Artificial lights that run all night, such as streetlights, can make leaves grow so tough that insects cannot eat them, which could threaten urban food chains, according to a new study. The study, ‘Artificial light at night decreases leaf herbivory in typical urban areas’, was published earlier this month in the journal Frontiers in Plant Science. The analysis was carried out by Yu Cao, Shuang Zhang, and Ke-Ming Ma of the University of Chinese Academy of Sciences (Beijing). How was the study carried out? The researchers wanted to examine how artificial lights impact the relationship between plants and insects. To do so, they focused on two common species of street tree in Beijing: Japanese pagoda and green ash trees. Although these trees are similar in many ways, Japanese pagoda trees have smaller, softer leaves which insects prefer to munch on. The researchers picked 30 sampling sites on main roads that are usually lit by streetlights all night. They then measured illuminance — the amount of light — at each site and collected around 5,500 leaves to evaluate the impact of light on their traits such as size, toughness, water content, and levels of nutrients and chemical defence compounds. If the leaves were larger, it would mean that plants directed their resources (such as nutrients, water, and energy) to growth. If the leaves were tougher and contained high levels of chemical defence compounds like tannins, it would indicate that the resources were allocated for defence. What were the findings? The researchers observed that for both species of trees, the higher the levels of illuminance, the tougher the leaves. The tougher the leaves, the less evidence of insects eating them. The researchers found no sign of insects munching on leaves in areas which were lit the brightest at night. They also noted that artificial lights altered the levels of nutrients and chemical defence compounds in the leaves that were analysed. For instance, Japanese pagoda trees which were exposed to more artificial light had lower levels of nutrients such as phosphorus in their leaves. Such leaves had less evidence of insects consuming them. According to the study, this could be a bad sign for the ecosystem. Shuang Zhang, co-author of the study, told The Guardian in an interview: “Decreased herbivory can lead to trophic cascading effects in ecology. Lower levels of herbivory imply lower abundances of herbivorous insects, which could in turn result in lower abundances of predatory insects, insect-eating birds, and so on. The decline of insects is a global pattern observed over recent decades. We should pay more attention to this trend.” Although researchers said they did not exactly know why trees are reacting to streetlights in this way, they suggested that trees exposed to artificial light at night might be extending their photosynthesis cycle. This can be really stressful for them as when a plant photosynthesises, it takes in energy, and if it does that all the time, the situation can be overwhelming and eventually kill the plant.
