Streetlights may make tree leaves tough and hard for insects to eat

Urban trees lit by streetlights tend to have tougher leaves that are eaten less by insects than trees that see dark nights. The researchers who discovered the pattern say it could have a detrimental effect on biodiversity in cities by preventing the flow of energy up the food chain.

Shuang Zhang and his colleagues at the Chinese Academy of Sciences decided to investigate the effect of artificial light on trees after Zhang noticed that Japanese pagoda (Styphnolobium japonicum) and green ash (Fraxinus pennsylvanicaa) trees on the streets of Beijing seemed to suffer relatively little insect damage compared with other trees in the city.

The researchers collected around 5500 leaves from 180 trees at 30 sites in Beijing, some near the characteristic orange glow of sodium streetlights and others where nights are dark, and measured their size, toughness, water content and nutrient levels. They also recorded any evidence of insect damage.

Leaves taken from under streetlights were tougher and had less insect damage. On Japanese pagoda trees, the amount of leaf damage was 2.1 per cent in areas with streetlights and 5.3 per cent in dark areas, while the amount of leaf damage on green ash trees was 2 per cent near streetlights and 4.1 per cent in dark areas.

The researchers weren’t available to answer questions, but they say in their paper that a decrease in leaves eaten by insects will mean less energy flowing up the food chain to insects and birds, leading to a cascading effect that further reduces biodiversity.

The team acknowledges that more investigation is needed because the mechanism leading to lower leaf damage isn’t yet understood. It could be that more light makes insects more visible to predators, reducing their numbers and therefore their effect on trees, for example.

Owen Lewis at the University of Oxford says the study is interesting but doesn’t show a causal link. He suggests future studies should take plants from areas with and without streetlights, place them in a controlled environment, and then observe insect habits to see if they show a preference for trees grown in dark conditions.

Lewis also points out that measuring herbivory is complex: more damage may mean a leaf has lower nutritional value and insects therefore have to eat more of it. Holes caused by insect damage can also grow as the leaf grows in size, he says.

“My hunch is that this could be quite a subtle effect,” he says. “In the middle of Beijing, the subtle effects of light pollution on insect herbivory, compared to the effects of how urbanised [the area is], how much pollution there is, how much semi-natural habitat there is – it’s likely to be trivial. It’s important, but it’s probably not the principal threat to insect diversity and ecosystem function.”