Subscribe
High levels of artificial light at night (ALAN) are making tree leaves tougher and harder for insects to eat, posing a threat to urban food chains, according to a new study.
Researchers from the Chinese Academy of Sciences found that ALAN can enhance leaf toughness while concurrently reducing leaf herbivory (feeding on living plant parts by insects and animals) levels in two of the most common street tree species in Beijing, China. This could potentially reshape the feeding preferences of herbivorous insects.
Light pollution, defined as the presence of unwanted, inappropriate, or excessive artificial lighting, can have numerous negative impacts. These range from disturbing migrating birds and newly hatched sea turtles to marring wilderness experiences and landscape beauty. Additionally, it poses risks to human health.
The study, published in in the journal Frontiers in Plant Science on August 5, 2024, highlighted that ALAN has species-specific effects on leaf nutrients, size and defence substances. These changes can significantly alter key functional traits and ecological processes like nutrient cycling and energy flow.
Plants experiencing high levels of artificial light focus on defence rather than growth, producing tougher leaves with more chemical defence compounds. In areas lit the brightest at night, the leaves were extremely tough, showing no signs of insect activity.
The study observed this phenomenon in two common street tree species in Beijing: The Japanese pagoda tree (Styphnolobium japonicum) and green ash (Fraxinus pennsylvanica). Researchers led by Yu Cao noted that in both species, ALAN led to higher leaf toughness and lower herbivory levels.
For Styphnolobium japonicum, herbivory decreased with increased leaf carbon content and leaf toughness, while it increased with leaf nitrogen content. Additionally, the interaction between specific leaf areas (SLA) and ALAN showed a significant negative effect on herbivory. The higher the SLA and ALAN, the lower the herbivory. For Fraxinus pennsylvanica, herbivory decreased with leaf toughness.
Leaf toughness, representing structural strength and mechanical defence, is one of the most crucial leaf traits. Higher leaf toughness implies intensified mechanical defence, hindering herbivore feeding and resulting in lower herbivory levels. Leaves with higher toughness also decompose at a slower rate, potentially affecting nutrient cycling negatively.
SLA can indirectly reflect the adaptive capacity of plants under different light intensities. High light conditions can limit the increase in SLA, leading to thicker leaves with reduced light capture efficiency and nutrition, which is less conducive to herbivore feeding, ultimately resulting in decreased herbivory. Overall, ALAN could potentially reshape the feeding preferences of herbivorous insects on leaves.
High ALAN intensity is detrimental to the energy flow from plants to higher trophic levels, posing a potential threat to the maintenance of biodiversity, including arthropod and bird diversity in urban ecosystems.
The 2016 report New World Atlas of Artificial Night Sky Brightness, published in Science Advances, found that more than 80 per cent of the world's population lives under light-polluted skies. The Milky Way is hidden from more than a third of humanity, including 60 per cent of Europeans and nearly 80 per cent of North Americans. A 2023 study published in the journal Science reported that the night sky is rapidly getting brighter, with citizen scientists documenting global rapid reductions in star visibility from 2011 to 2022.
There is a growing movement to establish “dark-sky places” --- areas independently certified to have a management plan to minimise light pollution. On January 11, 2024, the Pench Tiger Reserve in Maharashtra was designated as India’s first International Dark Sky Park, certified by the International Dark-Sky Association. It is the fifth such park in Asia, aiming to protect the night sky and promote astronomy.