Over the last two decades, night-time light emissions in North America and Europe have increased by more than 70 percent. The artificial light lures moths and other insect pollinators away from plants, a new study shows. This effect may also make daytime pollinators less efficient, posing a further threat to plants and global food security.
By Annie Roth
Populations of bees, bats, butterflies and other pollinators have been declining for decades due to habitat loss, disease, pesticides and climate change. Now, scientists have documented yet another threat to pollinators: night-time light pollution.
In a recent study in Nature, ecologists showed that plants growing near streetlights were pollinated far less often at night and produced fewer fruits than their unilluminated counterparts. In turn, this may compromise the efficiency of daytime pollinators in the same fields, the authors conclude.
“Even though daytime pollinators are usually more numerous than nighttime pollinators, they were unable to make up the difference in lost pollination of plants kept under artificial lighting,” said Eva Knop, an ecologist at the University of Bern in Switzerland and the study’s lead author. “Some studies have shown that nighttime pollinators seem to be more effective at transferring pollen between plants than their [daytime] counterparts,” Knop told Mongabay.
Scientists estimate that one-third of all cash crops depend upon animal-mediated pollination. Many plants receive most of their pollination after dark, especially in tropical and desert climates. These plants attract nocturnal pollinators by producing alluring fragrances and copious amounts of nectar.
Unfortunately, pollinators drawn to lights, such as moths, find artificial light more tantalizing than nectar. Such nighttime emissions have increased by more than 70 percent in North America and Europe over the last two decades, particularly in residential areas, according to published estimates.
To determine if night lights affect nocturnal pollination, Knop and her team found 14 ecologically similar cabbage thistle (Cirsium oleraceum) meadows in the Alpine foothills of Switzerland and set up mobile street lights in seven of them. Using night-vision goggles, the researchers closely monitored the behaviour of nocturnal pollinators in both the dark and illuminated meadows.
Light exposure reduced the number of pollinator visits by 62 percent, the team found. And while nearly 300 species of insects visited plants overnight in the dark fields, 29 percent fewer species came to the lighted fields, the results showed.
The team also measured how much fruit was produced by cabbage thistles under the two treatments. They found that illumination after hours caused a 13 percent reduction in cabbage thistle fruit production, which they attribute to decreased pollination.
The team’s analysis suggests that flower pollination during the daytime is more effective for plants visited by a greater number of nocturnal pollinators. As a result, lights at night may also drag down pollination by bees and other daylight visitors. How that connection might happen isn’t yet understood, Knop said.
Pollination is a fundamental ecosystem service that provides food, shelter and habitat to hundreds of thousands of species, so these nighttime impacts concern ecologists. “The effect of widespread use of nighttime artificial lights worldwide could have ecological and evolutionary effects that ripple through food webs in ways that we cannot yet predict,” John N. Thompson, distinguished professor of ecology and evolutionary biology at the University of California, Santa Cruz, who was not involved in the study, said in an email.
Indeed, the most severe impacts of light pollution on pollinators and other light-sensitive species have yet to be seen, Thompson and other ecologists worry.
Researchers point to several steps that citizens can take to help insects and other pollinators in their neighbourhoods: keep outdoor lighting off at night, plant more flowers that bloom at different times of the year, and never use pesticides.
CITATION – Knop, E., Zoller, L., Ryser, R., Gerpe, C., Hörler, M., & Fontaine, C. (2017). Artificial light at night as a new threat to pollination. Nature, 548(7666), 206-209.
(Annie Roth (AnnieRoth_AtSea) is a graduate student in the Science Communication Program at the University of California, Santa Cruz. Other Mongabay stories produced by UCSC students can be found here.)