Over the past two centuries, artificial lighting in our towns and cities has dramatically altered the night.
In urban environments, the soft dim glow of the sunlight reflecting off the moon is now outshone by bright streetlights, security lights and car headlamps. These artificial light sources can be more than 1000 times natural levels of moonlight – and their biological impact is increasingly visible.
So in brightly lit environments, song birds start singing earlier in the season and begin singing earlier each day, wallabies are less seasonal in their reproduction and nocturnal mammals forage for less time.
In the Urban Light Lab at the University of Melbourne, we are looking at these immediate effects and also asking: does light at night have more long-term evolutionary effects on species, and could it generate genetic change and even the formation of a new species?
Light as a constancy for life on earth
For the past 3.5 billion years, one of the constancies for life on earth has been the contrast between a bright day and a dark night.
Unsurprisingly, light is fundamental to many biological processes. For example, the intensity and duration of light present during the day is linked to the vertical movement of the plankton in our oceans, the commencement of reproduction in seasonally reproducing animals, and the onset of flowering and germination in plants.
In animals, as day turns into night, diurnal (day active) species reduce their activity or go to sleep and the nightshift takes over. Australia has a disproportionately large number of animals that are nocturnal, only venturing out in our cooler evenings.
Nocturnal species have evolved a suite of traits (including enormous eyes or a superior capacity to smell or hear) that are perfectly adapted for living in extremely low levels of light.
So, what happens when, after 3.5 billion years of evolution, light invades what should be darkness? This change is unprecedented in the history of life on earth and means that many species no longer experience true darkness.
And our latest research show the impact this can have. Our study, published in the Journal of Experimental Zoology-B, looks at the black field cricket, the Teleogryllus commodus, a nocturnal insect that lives in both urban and rural locations.
We found that exposure to artificial light at night is prolonging the time between the egg-to-adult stages of the cricket lifecycle. This might have important implications for the insects living in urban environments as it means they reach sexual maturity later and potentially, produce few offspring in a season.
It’s an unintended effect of our human need for light.
Light can alter who lives in a city
The presence of night lighting may determine which species or which individuals within a species are able to live in our cities. Many species will avoid the light altogether, leading to a decline in the diversity of species in urban areas.
On the other hand, many species are attracted to lights. Some predators, like spiders or geckos, may even benefit because if they hang around lights their next meal literally comes to them.
But, for others, like moths and birds, the lights cause them to become disorientated and trapped in the glare. This may lead to them dying through exhaustion, predation or collisions.
Because of this, some species may change. We predict that evolution should favour individuals in these species that are less attracted to the light.
In this scenario the effective gene pool of individuals living in light-affected areas is reduced, and this has implications for their genetic diversity and could make them more vulnerable to extinction.
Light reduces movement even within the city
While the lights in our parks and streets ensure safe human transport within and between our cities and rural areas, they often act as barriers to movement for other animals.
Night lights fragment the environment – creating a patchwork of light and dark areas that may result in populations of the same species becoming isolated in much the same way as habitat destruction, like tree clearing, has done in the past.
And bright lights at the edges of cities can reduce the likelihood of individuals moving into city areas.
Even in our city parks, which might be the only dark refuge for urban animals, pavement lighting creates boundaries and barriers that some animals just won’t cross.
This restriction of movement effectively isolates populations which may ultimately drive differences between them to such a degree that it leads to speciation – that is one species becoming two species that are no longer able to successfully breed together.
This is a problem in species with very low populations – the split could be enough to cause population collapse and extinction.
Light is more than skin deep
We can see how light at night is impacting animal behaviour, but there may also be hidden physiological changes that are equally, if not more, disruptive.
One of the ways light regulates our day-night rhythm is through its action on the melatonin pathway. Melatonin is an ancient chemical thought to have originated in the earliest life forms and remains present in everything from bacteria to fungi, plants and animals (including insects, birds, reptile and mammals).
The melatonin pathway is important because it is photo-sensitive – in the presence of light, especially blue light, melatonin production ceases. Under a natural day-night cycle this results in an oscillating pattern of melatonin that peaks at night when light levels are extremely low, and troughs during the day.
In humans, this corresponds to our sleep-wake cycle, and we all know what happens when we disrupt this cycle if we’re working night-shifts or taking long-distance flights that cross multiple time (and light) zones.
The problem for species living in artificially lit night-time environments (including in humans, when we use electronic devices after the sun goes down) is that the blue light emitted by this tech does the same thing as sunlight; melatonin production dips when its production should be at its highest.
This has inevitable consequences – more wakefulness and poorer sleep quality in animals that are active during the day and trying to sleep at night. But it also changes activity patterns for nocturnal animals that have evolved to live under the cover of darkness.
If we want to protect the wildlife living in our urban environments, we need to understand their physical, behavioural and evolutionary responses to our artificial lights.
Then we need to look at how our cities are built, particularly how they are lit, and think about how we can make adjustments that can help the animals living around us.
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