Light pollutants disrupt circadian rhythms and ecosystems around the world, but in the case of plants, which rely on light for photosynthesis, their effects may only be discovered. Now, scientists writing in Frontiers in Plant Science have found that exposure to elevated levels of synthetic light at night is producing tree leaves and for insects to eat, threatening urban food chains.
“We found that, compared to herbaceous ecosystems, tree leaves in urban ecosystems sometimes show few signs of insect damage. We were curious why,” said corresponding Dr. Shuang Zhang of the Chinese Academy of Sciences. “Here we show that in two of the most common tree species in Beijing, synthetic light at night caused an increase in leaf hardiness and a reduction in leaf herbivory levels. “
Artificial light has a higher nighttime brightness, up to a maximum of 10%: the majority of the world’s population is faced with light pollutants every night. Since plant houses interact with other plants and animals, any changes in plants caused by synthetic light can simply have a significant effect on the ecosystem.
“Leaves that are not damaged by insects may provide comfort to people, but to insects,” Zhang said. “Herbivory is an ecological herbal procedure that maintains insect biodiversity. “
Scientists suspected that plants subjected to high levels of artificial light would focus on their defense rather than their growth, generating tougher leaves that contain more defense chemicals. To test this, they chose two common species of street trees: the Japanese pagoda and the green ash. trees. Although those trees are similar in many ways, Japanese pagodas have smaller, softer leaves that herbivores prefer.
The scientists met 30 sampling sites spaced about a hundred meters apart on main roads, regularly 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 that could 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 hardness and higher levels of chemical defenses such as tannins would imply resources allocated to defense. Meanwhile, higher levels of water and nutrients mean higher quality nutrition to tempt herbivores.
For either tree species, higher levels of synthetic light meant sturdier leaves. The more potent the leaf, the fewer symptoms of insect herbivory. The more intense the light, the more leaves the scientists found that showed no symptoms of herbivory.
“The mechanism behind this trend is not yet fully understood,” Zhang said. “It is conceivable that trees exposed to synthetic light at night would prolong the duration of their photosynthesis. In addition, those sheets could allocate a greater proportion of resources to compound structural tasks, 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 Japanese pagoda leaves contained more nutrients, there were more herbivores. But green ash leaves were more influenced by higher light levels: they had higher levels of nitrogen, smaller leaves. and weaker chemical defenses.
This may simply be because green ash trees are less appreciated by herbivores, so they can allocate resources to their growth. Meanwhile, Japanese pagodas are devoting more resources to their defense, reducing their nutrient content.
“The reduction in herbivores can cause cascading trophic effects on the ecology,” Zhang said. “A lower degree of herbivory means 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 global trend observed in recent decades. We pay more attention to this trend.
While leaf strength is a mechanical defense against predation, it’s possible that other aspects contribute to reducing herbivory: for example, greater softness can also make insects more visual to their predators. More studies will be needed to fully perceive the effects of the synthetic mild.
“Our study was conducted in a single city and looked at only two tree species,” Zhang said.
“This limitation hinders our ability to generalize findings to broader spatial and taxonomic scales. Research on how urbanization affects insects and insect-related ecological processes is still in its infancy. “
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