If you live in the Global North or one of the rapidly urbanizing parts of the Global South, there may be a stark lack of darkness. The thing that has been a defining feature of the night is gone. Unceasing human activity — all those lights — means the night sky now has a slight red glow to it. This skyglow is the result of light pollution, and it has far reaching consequences on human health, ecosystems, and animal behavior.
One such behavior is nocturnal pollination. The world is currently facing the imminent threat of loss of pollinators. Lesser known, but equally critical, pollinating animals are nocturnal pollinators like elephant hawkmoths.
On top of being threatened by loss of habitat and the use of chemical pesticides, insecticides, and herbicides, these striking insects may also be threatened by artificial light at night. Since pollinators are usually part of a network of plants and pollinators, disrupting pollination, which elephant hawkmoths do at night, reduces plants’ overall ability to reproduce. This feeds back onto all the pollinators that depend on the plants.
Jolyon Troscianko is a visual ecologist, which means he studies how the world appears to animals and how animals use their vision to navigate. To understand the potential effects of artificial light on the visual ecology of elephant hawkmoths, his group turned to mathematical modeling. The results of their research were published earlier this year in Nature Communications.
There are several different types of artificial light, and Troscianko's group wanted to test the effects of commonly used sources of light on elephant hawkmoths' natural behavior. As the light source changes so does the wavelengths of the light available to be reflected off surfaces such as leaves and flowers. In turn, this shifts the perceived colors of these surfaces from the moth’s point of view.
Not only do elephant hawkmoths have a special place in plant-pollinator networks, they also fascinate scientists for being the only species that has been studied for its ability to see color at night. However, the results of this research are likely applicable to other nighttime pollinators.
We haven’t evolved to see it, but the natural night sky has a color of its own. That color may be essential for the survival of nocturnal animals like the elephant hawkmoth. Even at night, hawkmoths can detect colors of flowers, which enables them to serve as vital agents of pollination. But the moth's sensitive visual system also makes it vulnerable to subtle disruptions of its visual ecology. As the sun dips below the horizon, the proportion of red in the sky decreases. Since most artificial sources of light have higher proportions of red, the natural color of the night is skewed.
"The sheer number of scenarios we tested was quite a challenge — we had to limit ourselves because the number of different hypotheses we could test was vast," Troscianko said.
The researchers compared the ability of hawkmoths to perceive the color of various natural objects — flowers, leaves, and so on — under artificial lighting of different intensities with their ability to do so under full moonlight.
Some common types of light sources, like white LEDs, emit a wide range of wavelengths across the visible spectrum, while others, like orange LEDs, emit a narrower range of wavelengths. The latter are more likely to emit longer wavelengths — the ones that look red or oranges — while broad-spectrum light sources appear white.
Since the natural night sky contains lower levels of red than an artificially lit night sky, it isn’t surprising the researchers' model predicted that under redder sources of light, hawkmoths will have a harder time detecting flowers than under full moonlight. On the other hand, broad spectrum white lights actually enhance the moths' color perception. But what surprised Troscianko the most was that the effects of phosphor-converted amber LED lights, which have a mix of yellow, orange and red wavelengths, varied with the intensity of the light.
“You can think of it as lighting that has that eerie twilight property, where colors look odd, but you do still have some color vision. Imagine the moth flying around and trying to remember flower colors while moving between intense lights that enhance colors (but change them entirely), and then to an odd twilight world where some colors disappear, while others are still fairly strong. It's a difficult effect to describe, but similar things happen to humans when we move around in low light and colorful lights,” he explained.
They didn’t just test the ability of the moths to detect flowers. As prey of insectivorous birds, moths are known to camouflage themselves. If their ability to perceive color is disrupted, the moths may choose a hiding spot that wouldn’t camouflage them from the bird’s point of view.
To test this, the team also modeled the visual system of the blue tit — a day time hunter that might see the moth against whatever surface it chose to rest at the end of the night. They found that artificial light may hinder this anti-predation behavior, making the moths more likely to be eaten. In the color-polluted night sky, a surface may seem, to the moth, like a good spot against which to camouflage, but under daylight, the blue-tit would be able to detect it.
It sounds bad. But more work needs to be done before we can start making changes to our lighting.
For one, these effects need to be tested on actual animals. “I am most interested in seeing whether the disruptive color intensity effects that our model predicts might also interfere with moth flower handling behavior,” Troscianko said. Flower handling is the behavior when the moths get close to the flower and stick their proboscis in to take up nectar.
Anyone who has stared with dismay at the assortment of dead insects under a lamp may be forgiven for thinking that these effects of light on moth pollinating behavior are relatively unimportant. It is indeed hard to tell without further research whether the tendency of insects to be attracted to light could in itself be more damaging than subtle changes to their behavior.
But as Troscianko explains, “The effects we describe in our study might result in small effect sizes in terms of making flower handling slower or more inefficient, but the artificial light intensities involved could conceivably cover the majority of Europe. So [it is a] smaller effect, but over a much larger spatial scale (and therefore incredibly difficult to quantify). Our study also highlights multiple different potential problems, [such as that] some light types interfere with moth flower detection, make it difficult to color-match to substrates for camouflage, and aid the vision of predatory birds. So death by a thousand cuts.”