Streetlights make leaves tougher and harder for insects to eat, study says

(CN) —  Something as innocuous as a streetlight left on all night can have potentially wide-ranging effects on urban food chains, according to researchers in a new study.

The study, published in the journal Frontiers in Plant Science on Monday, focused on the effect of artificial light on street trees after researchers observed an odd phenomenon among urban foliage.

“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,” Shuang Zhang of the Chinese Academy of Sciences and corresponding author of the study said in a statement.

Scientists then found that artificial light at night resulted in increased toughness in the trees’ leaves, leading to a decreased levels of leaf herbivory, the act of animals eating plants.

Depending on one’s comfort level with insects, that may seem like a good thing, but the scientists disagree.

“Leaves that are free of insect damage may bring comfort to people, but not insects,” Zhang said. “Herbivory is a natural ecological process that maintains the biodiversity of insects.”

Zhang, working with the Chinese Academy of Sciences, investigated two of the most common tree species found along streets in Beijing: Japanese pagoda and green ash trees.

To conduct the study, the scientists found 30 sampling sites along main roads that were usually lit up in the evenings and through the night. The sites were spaced around 100 meters apart.

The scientists measured the illuminance at each site and collected nearly 5,500 leaves from the sample sites.  Each leaf was evaluated for traits that may be affected by artificial light, like levels of nutrients and chemical defenses, water content, size and toughness.

Tough leaves and high levels of chemical defenses, like tannins, would indicate the tree was allocating more resources to defense. Herbivores are tempted by high levels of water in nutrients.

In both tree species, the scientists observed that increased artificial light resulted in tougher leaves, which subsequently showed fewer signs of insect herbivory.

“The underlying mechanism for this pattern is not yet fully understood,” Zhang said. “It is possible that trees exposed to artificial light at night may extend their photosynthesis duration.”

Zhang also noted the leaves may be using their resources to support structural compounds, like fibers.

In the Japanese pagoda trees exposed to more artificial light, lower levels of nutrients like phosphorus were observed. When the leaves had more nutrients, there were more signs of herbivory.

In green ash trees exposed to more artificial light, the leaves were found to have higher levels of nitrogen, lower chemical defenses and smaller leaves.

One explanation behind the differences between the species could be that green ash trees are a less desirable meal to herbivores and could spend more resources on growth.

Zhang said the implications of the study extend beyond just trees and insects.

“Decreased herbivory can lead to trophic cascading effects in ecology,” Zhang said. “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.”

Researchers have been documenting the global decline of insects for some time, a pattern Zhang said needs to be focused on.

“We should pay more attention to this trend,” Zhang said.

Zhang also noted that more research is needed into the impact of artificial light on herbivory. He noted the study focused only on two types of trees isolated to one geographic area.

“Research on how urbanization affects insects and insect-related ecological processes is still in its infancy,” Zhang said.

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