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The impact of light on forests

Light pollution is becoming more intense and could affect Brazilian ecosystems

Léo Ramos Chaves Vegetation near urban centers is most affected by the excess of artificial lightLéo Ramos Chaves

Populations in some regions of Brazil, such as the Amazon, have scarce public illumination. In most of that region, on clear nights, one can still see the band of stars in the Milky Way as clearly as one might perhaps have seen it before the arrival of Europeans to the Americas. A study published in the journal Science in June 2016 estimated that excessive artificial light at night might be preventing one-third of the world’s population from seeing the Milky Way. While entire populations of European countries are already deprived of naturally dark nights, light pollution in Brazil only reaches the same level in its large urban centers, concentrated along the coast. In a paper published in February 2017 in the journal PLOS ONE, researchers from São Paulo and the United Kingdom performed the first space-time assessment of the presence of artificial light on different types of Brazilian vegetation.

Biologist Juliana de Freitas and ecologist Waldir Mantovani of the Institute of Energy and Environment of the University of São Paulo (USP), in collaboration with the ecologists Jonathan Bennie and Kevin Gaston of the University of Exeter, in the United Kingdom, evaluated the percentage of area of each type of Brazilian vegetation exposed to artificial lighting or increases in luminous intensity over time. In order to do this, maps containing data on artificial illumination obtained from U.S. National Oceanic and Atmospheric Administration (NOAA) satellites from 1992 to 2012 were superimposed on maps detailing the 52 types of native vegetation mapped by the Brazilian Institute of Geography and Statistics (IBGE). In this way, the researchers obtained the percentage of area of each type of vegetation affected by detectable artificial light and by an increase in luminous intensity over 20 years.

The study published in PLOS ONE indicates that the area of each of these types of vegetation affected by artificial lighting varies from 0 to 25%. As expected, the less-affected vegetation is located in the less-populated regions of Brazil. The types of vegetation for which the percentage of area illuminated by detectable artificial light was close to zero were rocky outcrops and the edaphic savannas—vegetation that occurs on patches of poor, sandy soil in the Amazon, which can vary from fields to forests.

Being able to see fewer stars in the sky is the least of the harm caused by light pollution. Studies have confirmed the effects of excess artificial light on human health: it suppresses production of the hormone melatonin, which can cause anything from sleep disorders to diseases like diabetes and depression. Light pollution also affects the health and changes the habits of many animals, as well as altering the biological rhythm of plants.

As expected, the Atlantic Forest, the most devastated Brazilian ecosystem, is also the most subject to light pollution. Originally distributed in a narrow band along almost the entire coast of Brazil, from the states of Rio Grande do Norte to Rio Grande do Sul, this biome has already lost more than 70% of its native cover and its lands are now occupied by the most populous cities in Brazil. The two types of Atlantic Forest vegetation most affected by the artificial light of the Brazilian metropolises are the Restinga coastal woodland vegetation and the mangroves. “This result is not surprising,” says Freitas. “But no one had measured this impact.”

An earlier study by Bennie and colleagues using the same satellite data had already shown that, from 1992 to 2012, artificial illumination increased by 9% in the area occupied by mangroves on the planet. During the same period, 17% of the area occupied by Brazilian mangroves suffered from an increase in artificial illumination, almost twice the world average. Other coastal woodland vegetation, the Restinga, is even more affected. About 25% of the area occupied by this vegetation, consisting of low trees and shrubs that grow in acidic, nutrient-poor soils, is subject to artificial illumination. The Restingas are home to several species of bat, a mammal with very sensitive eyes that avoids light at all costs. Those who eat fruit spread seeds of various species of plants and collaborate to renew the forests near the Restingas (see article). With these animals repelled by the light, the forests that they abandon could suffer a loss of diversity. To understand the impact of artificial illumination, however, we must investigate further.

In temperate and tropical ecosystems, there are many species of insects that are irresistibly attracted by artificial lights. Brazil has immense diversity, especially of beetles. Some of them naturally emit light—they are bioluminescent, like fireflies—and cannot reproduce in the presence of intense artificial light. Other species use the star cluster of the Milky Way to orient themselves during night flights. The imbalance in the populations of these animals could generate consequences that are difficult to predict, since different beetle species play different, distinct roles in a forest, from organic matter decomposition to plant pollination, explains Brazilian entomologist Bruno de Medeiros, from Harvard University.

In a study published in 2016 in the Revista Brasileira de Entomologia, Medeiros, entomologist Sérgio Vanin of the USP Institute of Biosciences, and Italian anthropologist Alessandro Barghini, who completed his PhD in biology at USP and is an expert in light pollution, showed how different bulbs attract different beetle species. “The bulbs that emit more ultraviolet radiation attract more species of insects,” says Medeiros.

One solution to minimize the impact of light pollution on beetles, therefore, would be to use LED bulbs, which do not emit ultraviolet radiation. The problem is that the light emitted by the most common—white—LED lamps is the type that most interferes in the production of melatonin, fundamental for the health of vertebrates. “We should create multidisciplinary groups to study the subject,” recommends Barghini.

Scientific articles
FREITAS, J. R. et al. Exposure of tropical ecosystems to artificial light at night: Brazil as a case study. PLOS ONE February 8, 2017.
MEDEIROS, B. A. S. et al. Streetlights attract a broad array of beetle species. Revista Brasileira de Entomologia. December 14, 2016.