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Harsh light causes ecological damage, disrupting plants’ seasonal rhythms and their fragile relationship with pollinators.
Item Information
- Author: Ally Hirschlag
- Rolle, BBC Future
May 12, 2023
On a summer night in 2014, a group of biologists and ecologists observed several suburban areas with streetlights near Wallingford, UK.
The presence of these people on the streets at night may have raised some concerns among locals. In fact, they weren’t there to commit robberies, but to accompany moths on their nocturnal pollination routine.
Scientists studied how streetlights affect moth behavior. His theory was that artificial light at night would disrupt moth flight patterns and decrease the quantity and quality of pollination.
And of the hundreds of moths observed, more than 70% were drawn up towards lights and away from flowers. This resulted in a significant reduction in pollination as well as the amount of pollen species transmitted by insects.
The same results were obtained in several moth species and more than 28 plant varieties, suggesting a ripple effect that could have wider implications.
This is just a specific form of damage caused by light pollution in nature.
Moths aren’t the only pollinators harmed by bright city lights. Bees are also feeling the effects, as are the plants they pollinate.
Light pollution is already felt in about a quarter of the world and is increasing by about 6% each year as urbanization increases. Global light pollution has increased by at least 49% over the past 25 years.
Its effects are still being studied and vary widely between species, but scientists already know that light pollution affects plant growth and reproduction.
It affects their seasonal rhythm, their ability to recognize and respond to natural light, and their fragile relationships with pollinators.
The more tense this relationship is, the greater the risks to our food production. And a single broken bond can have unpredictable physiological consequences throughout the food chain.
How plants “feel” the world.
A major reason for the effects of light pollution or artificial night lights on most living things on the planet is the circadian rhythm the natural cycle of sleep and wakefulness that repeats itself every 24 hours and is governed by day and night. Night.
Most living beings, regardless of whether they have day or night habits, carry out their survival processes according to this cycle.
For example, when it is compromised by the perception of a shorter night, it can cause undue stress on the ability of internal systems to function. And the longer this interference lasts, the greater the potential for harmful effects.
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Light pollution harms many different ways
Circadian clock disruptions can also affect the regulation of substances in humans, plants, and animals, most notably melatonin — the molecule produced in response to darkness that helps regulate circadian rhythms.
“It’s thought to be elevated at night, but light pollution suppresses melatonin, which over time can have a whole host of adverse health effects,” said researcher Valentina Alaasam of the University of Nevada in the United States.
Health effects on humans include an increased risk of sleep disorders, diabetes, depression, heart problems and breast cancer.
The effects are a bit reminiscent of jet lag. Let’s say you are flying from London to New York in the United States.
By traveling five hours behind your departure point, you actually gain five hours a day or night (depending on when you travel), causing your circadian rhythm to become unbalanced.
The resulting jet lag can leave you feeling confused, sleepy, or uncomfortable for days. It is the result of a disruption in the circadian rhythm.
Now imagine you are a tree on a city sidewalk, constantly exposed to artificial light, such as from street lights, cars, and buildings. When your internal system thinks it’s always daytime, it can turn your life upside down.
“Plants are like animals. They need a sleep cycle to process things and what they do is do different activities at different times of the day. To do this, they need to know the time of day,” explains Professor Joanne Chory from the Salk Institute for Biological Studies in San Diego, California (USA).
Plants use their photoreceptors we know of 13 of them to determine information such as day length.
This lets them know when to flower and, in the case of deciduous trees, to shed their leaves—certainly two of the most important times for trees.
Five of the photoreceptors absorb nearinfrared light coming from the moon and stars, while another eight absorb some type of ultraviolet light. If we artificially increase the length of the day, light pollution can cause these photoreceptors to change the flower pattern of the plant.
“Plants are stressed by light [artificial]. They can increase photosynthesis and thus increase stress,” says ecologist Brett Seymoure, professor of biological sciences at the University of Texas at El Paso in the US. “It’s like a bodybuilder who works out all the time.”
When a plant performs photosynthesis it absorbs energy, and if it does so constantly it can become overloaded, creating a reactive type of oxygen that eventually kills the plant.
Add to that warmer winters and longer summers due to climate change, and trees grow completely out of their natural rhythm, which can lead to them becoming weaker.
seasonal changes
Flowering, or budding, typically occurs in spring when higher temperatures arrive, accompanied by longer days and greater amounts of ultraviolet radiation. But Seymoure says light pollution mistakes these cues for plants.
“The seasonal changes in plants were influenced by artificial lighting at night,” says Seymoure. “They bud earlier or keep their leaves longer because the photoperiod the relationship between day and night is obscured by artificial light at night.”
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Streetlights radiated for miles around a city can disrupt the circadian rhythms of plants and animals
A study analyzed 13 years of data on bud formation of deciduous trees in the UK and concluded that buds form up to 7.5 days earlier in lighter regions at night due to light pollution.
And for the same reason, budding can change when trees shed their leaves in the fall—artificial light interferes with photoperiod perception, and days get shorter as winter approaches.
Normally, as the days get shorter, deciduous trees stop producing chlorophyll, the primary pigment for photosynthesis, and draw nutrients from the leaves. Therefore, they change color and eventually fall to the ground.
“But because they’re constantly bombarded by streetlights, they don’t get that photoperiod, so they keep their leaves much longer and potentially lose any leaves that store chlorophyll — which is actually very expensive energetically,” he says.
Light pollution affects the daily rhythm of pollinators and their overall standard of living. This ultimately prevents the plant from multiplying.
Several studies, including analysis of British moths, concluded that artificial light reduced pollination.
A 2017 study concluded that nocturnal interactions between plants and pollinators are reduced by 62% in brightly lit areas compared to dark locations. And when these interactions decrease, plants are less encouraged to produce pollen, reducing the amount of pollen available to daytime pollinators like bees.
Just as jet lag affects humans, light pollution can also take a toll on pollinators’ bodies, reducing their sleep and recovery time and ultimately making it more difficult for them to pollinate and reproduce.
Light pollution even harms migration. It diverts insects (and other types of flying animals) from their natural path and into dangerous and often deadly scenarios.
“That has already been shown [ela] confuses the migration routes [dos pássaros] and changes the species’ breeding grounds or where they spend the winter,” says Alaasam.
And these migratory shifts can damage plants that are pollinated by birds or plants whose seeds are dispersed by birds.
As for insects, Seymoure calls this attraction to light the “trap effect,” in which “insects fly to lights but never pollinate because they’re just too busy flying around the light.”
Other pollinators, such as certain beetle species, are photophobic and simply avoid vegetation exposed to artificial light. It is the “repulsive effect”.
About a third of insects attracted to artificial lights at night do not survive until the next morning, whether due to predation, starvation or exhaustion, according to an analysis of 229 studies coauthored by Seymoure.
“Insect Apocalypse”
The pollinator population has systematically declined. Ecologists like Seymoure fear this is the result of a confluence of stressors that include light pollution, chemical pollution, climate change and habitat loss.
Insects are fundamental to life on the planet, and the more we lose them, the more pronounced the domino effect on species becomes.
For example, if native bees die in the United States, all of the plant species they pollinate could soon disappear if left unchecked. And these species are responsible for 80% of the flowers in the country.
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The artificial lengthening of day length caused by light pollution can alter the flowering pattern of plants.
The US Department of Agriculture estimates that these crops make up 25% of the food Americans eat every day.
“About 200 million years ago, flowering plants and insects began to evolve together. If you start changing the insect behavior or the behavioral physiology of the plants, it affects both of them,” explains Seymoure.
“[Com a poluição luminosa] They have this strange new situation that will have ramifications for the entire planet because almost everything depends on insects and flowering plants 80% of our food species are pollinated by insects.”
Plants and pollinators depend on changes in light and temperature to know how and when to carry out their survival processes. And as the planet warms and light pollution increases, these processes become increasingly unbalanced. It is unclear whether the affected species will adapt quickly enough to survive.
“Light pollution and climate change provide these clues.” [luz e temperatura] less reliable and causes a degree of ecological chaos,” says Alaasam.
On the other hand, there are pollinators that are not as badly damaged by light pollution. In fact, some have evolved to use it to their advantage.
One study looked at “city moths” from ten different populations and concluded that they have adapted not to fly towards artificial lights at night.
“Moths that were exposed to artificial light in the last 20 to 50 years no longer had the genes that enabled them to fly towards the light,” Seymoure says.
Pollinating bats also seem to have found a positive solution. Not only does artificial light illuminate the insects they like to eat, but a 2022 study found that it also boosts pollination due to the greater amount of fruit and seed in the artificially lit trees (possibly caused by overflowering).
However, this can only be a temporary benefit. Seymoure worries that too much artificial light could overwhelm the trees and weaken them over time.
Many other insect species have never been attracted to light and are therefore not directly harmed by light pollution. However, that does not mean that they are not directly affected in other ways.
One example is bees, which are not attracted to light but are harmed by light pollution because plants they pollinate are less likely to produce pollen from nocturnal pollinators that move away due to the artificial light.
What can we do?
There are some simple steps we can all take to reduce light pollution. Many of its positive effects are immediate.
• Replace fluorescent lamps with LED lamps. Not only do they cause less light pollution, they are also more economical. Be sure to use warm rather than cool light bulbs (up to 3000 K the higher the K value, the cooler the light).
• Get rid of excess lighting. It throws a lot of light into the air, which can disorient birds and cause them to get lost or fly into structures.
• Lower the blinds at night or install covers over your outdoor light sources.
• Replace switches with motion sensors.
It may seem that these small changes will not have an overwhelming impact, but they all can also result in savings from lower energy bills.
And Alaasam points out that this factor can help convince your office manager, your landlord, or local officials to reduce light pollution.
These are changes that are beginning in small areas of ecological consciousness around the world. And the benefits these communities achieve could perhaps attract others, pardon the expression, like a moth to a flame.