Study reveals effects of light pollution on ecosystem

Light pollution is a growing problem that is putting increasing pressure on natural ecosystems, a new Chinese study has revealed.

A study has shown that leaving street lights on all night causes tree leaves to become too tough for insects to eat, threatening the food chain.

Scientists noted that trees in urban ecosystems showed much less damage than those in rural areas.

“We noticed that compared with natural ecosystems, tree leaves in most urban ecosystems generally showed no signs of insect damage,” said study author Dr. Shuang Zhang of the Chinese Academy of Sciences. “We were curious to know why this was the case. In two of the most common tree species in Beijing, artificial light at night increased leaf stiffness and reduced leaf herbivory.”

The scientists tested two common types of street trees in Beijing: Japanese pagoda trees and green ash trees.

Japanese sophora trees have smaller, softer leaves that insects prefer to eat. Scientists believe that plants in areas with high levels of artificial light may focus on defense rather than growth, meaning their leaves will be tougher, with more defensive chemicals.

To test the trees, they found 30 sampling sites on major roads that are normally lit by street lights at night.

They measured the amount of light (illumination) at each site and then tested the trees' leaves for stiffness.

They tested nearly 5,500 leaves for properties, including size, stiffness, water content, nutrient levels and chemical defenses.

If the leaves are larger, it indicates that the plants have directed their energy into growing the leaves, but if they are tough and contain tannins or other chemical defenses, it indicates that they have devoted their energy to defending themselves.

They found that the more light there was, the tougher the leaves were. In the more lit areas at night, the leaves were very tough and showed no signs of insects chewing on them.

Although they don't fully understand why this happens, the researchers suggest that trees exposed to artificial light could extend their photosynthesis time.

Although a tree with virgin leaves may be more pleasant for some people to look at, it can be a bad sign for the ecosystem.

“Leaves free of insect damage may bring comfort to people, but not insects,” Zhang said. “Herbivory is a natural ecological process that maintains insect biodiversity. Decreased herbivory can lead to trophic cascade effects in ecology. Decreased levels of herbivory mean lower abundances of herbivorous insects, which in turn may lead to lower abundances of predatory insects, insectivorous birds, etc. The decline of insects is a global pattern observed over recent decades. We should pay more attention to this trend.”