pollution is a rapidly growing threat to nocturnal wildlife around the world, particularly for bats. Artificial light falling on or close to a bat roost can have a significant impact on their behaviour and survival. Some lights are more harmful to bats than others, and it is important to understand these differences to minimise the negative impact on bats and their ecosystems.

High-intensity, elevated, and upward-facing lights have the strongest effect on bats. These types of lights can disrupt bat behaviour the most, affecting their navigation, roosting, and feeding patterns. White lighting, in particular, is harmful to a wide range of bat species. The switch to LED lights in streetlamps, which do not emit UV wavelengths, has reduced the number of insects attracted to streetlights. This has had a knock-on effect on the bats that feed on these insects, with some species being affected more than others.

Research has shown that light-averse North American bat species, such as the little brown bat and the big brown bat, will go to great lengths to avoid artificial light. These bats were detected on significantly fewer nights when residential LED floodlights were present, even at distances of 75 meters or more. The presence of artificial light reduced their foraging activity, with little brown bats averaging only 43% of their usual activity levels.

On the other hand, some bat species are less affected by artificial light and may even benefit from the switch to LED lights. Species such as the Common pipistrelle were found to be relatively unaffected by the lights, with only a slight reduction in their activity around streetlights. Other species, like the Nathusius pipistrelle, showed no change in behaviour and were just as active around the lights as before.

To minimise the harm caused by artificial lighting, it is recommended to preserve naturally dark roosting and feeding sites for bats, connected by dark corridors. If lighting is necessary, low-intensity red or amber lighting is the least disruptive to bats and should be installed at least 50 meters away from bat habitats. Pointing lights downwards and using shielding can also help to reduce the impact on bats.

Trees can help reduce the impact of light pollution on bats

Light pollution, or artificial light at night (ALAN), is a significant threat to nocturnal wildlife, especially bats. Artificial light disrupts their navigation, roosting, and feeding, and can even expose them to predators.

The presence of trees can also contribute to the preservation of naturally dark areas, which are crucial for bats. By providing shade and blocking direct artificial light, trees can help maintain darker conditions within their immediate surroundings. This is particularly beneficial in urban areas, where the presence of trees can mitigate the impact of streetlights and other artificial light sources on bat habitats.

Furthermore, trees offer roosting and resting sites for bats, providing them with shelter and safety. Bats are less likely to be disturbed by artificial light when they have access to safe and protected roosting sites within tree cavities or foliage. The availability of suitable roosting sites can also help reduce the impact of light pollution on bat behavior, as bats may be less inclined to abandon their roosts due to artificial lighting.

In conclusion, trees play a vital role in mitigating the effects of light pollution on bats. They provide physical barriers that block or reduce artificial light, create safe travel corridors, preserve dark conditions, and offer essential roosting sites. By incorporating trees into landscape planning and design, we can help reduce the negative impact of light pollution on bat populations and support their overall conservation.

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