By Stephen Beech via SWNS
Streetlights running all night are making leaves too tough for insects to eat, warns new research.
The problem is a threat to the food chain, say scientists.
Chinese researchers found that trees on streets exposed to artificial light at night have tougher leaves and are less likely to be eaten by insects, potentially damaging urban biodiversity
Light pollution disrupts body clocks and ecosystems around the world – but for plants, dependent on light for photosynthesis, its effects could be profound.
The study, published in the journal Frontiers in Plant Science, shows that exposure to high levels of artificial light at night makes tree leaves grow tougher and harder for insects to eat.
Corresponding author Professor Shuang Zhang, of the Chinese Academy of Sciences, said: “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.
“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 levels of night-time brightness by almost 10% with most of the world’s population experiencing light pollution every evening.
Because plant properties affect their interactions with other plants and animals, the researchers explained that any changes to plants caused by artificial light could have a “significant” impact on the ecosystem.
Zhang said: “Leaves that are free of insect damage may bring comfort to people, but not insects.
“Herbivory is a natural ecological process that maintains the biodiversity of insects.”
The researchers suspected that plants experiencing high levels of artificial light would focus on defense rather than growth, producing tougher leaves with more chemical defense compounds.
To test their theory, the team selected two common species of street tree: Japanese pagoda and green ash trees.
Although the trees are similar in many ways, Japanese pagoda trees have smaller, softer leaves which herbivores prefer.
The research team identified 30 sampling sites on main roads which are usually illuminated all night.
To determine the level of exposure to artificial lighting, the researchers measured illuminance at each site.
More than 5,000 leaves were collected and evaluated for insect herbivory and traits that could be affected by artificial light – such as size, toughness, water content, and levels of nutrients and chemical defenses.
Larger leaves would indicate resources allocated to growth, while toughness and higher levels of chemical defenses, such as tannins, would indicate resources allocated to defense.
Higher levels of water and nutrients indicated higher-quality nutrition to tempt herbivores.
The researchers found that, for both species of tree, higher levels of artificial light meant tougher leaves. And the tougher the leaf, the less evidence of insect herbivory.
The more intense the light, the more frequently scientists encountered leaves that showed no signs at all of herbivory.
Zhang said: “The underlying mechanism for this pattern is not yet fully understood.
“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.”
The team found that Japanese pagoda trees exposed to more artificial light had lower levels of nutrients such as phosphorus: where Japanese pagoda leaves had more nutrients, more herbivory occurred.
But green ash leaves were more strongly influenced by higher light levels: they had higher levels of nitrogen, smaller leaves, and lower chemical defenses.
The researchers say that could be because green ash trees are less preferred by herbivores, so they can afford to allocate resources to growth.
Japanese pagoda trees put more resources into defense, lowering their nutrient content.
Zhang “Decreased herbivory can lead to trophic cascading effects in ecology.
“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.”
Although leaf toughness is a mechanical defense against predators, the Chinese research team said that it is possible that other factors contribute to decreased eating of leaves. For example, more light could make insects more visible to their predators.
They say further research will be needed to fully understand the effects of artificial light.
Zhang added: “Our study was conducted in only one city and involved just two tree species.
“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.”
