Bye-bye dark sky: Is light pollution costing us more than just the night-time?

Hein van Grouw, our Senior Curator of Birds, and UK Biodiversity Training Manager Steph Holt, reveal the impacts of light pollution on British wildlife and provide a few tips for reclaiming your slice of the night sky.

What is light pollution?

Also known as photo pollution, light pollution is the presence of artificial light in the night environment. It’s an anthropogenic pollutant – meaning it originates in human activity – that can be disruptive and harmful to both wildlife and humans.

In well-lit areas such as towns and cities, sky glow can be enough to entirely obscure our view of the stars. A 2016 study found that 77% of the UK are unable to see the Milky Way.

But there’s a lot more to light pollution than an obstructed view of the stars.

How bad can it be?

“The rate at which artificial lighting has increased across the globe is exponential. If you look at satellite footage of street lighting and how that’s changed over the past two decades, it’s almost frightening to watch,” Steph explains.

“Year-on-year global artificial lighting has increased by about 10% and it’s not just the amount of light that’s changing – the location and type of lighting matters to wildlife as well. Much of that light is directed or reflected upwards and is clearly visible from space.”

Research undertaken in association with UNESCO found that light pollution visible from space has reduced by 28% since the early 1990s, but sky glow is only part of the problem. Globally, between 1992 and 2009 there was a 39% increase in dimly lit areas and a 19% increase in brightly lit areas.

“There’s very little control over the nature and use of artificial lighting.”

“It’s being looked at in highly developed areas like the UK and USA, but the driver to reduce light pollution isn’t coming from the need to protect wildlife, it’s more about energy efficiency and keeping costs down,” Steph says.

It’s not just about how much light there is but what type of light is being produced.

In some cases, energy-efficient lighting such as blue-rich LEDs can be just as bad, or even worse, for wildlife as less energy efficient options.

“Some types of energy efficient lighting produce more UV light than older styles of lighting. Humans can’t see it so it’s of no use to us but can disturb wildlife.”

How is light pollution affecting British wildlife?

The UK is home to 18 species of bat, all of them protected. Each species also responds differently to light.

“The thing that they do have in common is very sensitive eyes that are very efficient in low-light situations,” says Steph.

Bats navigate thanks to a combination of powerful night vision and echolocation, switching between the two as needed. In low-light situations they may stop echolocating to conserve energy and disguise themselves from some types of prey that have adapted to hear their echolocation.

For bats, encountering an artificially lit area is like staring into car headlights on full beam. If they suddenly encounter something glary or reflective they might not see the object in their path, potentially leading to impacts with obstacles.

Interrupted commutes

Some types of bats, such as brown long-eared, greater horseshoe and Natterer’s bats, are used to operating in complete darkness very late at night.

“They use echolocation calls to build up a picture of their landscape and commute through that landscape from roost to preferred feeding site,” says Steph.

“Bats fly at night for a pretty specific reason and that’s predator avoidance. Their natural predators are mostly avian, which with the exception of owls which will predate bats, mostly come out during the day.”

Many bats will avoid lit areas entirely, as they associate them with the risk of predators.

“If you create a long, linear arrangement of street lights, like a highway, it can be very difficult for bats to navigate, particularly if there’s a lot of upwards light spill,” Steph adds.

“A well-lit road can effectively become a permanent barrier to bats trying to commute to their preferred feeding sites – which can lead to the deterioration or loss of bat roosts.”

Bug buffets

Pipistrelle bat species have learned that their prey are attracted to artificial light and will in turn be drawn to well-lit bug buffets.

“This can offer them a short-term competitive advantage, but also adds a predation loading to certain kinds of insects,” explains Steph.

“Lights can draw insects away from their usual activities so they won’t be breeding as efficiently as they could, which can lead to drops in insect numbers that in turn bats need.”

Studies show artificial lighting leaves moths and their caterpillars more vulnerable to predation and distracts the adults from breeding. Moth numbers have fallen by a third in the UK since the late 1960s.

In addition, caterpillar numbers are 52% lower in hedgerows and verges under streetlights.

It’s a concern for any animal that relies on these insects as a food source.

For the birds

While bats might be having a hard time, some birds are really making the most of the extended operating hours offered in big cities.

Our bird expert Hein has observed how pigeons and their predators, peregrine falcons, are thriving in human-built environments.

“All pigeons, both feral and domestic, evolved from rock doves that lived on cliffs,” Hein explains.

“For hundreds of years now, city pigeons have been roosting successfully in cities. More recently, peregrine falcons have worked out it’s not a bad idea to do the same, moving to towns and cities and establishing themselves on the faces of buildings which are not unlike cliff faces.”

Working longer hours

Birds rely very heavily on their eyes to navigate and hunt, and artificial light can affect their sleeping and eating patterns.

It’s normal for birds to wind down with the light. At the end of the day, dimming light prompts birds to fill their stomachs and return to their roosts. It’s been observed that dimmed sunlight from an eclipse in the middle of the day can also trigger pigeons to eat.

Artificial light disrupts this. “In the evenings, when country pigeons would have gone to bed, city ones are often still very active,” says Hein.

“In the centre of cities, it’s almost never truly dark and peregrines have discovered that it might be easier to get a pigeon in town after hours.”

“Sky glow might not be as bright as daylight, but it does extend the time when they can hunt.”

The negative impacts for birds

At night, migrating birds sometimes crash into buildings because they’re attracted to artificial lights, but that’s not the only negative impact.

“Many animals experience changes in daily or seasonal rhythms in artificially lit areas as well as changes in their behaviour, from breeding to feeding. While some will be able to forage or hunt more, others will find themselves more vulnerable to predation. Some species actively avoid light, impacting their ability to use and cross urban areas,” Steph explains.

For example, tawny owls avoid lit places, possibly because the small mammals they prey on do too.

Tips to reduce light pollution at home

“There are things we can all do to minimise the impact of artificial lighting on biodiversity, without impacting our ability to work, travel or feel safe,” reassures Steph.

Prior to joining the Natural History Museum, Steph worked as an ecological consultant advising on lighting requirements for new developments. Here are her top tips based on the latest guidance on artificial lighting and wildlife from the Bat Conservation Trust.

1. Light only what you need

Avoid wasting light and energy by lighting only the features you need to see, such as steps and the lock on the front door. Avoid lighting up trees that could be home or feeding sites for bats or other nocturnal wildlife.

2. Get directional

Use cones or baffle lights to focus the light down onto the ground or on the object you need to see. This will limit your contribution to sky glow and improve star visibility in your area.

3. Avoid floor lights and bollards

Although ground lighting seems bat-friendly, bollards near reflective surfaces like concrete, water or pebble pathways can be a double whammy for wildlife. Instead, Steph advises placing lights higher and directed downwards.

4. Keep it warm

Most domestic outdoor lighting is now LED, so look for warm white options rather than the cool white and blue wavelengths of the light spectrum. Warm whites, oranges and reds are all better options for most wildlife as well as our own circadian rhythms.

Help local wildlife, see more stars and maybe even get better night’s sleep – all fairly compelling reasons to join the dark side.