Street lights can make tree leaves tough, making them harder for insects to eat.

Urban trees lit by streetlights tend to have tougher leaves and be less eaten by insects than those that spend the dark nights, a pattern the researchers say could disrupt the flow of energy up the food chain and have negative effects on urban biodiversity.

Zhang Shuang and his colleagues at the Chinese Academy of Sciences say that Zhang is a Japanese five-story pagoda (Styphnolobium japonicum) and Green Ash (AshBeijing’s street trees appear to be relatively free of insect damage compared to other trees in the city.

The researchers collected around 5,500 leaves from 180 trees at 30 locations in Beijing, including near the distinctive orange glow of sodium streetlights and in dark areas at night, and measured the leaves’ size, firmness, moisture content and nutrient levels. They also recorded any evidence of insect damage.

Leaves taken from under streetlights were stronger and less affected by insects: for Chinese sophora trees, 2.1% of leaves were damaged in the lit areas and 5.3% in the dark, while for ash trees, 2% of leaves were damaged near streetlights and 4.1% in the dark.

The researchers couldn’t answer that question, but they did say in their paper that with fewer leaves for insects to eat, less energy flows up the food chain to insects and birds, which could have a knock-on effect of further reducing biodiversity.

The researchers acknowledge that the mechanisms by which leaf damage is reduced are still unclear and require further investigation — for example, increased light could make insects more visible to predators, reducing their numbers and their impact on trees.

Owen Lewis of the University of Oxford says the study is intriguing but doesn’t prove causation, and suggests that future studies should take plants from areas with and without street lighting, place them in controlled environments, and observe the insects’ behavior to see whether they prefer trees that grow in dark conditions.

Lewis also notes that measuring herbivores is complicated: Heavy damage can mean leaves are less nutritious, forcing insects to eat more of them, and holes made by insect damage can also get bigger as leaves get bigger, he says.

“My intuition is that this may be a fairly subtle effect,” he says. “In central Beijing, the subtle effects of light pollution on insect feeding are likely to be insignificant compared with the effects of the degree of urbanization, the degree of pollution, the degree of semi-natural habitat, etc. Light pollution is important, but it is not likely to be the main threat to insect diversity and ecosystem function.”