A Brighter Future? The Consequences Of Street Lighting On Insect Populations
Artificial light at night—or “ALAN,” as scientists refer to it—is known to have substantial consequences for biodiversity and ecosystem processes. Amongst other things, birds’ migration patterns can be disrupted, various nocturnal animals’ rhythms thrown off, and insect behavior altered. In a recently published study, a team of researchers from the U.K. tested the effects of ALAN on insect populations. Specifically, they wanted to know, in addition to the abundance of evidence documenting the aforementioned effects of ALAN on insect behavior, whether it also affected the population size of nocturnal insects. While previous work had documented both the effects of ALAN on insect behavior, as well as a general decline of insect populations, little research has looked at whether ALAN itself affected this decline.
To answer this question, the researchers set up a series of experiments. Using what’s known as a matched-pairs design, they tested the impacts of ALAN on wild moth caterpillars by comparing the number of caterpillars in a habitat directly lit by streetlights to an otherwise similar habitat situated at least 60 meters from the nearest streetlight. They also divided the streetlights at the two unlit sites between LED and conventional yellow sodium (HPS) lamps since there could be differences between the two kinds of lamps. In counting the caterpillars, the researchers also weighed them to look at normal development and how that may also be impacted by the nighttime lighting. In addition to looking at areas where lights were already placed, the team installed new, temporary lights in areas with no history of lighting to test whether ALAN would disrupt the feeding behavior of nocturnal caterpillars.
What they found was that there were substantially fewer caterpillars in the lit areas—specifically, 52% fewer in LED-lit hedges and 41% fewer in hedges illuminated by HPS. For grassy areas, there were 43% fewer caterpillars in areas lit by LEDs and 24% fewer for HPS (although, importantly, the effect for HPS-lit habitats was not statistically different for no effect). Additionally, caterpillars sampled from lit areas were typically heavier than those from unlit areas, which the researchers hypothesize is due to heightened developmental rates (triggered by eating behaviors) from ALAN exposure. Using statistical models, the team found that this was the case for hedges lit by both LEDs and HPS, and grassy areas lit by HPS.
The scientific importance of these findings is that the researchers here showed that entire life cycles and not just single parts of these insects’ lives are affected by human activity. More generally, this implies that we collectively should try to take the longer-term impacts of even local actions on non-human animals, their (and our) environments, and biodiversity into account. In particular, as technology develops, we need to understand the relative tradeoffs and their effects on ecosystems. While LEDs are more eco-friendly in that they use less energy, this study demonstrates that a gain in energy efficiency may not outweigh the costs in terms of fauna populations.
Where previous population-level research focusing on the impacts of ALAN has largely examined bats, birds, and other vertebra, animals occupying the base of the food chain (such as insects) play their own crucial role and may be impacted in unique ways. While the researchers acknowledge a need for further work to parse the relative importance of ALAN for insect population declines more globally (independent of other drivers such as climate change and habitat loss), they show clearly that ALAN nonetheless presents a critical issue for insect populations at the local level. This likely has upstream consequences for other species, such as moths’ predators and pollination.
To that end, we need to quickly and scientifically understand how to mitigate the worst effects of nighttime lighting and light pollution on insect populations. Although technology is part of the problem here, it can also be part of the solution. LEDs can be dimmed, businesses can shut off lights at night altogether, and groups such as the International Dark-Sky Association could get more attention from funders and governments. Inasmuch as the negative impacts of ALAN are an environmental concern, they are also an animal welfare concern, and advocates must not neglect the less “charismatic,” albeit crucial, species.