See and be seen: Understanding trade-offs in bioluminescent signalling and how it is affected by artificial lighting at night

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Almost all animals are sensitive to their light environment. They use light for diverse functions including entraining their biological rhythms and vision. Consequently, any alteration to light levels within the environment has potentially enormous consequences for animals, shifting activity cycles and affecting vision. Even if animals can adapt rapidly to such profound changes to their environment, they may still incur severe costs because they cannot fully compensate. For instance, the presence of bright light in formerly dark environments may disrupt the activity and sleep patterns of animals. Humans have dramatically altered the light regimes in the environment through the introduction of artificial lighting at night (ALAN), which is now present across vast areas of the planet. The most prevalent form of ALAN is road lighting, which proliferates as road networks expand. Satellite images of the Earth at night attest to the extent of road lighting across much of the planet. Indeed, the newest LED street lighting means that ALAN is cheaper and more energy efficient than ever before, promoting widespread adoption. These LED lights are brighter and whiter than ever before, quite different from the orange sodium lamps they replace. Yet the presence of ALAN disturbs nocturnal animals, potentially displacing them to darker 'refuge' habitats. Moreover, day-active animals may extend their activity into the night, creating additional pressures on nocturnal species. Although it is clear that ALAN affects the activity patterns of animals, there is little understanding of the mechanisms by which their behaviour and physiology is altered, or of larger scale effects upon populations.Our aim is to understand how ALAN affects animals at the behavioural, physiological, and population levels. We will use glow worms as an indicator species, a proverbial 'canary in the mine'. Our choice of indicator species is determined by their unique mating behaviour in which flightless female glow worms generate a bioluminescent glow in their abdomen that attracts males, which fly to them to mate. Our strategy will be to exploit this mating behaviour to quantify the effects of ALAN upon glow worms. Our data indicate that these effects are likely to be severe because female glowing consumes substantial amounts of energy that cannot be replenished because they do not feed as adults. Our preliminary data also show that far fewer males are attracted to females when they are illuminated by artificial lighting, so females glowing under ALAN use valuable energy for little benefit. Moreover, lighting along roads may act as a barrier by preventing males from dispersing through the environment to find females, affecting the structure and ultimately the survival of glow worm populations.We will use a combination of laboratory and field experiments to directly test hypotheses about the effects of ALAN upon glow worm reproduction. We will couple this with assessing the genetic structure of glow worm populations to determine how fragmented they have become already. We will then use statistical and modelling techniques to map the distribution of populations, the gene flow among them, and the presence of ALAN at both the local and broader scales. By doing so, we will identify the mechanistic cause of the effect of ALAN upon glow worm biology, with potentially broad implications for other nocturnal animals. We will also explicitly test the hypothesis that ALAN has had, and continues to have, a severe impact upon glow worm populations, potentially isolating them and driving them to extinction. By maintaining close links with policy makers and nature conservation practitioners throughout (see Pathways to Impact), our project has the potential to inform policy around ALAN and the extent to which its effects on wildlife can be mitigated.