Scientists have found that exposure to high levels of artificial light at night makes tree leaves tougher and harder for insects to eat. This change threatens urban food chains by disrupting the natural process of herbivory, said the research in Frontiers in Plant Science.
DISCOVERING THE IMPACT OF ARTIFICIAL LIGHT
Dr. Shuang Zhang of the Chinese Academy of Sciences noticed a curious phenomenon: tree leaves in urban ecosystems show little sign of insect damage compared to natural ecosystems. This observation led to a study investigating the effects of artificial light at night on tree leaves. The findings revealed that in two common tree species in Beijing, artificial light increased leaf toughness and decreased leaf herbivory.
THE GROWTH OF LIGHT POLLUTION
Artificial light has increased night-time brightness by nearly 10%. Most of the world’s population experiences light pollution every night. Plant properties affect their interactions with other plants and animals, so changes caused by artificial light can significantly impact ecosystems.
“Leaves that are free of insect damage may bring comfort to people, but not insects,” said Zhang. “Herbivory is a natural ecological process that maintains the biodiversity of insects.”
DEFENSE OVER GROWTH
The researchers hypothesized that plants exposed to high levels of artificial light would focus on defense rather than growth, producing tougher leaves with more chemical defense compounds. They selected two common street tree species for their study: Japanese pagoda and green ash trees. Although similar in many ways, Japanese pagoda trees have smaller, softer leaves preferred by herbivores.
METHODOLOGY: MEASURING LIGHT EXPOSURE AND LEAF TRAITS
The scientists identified 30 sampling sites along main roads in Beijing, which are usually illuminated all night. They measured illuminance at each site to determine the level of artificial light exposure. Nearly 5,500 leaves were collected and evaluated for insect herbivory and traits affected by artificial light, such as size, toughness, water content, and levels of nutrients and chemical defences.
Larger leaves indicate resources allocated to growth, while toughness and higher levels of chemical defences indicate resources allocated to defense. Higher levels of water and nutrients suggest higher-quality nutrition to tempt herbivores.
TOUGHER LEAVES, LESS HERBIVORY
For both tree species, higher levels of artificial light resulted in tougher leaves and less evidence of insect herbivory. The more intense the light, the more frequently leaves showed no signs of herbivory.
“The underlying mechanism for this pattern is not yet fully understood,” said Zhang. “It is possible that trees exposed to artificial light at night may extend their photosynthesis duration. Additionally, these leaves might allocate a greater proportion of resources to structural compounds, such as fibers, which could lead to an increase in leaf toughness.”
SPECIES-SPECIFIC RESPONSES
Japanese pagoda trees exposed to more artificial light had lower levels of nutrients like phosphorus. Where these leaves had more nutrients, more herbivory occurred. Green ash leaves, however, were more strongly influenced by higher light levels: they had higher levels of nitrogen, smaller leaves, and lower chemical defences. This difference could be because green ash trees are less preferred by herbivores, allowing them to allocate resources to growth. Meanwhile, Japanese pagoda trees put more resources into defense, lowering their nutrient content.
ECOLOGICAL IMPLICATIONS: INSECTS GOING HUNGRY
“Decreased herbivory can lead to trophic cascading effects in ecology,” said Zhang. “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 leaf toughness is a mechanical defense against predation, other factors might also contribute to decreased herbivory. For instance, more light could make insects more visible to their predators. Further research is needed to fully understand the effects of artificial light.
LIMITATIONS AND FUTURE RESEARCH
“Our study was conducted in only one city and involved just two tree species,” cautioned Zhang. “This limitation hinders our ability to generalize the conclusions to broader spatial and taxonomic scales. Research on how urbanization affects insects and insect-related ecological processes is still in its infancy.”
The findings from this study highlight the complex and far-reaching impacts of artificial light at night on urban ecosystems. By toughening tree leaves and reducing insect herbivory, artificial light disrupts natural food chains and threatens biodiversity. As urbanization continues to grow, understanding and mitigating these impacts becomes increasingly important for maintaining ecological balance.
The Way Forward
Continued research and collaboration across various fields are essential to fully grasp the extent of artificial light’s impact on urban ecosystems. By exploring how different species and regions are affected, scientists can develop strategies to mitigate the negative effects and preserve biodiversity in our increasingly illuminated world.
The toughening of tree leaves under artificial light serves as a powerful reminder of the interconnectedness of urban environments and the natural world. Addressing the challenges posed by light pollution is crucial for protecting urban ecosystems and the myriad species that depend on them.
