By Hannah Corsini, Freelance Writer

Published February 23rd, 2023

Artificial light during night-time hours has become commonplace but it is known to negatively impact wildlife. What insights can a 2021 study give into how it affects migratory species such as the monarch butterfly?

For centuries, the night sky around the world has been transformed by artificial light. Whilst unrestrained access to light has had clear benefits for humankind, the ramifications of night-time light pollution have also been evident.

The ability of artificial light to extend daylight hours and shorten the night has shown to disrupt both the circadian rhythms of humans and other species, which has knock-on effects on various physiological and behavioural traits.

Our circadian rhythms are the 24-hour cycles set in our body, functioning as part of the body’s ‘internal clock’. They are governed by genes throughout almost every cell of our body and by the proteins which these genes produce. Although the genes are not exactly the same in each animal, they remain relatively similar across a broad spectrum of organisms, including non-animals such as plants and fungi.

These genes respond to signals from the environment, such as light, which means that external signals have the power to alter specific behaviours controlled by our circadian rhythms.

‘External signals have the power to alter specific behaviours controlled by our circadian rhythms.’

One of such traits affected by circadian rhythm is migration. The negative effects of night-time light pollution on animals which migrate at night has previously been shown. For example, the migration patterns of birds, which rely on navigational cues from the sky, have been disrupted by artificial light during the night.

However, little research up to this point has been conducted to demonstrate the effects which night-time light pollution can have on the migration of non-nocturnal (also known as diurnal) species, like butterflies.

Like birds, butterflies use navigational cues from the sky. With the sun as their compass, they orient themselves using intercommunication between their antennae and the central complex of their brains. Their circadian clock allows them to adjust their flight path as the sun moves across the sky, realigning themselves to fly in a southerly direction.

Perhaps the most famous example of butterfly mass migration is that of the North American monarch butterfly, which travels up to 4000 kilometres every autumn in its journey from the northern United States to central Mexico.

The migratory corridors of monarch butterflies include cities in Texas and parts of Mexico, areas where they could be exposed to night-time light pollution. Monarchs roost at night in large groups, and when this roost site is near a streetlight or well-lit building, the monarchs could be exposed to night-time light pollution.

To test the effects of night-time light pollution on monarch butterflies, scientists isolated 78 of the butterflies and conducted flight simulator studies. Although the experiment was conducted at night, 98% of the butterflies immediately started to fly when an artificial light source was turned on, all flying in an equivalent manner to monarchs which had been tested in the flight simulator during the daytime.

‘The North American monarch butterfly travels up to 4000 kilometres every autumn.’

Sixty-three percent of these monarchs flew continuously for at least five minutes, and 93% of those flew directionally—leaving a grand total of 42 of the original 78 monarch butterflies that flew directionally when exposed to an artificial light source.

The monarchs tested in the early stages of the night flew as if it were late afternoon, suggesting that night-time light pollution has the potential to prolong their day. Meanwhile, monarchs tested later at night flew as if it were the morning, perceiving an earlier start of the day.

There are no documented cases of monarch butterflies actively migrating as a response to artificial light in the wild, and autumn night-time temperatures—although set to increase as a result of climate change—are currently often too low for night-time flight to occur anyway.

However, the effects of night-time light pollution on the monarch butterfly should not be diminished. Night-time light pollution could disrupt their circadian rhythms, leading to misreadings of the sun’s navigational cues during the day. It can also cause increased energy expenditure due to premature activity when temperatures are high enough for flight. All of this has the potential to negatively impact migratory success.

Migratory species across a breadth of taxa have been experiencing declines in abundance, largely due to human activity negatively impacting their ability to migrate. These species provide an invaluable network of connectivity between ecosystems and shape the diversity of different areas by widening their gene pools.

This study, among others, suggests that artificial light at night could be contributing to their inability to traverse great distances in the same numbers as they used to.

‘Migratory species across a breadth of taxa have been experiencing declines in abundance.’

The monarch butterfly itself has fallen in abundance in North America over the last few decades. Consequently, the conservation efforts for the species have heightened. The findings of this study suggest that future conservation plans should take into account the position of light sources and their proximity to monarch butterfly migratory routes, and avoid building pollinator gardens in areas susceptible to night-time light pollution.

Moreover, the amount of land covered by artificial light at night is estimated to increase by 5% to 10% each year in the western world. How will we navigate this whilst meeting the needs of the species we cohabit this Earth with?

Featured Image: Mariah Hewines | Unsplash

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