Light Pollution Keeps Rainforest Bats from Eating and Disbursing Seeds

Nocturnal, seed-disbursing bats are being thwarted from preforming their role in supporting the growth of rainforests because of an increasing amount of light pollution, according to a new study.

Writing in the Journal of Applied Ecology, a team of scientists from the German Leibniz Institute for Zoo and Wildlife Research in Berlin (IZW) reports that seed-distributing bats avoid feeding in areas where there is light pollution.

Light pollution is an overabundance of excessive, obtrusive or misdirected artificial light. In many tropical countries, light pollution is increasing rapidly as human populations grow, the researchers said in a statement.

Led by Daniel Lewanzik from the IZW, the researchers theorized that as more and more areas become saturated in artificial light, bats in these tropical areas will avoid them, thus robbing the area of bats' natural function as a pollinator and a seed distributor.

To test this they ran an experiment using Sowell's short-tailed bats, a type of fruit bat common in tropical forests. Lewanzik and his team gave the bats a simple choice: choose food in a light or dark cage. The bat's cage was partitioned, with one section being naturally dark and the other section being lit by a sodium street lamp, the most common form of street lighting in the world.

Either side of the partition was stocked with the bats' favorite treats: pepper plants, nightshade and figs.

The experiment revealed that the bats flew into the dark compartment twice as often as they flew into the artifically lit one.

In a second experiment, Lewanzik illuminated pepper plants growing in the wild with a common street lamp and measured the amount of fruit harvested from the light-bathed plant to others nearby that were not bathed in artificial light.

"While bats harvested 100 percent of the marked, ripe fruit from the plants in the dark, only 78 percent were taken from the lit plants," the researchers said in a statement. "Although insect-eating bats have been shown to avoid foraging in light-polluted areas, this is the first study to show that fruit-eating bats also avoid lit areas."

The findings have important implications for forest regeneration in the tropics, the researchers said.

"In tropical habitats bat-mediated seed dispersal is necessary for the rapid succession of deforested land because few other animals than bats disperse seeds into open habitats," Lewanzik said.

This seed-disbursal occurs as the bats defecate mid-flight, producing what the researchers called a "copious seed rain"

Lewanzik said this seed rain, which is beneficial to forest growth, is likely reduced by light pollution. As a solution, Lewanzik suggested changes in lighting design could be the answer.

 "The impact of light pollution could be reduced by changes in lighting design and by setting up dark refuges connected by dark corridors for light-sensitive species like bats," he said.