Light pollution in areas around rainforests may discourage growth of the wild areas, due to its effect on fruit bat populations. 

Researchers from the Leibniz Institute for Zoo and Wildlife Research Berlin (IZW) conducted a study into the effect of artificial light on the flying mammals.

Daniel Lewanzik of the IZW directed a surprisingly simple test. In Costa Rica, he and his team divided a flight cage, containing Sowell short-tailed bats (Carollia sowelli), into two compartments. Each of the sides was stocked with the bat's favorite foods, including figs, nightshades and peppers. While one side was kept dark, the other was lighted by a sodium street lamp. This design is the most common type of outdoor light in the world. 

Bats flew into the dark side of the cage twice as often as they would go into the lighted compartment. Lewanzik concluded from his study bats avoid eating in areas illuminated by artificial light.

As a follow-up to that experiment, the researcher illuminated wild, ripe peppers with artificial light. He then recorded the rate at which the bats harvested the food from those plants, compared to a control group, of plants in a normal day/night cycle. He found bats harvested 100 percent of the ripe fruit kept under normal standard conditions, but they took just 78 percent of the illuminated vegetables. 

Rainforests depend on bats for pollination, as well as to spread seeds around the forest floor. As bats fly over the rain forest in vast numbers, their feces fall in a process known as "seed rain."

"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

Many areas of the rainforest are developed by humans, farmed, then abandoned when crops no longer grow. Bats are a leading source of seeds for these largely-barren areas. 

Beyond just the sheer size of the forest, a more vigorous rainforest also provides improvements to the quality of the land and soil near it, as well as numerous animal species. If the recovery of rainforests from fires or other disaster is hindered by a lack of bats, the effect could be severe. 

"This could have negative impacts on biodiversity and consequent effects on land erosion, particularly in developing countries of the tropics where light pollution increases rapidly with growing economies and human populations," researchers wrote in the article announcing their results.

Earlier studies showed bats who eat insects also avoid lit areas. Study of the effects of light pollution on fruit-loving bats was published in the Journal of Applied Ecology, managed by the British Ecological Society. 

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