Streetlights left on all night cause leaves to become so tough that insects cannot eat them, threatening the food chain, a study has found.
Scientists noticed that trees in urban ecosystems showed far less damage than those in more rural areas. Their research, published in Frontiers in Plant Science, has shed light on a potential reason.
“We noticed that, compared with natural ecosystems, tree leaves in most urban ecosystems generally show little sign of insect damage. We were curious as to why,” said the study’s author, Dr Shuang Zhang of the Chinese Academy of Sciences. “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 from streetlights had a surprising effect on the leaves of trees surveyed. The scientists tested two common species of street tree in Beijing: Japanese pagoda and green ash trees. The pagoda trees have smaller, softer leaves that insects prefer to munch on. Researchers thought plants in areas with high levels of artificial light may focus on defence rather than growth, which would mean their leaves would be tougher, with more chemical defence compounds.
To test the trees they found 30 sampling sites on main roads that are usually lit by streetlights all night. They measured the amount of light – illuminance – at each site and then tested the leaves of the trees for toughness. They tested almost 5,500 leaves for properties including size, toughness, water content, and levels of nutrients and chemical defences.
If the leaves were larger, this would indicate that plants directed their energy to growth of leaves, but if they were tough and contained tannins or other chemical defences this would indicate that they had allocated their energy to defend themselves.
They found that the more illuminance there was, the tougher the leaves. In the areas lit the brightest at night, the leaves were extremely tough and showed no sign of insects munching on them.
Researchers said that while they did not completely understand why this happened, they suggested trees exposed to artificial light could extend their photosynthesis duration.
Though a tree with pristine leaves may be more enjoyable for some people to look at, it could be a bad sign for the ecosystem.
“Leaves that are free of insect damage may bring comfort to people, but not insects,” said Zhang. “Herbivory is a natural ecological process that maintains the biodiversity of insects.”
Zhang added: “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.”