Believe it or not, streetlights are ruining the environment. Light pollution is harming plant life as well as other aspects of the ecosystem.
Artificial light at night is disrupting life on Earth, from human sleep patterns to delicate ecosystems. Plants, which rely on sunlight for survival, are particularly vulnerable. New research published in Frontiers in Plant Science reveals that excessive nighttime lighting is causing tree leaves to develop tough, unappetizing defenses. This change could have far-reaching consequences for urban food chains.
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“We noticed that, compared to natural ecosystems, tree leaves in most urban ecosystems generally show little sign of insect damage. We were curious as to why,” said corresponding author Dr Shuang Zhang of the Chinese Academy of Sciences. “Here we show that in two of the most common tree species in Beijing, artificial light at night led to increased leaf toughness and decreased levels of leaf herbivory.”
Artificial light has intensified nighttime brightness by nearly 10%, casting a glow over most of the world’s population. Since plants interact closely with other organisms, any alterations to their biology due to artificial light could have far-reaching consequences for entire 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.”
The scientists suspected that plants experiencing high levels of artificial light would focus on defense rather than growth, producing tougher leaves with more chemical defense compounds. To test this, they selected two common species of street tree: Japanese pagoda and green ash trees. Although these trees are similar in many ways, Japanese pagoda trees have smaller, softer leaves which herbivores prefer.
Researchers selected 30 locations along brightly lit main roads, spaced about 100 meters apart. To assess light exposure, they measured illumination levels at each site. Nearly 5,500 leaves were collected for analysis of insect damage and properties influenced by artificial light, such as size, hardness, water content, nutrient levels, and chemical defenses. Larger leaves typically signal growth investment, while tough leaves with high levels of protective compounds like tannins suggest defense prioritization. Conversely, leaves with abundant water and nutrients are more appealing to insect herbivores.
“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, it is possible that other factors contribute to decreased herbivory: for instance, more light could make insects more visible to their predators. Further research will be needed to fully understand the effects of artificial light.
“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.”
