Pollination threat from artificial light

Worsening night-time light pollution is hitting pollination rates and denting plant fruit yields, a new study has shown for the first time.

Artificial light at night - ALAN for short - is a worsening global problem which is growing, some studies suggest, at 6% per year. Despite this, the impact of nocturnal illumination on the behaviour of pollinators, which conservatively contribute nearly US$400 billion per year in productivity terms to the global economy, has never been studied. 

Now, Swiss scientist Eva Knop, working at the University of Bern, has shown that the effect is significant. Writing in Nature, she and her colleagues show that patches of meadow subject to light pollution through the night see a 62% drop in visits by pollinators and a corresponding 13% drop in plant productivity compared with identical patches of meadow that remained unlit. The range of pollinator species was the same, they found, but the total number was dramatically lower in the lit environments.

The team collected their data from 14 meadow sites that had never been subject to night-time illumination. In 7 of these sites mobile street lights were erected to illuminate the wild plants growing there while the other 7 sites served as controls. The researchers used nets to capture pollinators visiting the plant species, and also sampled the quantity of fruits produced by one common plant species, called a cabbage thistle, in 5 lit and 5 unlit sites.

Pollination visits and berry yields were significantly lower in the lit sites, although the researchers aren't sure why. Nor do they know why illumination at night should prevent the day-active pollinators from compensating and maintaining the fruit yield.

They speculate that the light may have directly affected the behaviour of noctural insect visitors, like moths, or was influencing the plants to render them less attractive under lit conditions. Night-active species tend to have highly sensitive vision, so flood-lighting could be saturating their ability to pick up on subtle plant signals that would normally attract them at night. And flood-lighting plants at night might in turn affect their attractiveness during the daytime.

This effect has been overlooked to date, say the team, who point out that the worldwide decline in pollinators numbers, and the plight of species like honeybees hit by colony collapse disorder, has received significant attention. But night-time light pollution, they say, "is a threat to pollination that is rapidly spreading globally..."