Light disrupts circadian rhythms and ecosystems around the world.
For plants, which rely on light for photosynthesis, its effects can be profound.
Scientists writing in Frontiers in Plant Science have found that exposure to high levels of synthetic light at night makes tree leaves tougher and more difficult for insects to eat, threatening urban food chains.
“We found that compared to herbaceous ecosystems, tree leaves in maximum urban ecosystems sometimes show few signs of insect damage. We were curious why,” said corresponding Dr. Shuang Zhang of the Academy of Sciences. Chinese Sciences.
“Here we show that in two of the most common tree species in Beijing, synthetic light at night led to increased leaf resistance and reduced levels of leaf herbivory. “
Artificial light has higher nighttime light levels, about 10%, and the majority of the world’s population faces light pollutants every night.
Since the plants’ home has its interactions with other plants and animals, any changes in plants caused by synthetic light could have a significant impact on the ecosystem.
“Leaves that are not damaged by insects may bring comfort to people, but to insects,” Zhang said.
“Herbivory is an ecological herbal procedure that maintains insect biodiversity. “
Scientists suspected that plants exposed to high levels of synthetic light would prioritize defense over growth, generating more potent leaves that contain more chemical defense compounds.
To prove this, they decided on two common species of street trees: the Japanese pagoda and green ash trees.
Although those trees are in many ways, Japanese pagodas have smaller, softer leaves that herbivores prefer.
The scientists learned about 30 sampling points located about a hundred meters away on the main roads, illuminated throughout the night.
Up to the point of exposure to synthetic lighting, they measured the illuminance at each site.
Nearly 5,500 leaves were collected and evaluated to determine their herbivorous nature and characteristics that can be affected by synthetic light, such as size, resistance, water content, and levels of nutrients and chemical defenses.
Larger leaves would imply resources allocated to growth, while hardness and higher levels of chemical defenses such as tannins would imply resources allocated to defense.
Meanwhile, higher water and nutrient levels mean higher quality nutrition to tempt herbivores.
For either tree species, higher levels of artificial light meant tougher leaves.
The more potent the leaf, the lower the insect herbivory.
The more intense the light, the more leaves the scientists found that showed no symptoms of herbivory.
“The mechanism of this trend is not yet fully understood,” Zhang said.
“It is possible that trees exposed to artificial light at night increase the duration of their photosynthesis.
Furthermore, those sheets can allocate a greater proportion of resources to structural compounds, such as fibers, which can lead to an increase in the strength of the sheets.
Japanese pagoda trees exposed to more synthetic light had lower levels of nutrients such as phosphorus.
Where the leaves of Japanese pagodas contained more nutrients, there are more herbivores.
But green ash leaves were more influenced by the upper soft levels.
They had higher nitrogen levels, smaller leaves, and weaker chemical defenses.
This may simply be because green ash trees are less valued by herbivores, so they may allocate resources to their growth.
Meanwhile, Japanese pagodas are devoting more resources to their defense, to their nutrient content.
“Declining herbivore numbers can cause a trophic cascade in the ecology,” Zhang said.
“Lower degrees of herbivory mean a lower abundance of herbivorous insects, which in turn can lead to a lower abundance of predatory insects, insectivorous birds, etc.
The decline of insects is a trend that has been observed in recent decades.
We pay more attention to this trend. »
Although leaf hardiness is a mechanical defense against predation, it is possible that other factors may be contributing to a decrease in herbivory.
For example, being gentler can make insects more visual to their predators.
Further studies will be conducted to fully perceive the effects of synthetic light.
“Our study was conducted in a single city and only looked at two species of trees,” Zhang said.
“This limitation hampers our ability to generalize the findings to broader spatial and taxonomic scales.
Research on how urbanization affects insects and the ecological processes related to them is in its early stages. »