Are streetlamps really such a bright idea? Light pollution is affecting plant growth and and insect numbers
- Researchers from the University of Exeter simulated the effects of street lighting on artificial grassland plots containing insects
- They exposed them to two types of light spewed out by streetlamps
- Found amber light stopped a wild relative of peas flowering
- Plant is a key source of food for aphids and their numbers declined
- Other studies have shown that bats are less active around artificial lighting and that blackbirds reach sexual maturity faster
They may light our paths at night, but streetlamps and other sources of artificial light at night time affect the growth and flowering of plants as well as the number of insects, according to a new study.
Researchers have shown that light pollution can impact the natural environment in complex ways that may be hard to predict.
Given that cities are lit up 24-hours-a-day in many developed countries, there are fears that such ecological impacts may be widespread.
Researchers have shown that light pollution can impact the natural environment in complex ways that may be hard to predict. This image shows the two different types of light treatment used - ‘white' light similar to newer commercial LED street lighting systems and an 'amber' light simulating a type of sodium street lamp
Researchers from the University of Exeter simulated the effects of street lighting on artificial grassland plots containing a community of invertebrates at night.
They exposed them to two different types of light treatment - a ‘white' light similar to newer commercial LED street lighting systems and an 'amber' light simulating the type of sodium street lamp still found in much of the UK and further afield.
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The experiments investigated the effects of the lighting on the population density of a species of pea aphid.
The low intensity amber light was shown to prevent flowering in a wild relative of peas and beans called greater bird's foot trefoil, which is a key source of food for the pea aphid in grasslands and road verges, especially in mid-summer.
The study showed that low intensity amber light caused by street lighting in cities (a stock image of London at dusk is shown) prevented flowering in a wild relative of peas and beans called greater bird's foot trefoil, which is a key source of food for the pea aphid in grasslands and road verges, especially in mid summer
They found that the number of aphids declined under the light treatment in mid-August due to the limited amount of food available.
Professor Kevin Gaston, Director of the Environment and Sustainability Institute (ESI) said: ‘These are the first findings from major long-term experiments … and already reveal how profound the impacts of artificial night time lighting can be on even simple communities of organisms.’
Dr Jonathan Bennie added: ‘Our results suggest that by lighting up our night time environment we trigger complex effects on natural food webs.
‘While we are all aware that street lights often attract insects at night, we show that they may have more permanent, widespread impacts on wildlife and ecosystems.’
Another study carried out at the university, along with Bat Conservation Ireland, revealed that bats don’t like streetlights either and that activity was generally lower in street-lit areas than in dark locations with similar habitat.
The findings have important implications for conservation, overturning the previous assumption that common bats benefited from street-lights because they could feed on the insects that congregated around them.
The research, published in the journal Philosophical Transactions of the Royal Society B, found that the activity of soprano pipistrelle, noctule and serotine bats was similar or lower in areas with street lighting compared with dark areas.
An increase in the activity of the UK’s most common bat, the common pipistrelle, was only seen in locations where there was also a high amount of shelter from trees or hedgerows.
The only species for which lighting appeared favourable was Leisler’s bat, a species common in Ireland but rare in Britain.
Dr Fiona Mathews said: ‘People rarely see bats, and when they do it is usually because they are silhouetted by a light.
‘Because clouds of insects accumulate around lights, there has been an assumption that the bats were getting an easy lunch.
Another study carried out at the university, along with Bat Conservation Ireland, revealed that bats don’t like streetlights either and are less active around them. An increase in the activity of the UK’s most common bat, the common pipistrelle (stock image), was only seen in sheltered locations
‘What our work shows is that they are actually usually just as active, if not more so, in adjacent dark areas.
‘We already knew that lighting was bad news for rare species such as horseshoe bats.
‘Now we have demonstrated that, for the common species of vital importance to our ecosystem, lighting is not helpful. Over recent decades, the number of streetlights, and the brightness of lighting, has grown enormously.
‘We also use increasingly powerful lights to illuminate outdoor areas around our homes. We urgently need to reverse this trend.’
The research analysed large-scale surveys conducted in Britain and Ireland, involving more than 265,000 bat calls at over 600 locations.
The links between lighting and bats were explored at several spatial scales including car-surveys conducted by volunteers across Ireland, to shorter surveys conducted by bicycle, and detailed monitoring over multiple nights at specific sites.
Despite frequently being depicted as blind, bats have good eyesight that is adapted for low light conditions.
Dr Mathews explained: ‘When we walk out of a lit house into the dark, it takes a while for our eyes to adapt to the darkness.
‘The same is true in bats – they are dazzled by bright light and it takes time for their eyes to re-adjust. This could affect their ability to navigate.
‘In addition, it seems that their ability to hunt insects is reduced in the light. So although a bat may be seen flying round and round a streetlamp, it may actually be struggling to catch anything.’
ARE CITIES CREATING FREAK ECOSYSTEMS?
In Februrary, a study claimed that humans and the urban enviornments we've created have affected the evolution of a whole host of species - triggering the growth of enormous spiders, the shrinking of fish and the emergence of birds with no fear of man.
And the changes are happening faster than previously thought which ‘may have significant implications for ecological and human well-being,’ according to one expert.
Maria Alberti, of the University of Washington College of Built Environments, said that until recently, it was assumed that evolutionary change would take too long to affect ecological processes immediately.
‘We now have evidence that there is rapid evolution. These changes may affect the state of the environment now. This is what's called eco-evolutionary feedback,’ she said.
A new study claims that animals living in cities have evolved rapidly because of human influence. For example, Pacific salmon (b) have become smaller, blackbirds (d) have changed their migratory behaviour, Crespis sancta (e) disperses less seeds in built-up areas, earthworms (f) have become tolerant to metal, while white-footed mice (c) are carrying more ticks, leading to increased cases of Lyme Disease in humans
Professor Alberti explained that the number and variety of animals living in cities has not simply decreased, but humans are causing them to evolve rapidly.
‘Many species of mammals, birds, fishes, and insects have learned novel behaviours, such as diet, foraging skills, and anti-predator behaviour,’ the study says.
For example, it cites research that found the bodies of fish changed in response to the construction of dams and that Pacific salmon have become smaller in urban areas.
In contrast, a study published in August last year found that spiders are growing larger in cities than in rural environments.
Professor Alberti said that changes in animals have an effect on ‘ecosystem functions such as biodiversity, nutrient cycling, seed dispersal, detoxification, food production and ultimately on human health and well-being.’
In addition to the growing spiders and shrinking salmon, she cited earthworms with increased tolerance to metals and seeds of some plants dispersing less effectively.
Earthworms have adapted genetically to a series of soil contaminates, including arsenic and certain plants also have also developed an increased tolerance for heavy metals in brownfield sites in cities.
Worryingly, the study also points to evidence of changes in a type of urban mouse, that may have health consequences for humans.
Some white-footed mice in New York were found to be a ‘critical host’ for black-legged ticks that carry the bacterium that causes Lyme disease.
Professor Alberti’s study says that thriving mouse populations allow more ticks to survive and lead to predictable spikes in human exposure to the illness.
It also discusses how songbirds are becoming tamer and bolder, as well as changing their tunes to ensure their acoustic signals are not lost in the noisy urban background.
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