08-05-2024

Streetlights make tree leaves too tough for insects to eat

Earth.com staff writer

Light pollution disrupts circadian rhythms and ecosystems worldwide, with potentially profound effects on plants that rely on light for photosynthesis.

A new study published in Frontiers in Plant Science has revealed that high levels of artificial light at night can make tree leaves tougher and harder for insects to eat, which poses a threat to urban food chains.

Tough leaves under artificial light

“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 Shuang Zhang, a scientist at the Chinese Academy of Sciences and the study’s corresponding author.

“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 increased night-time brightness by almost 10%, affecting most of the world’s population. Changes in plant properties due to artificial light can significantly impact ecosystems, as plant interactions with other plants and animals are altered.

“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.”

Tougher leaves as a defense mechanism

The researchers hypothesized that plants exposed to high levels of artificial light would prioritize defense over growth, resulting in tougher leaves with more chemical defense compounds.

To test this, they focused on two common street trees: Japanese pagoda and green ash trees. These trees, though similar in many ways, differ in leaf characteristics, with Japanese pagoda trees having smaller, softer leaves preferred by herbivores.

The team selected 30 sampling sites along main roads in Beijing, spaced roughly 100 meters apart and usually illuminated all night. They measured the illuminance at each site to determine exposure to artificial light.

Nearly 5,500 leaves were collected and analyzed for insect herbivory and traits potentially affected by artificial light, such as size, toughness, water content, and levels of nutrients and chemical defenses.

Higher light levels lead to tougher leaves

For both tree species, higher levels of artificial light resulted in tougher leaves, with tougher leaves showing 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,” Zhang said. “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.”

Japanese pagoda trees exposed to more artificial light had lower levels of nutrients like phosphorus, with higher nutrient levels correlating with more herbivory.

Conversely, green ash trees, less preferred by herbivores, showed stronger influences from higher light levels, displaying higher nitrogen levels, smaller leaves, and lower chemical defenses.

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

Far-reaching impacts of light pollution

While leaf toughness is a mechanical defense against predation, other factors may also contribute to decreased herbivory. For example, increased light might make insects more visible to predators. Further research is needed to fully understand the effects of artificial light.

“Our study was conducted in only one city and involved just two tree species,” Zhang cautioned. “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 study highlights the complex and far-reaching impacts of light pollution on urban ecosystems, particularly on the interactions between plants and insects. It underscores the need for further research to develop comprehensive strategies for mitigating these effects and protecting urban biodiversity.

“These results emphasize that high ALAN [artificial light at night] is detrimental to the flow of energy from plants to higher trophic levels, and further suggest that ALAN may have far-reaching effects on the maintenance of urban biodiversity and ecosystem functioning,” the authors concluded.