Light alters 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 artificial light at night produces tree leaves and insect food, 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 caused an increase in leaf resistance and a reduction in leaf herbivory levels. “
Artificial light has higher nighttime light levels, up to a maximum of 10%, and the majority of the world’s population experiences light pollutants every night.
Since the home of plants 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 illuminance at each site.
Nearly 5,500 leaves were collected and evaluated for herbivory and characteristics likely to be affected by synthetic light, such as size, strength, water content, and levels of nutrients and chemical defenses.
Larger leaves would imply resources allocated to growth, while toughness 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 sturdier 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 underlying this trend is still fully understood,” Zhang said.
“It is possible for trees exposed to artificial light at night to prolong their photosynthesis time.
In addition, those sheets may simply allocate a higher proportion of resources to structural compounds, such as fibers, which can lead to an increase in the strength of the sheet. »
Japanese pagoda trees exposed to more synthetic light had lower levels of nutrients such as phosphorus.
Where Japanese pagoda leaves 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 devote more resources to their defense and thus to their nutrient content.
“The presence of herbivores can cause cascading trophic effects on 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 the strength of the leaves provides a mechanical defense against predation, it is imaginable that other points contribute to reducing 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 looked at only two species of trees,” Zhang said.
“This limitation hinders our ability to generalize 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. “