John Metcalfe was CityLab’s Bay Area bureau chief, covering climate change and the science of cities.
And it's probably not in the way you think.
When picturing the killers of rainforests, one might imagine sweaty loggers swinging big chainsaws.
But maybe we should consider another possible destructive force: Municipal work crews, who make the forest's periphery safer and easier to walk by installing street lights.
Two Berlin researchers, Daniel Lewanzik and Christian Voigt, have spent months in Costa Rica investigating how light pollution could be altering the health of a rainforest. They were curious about the effect this ever-growing source of illumination has on fruit-eating bats, like these handsome guys:
These nocturnal animals are an important source of pollination and also seed dispersal, as they literally poop out new generations of forest (and what have you done for the environment recently?). Throughout South America and parts of Central America, the bats encourage the growth of hundreds of plant species, ranking second only to birds as the most crucial agent of seed-spreading.
Fruit-eating bats also happen to be extraordinarily photophobic, so much that they'll change their feeding behavior if the moon gets a bit too bright. So Lewanzik and Voigt set up an experiment in a natural preserve to see if the bats were repeled by sodium-vapor street bulbs, a cheap form of illumination that dominates more than three-quarters of the world's light market. Their observations revealed that the bats were twice as likely to enter a dark environment than a lighted one. Also, they were only half as likely to eat fruit in a well-lit space.
If street lights are cutting down on the bats' ability to drop "seed rain" – the delightful term for their flying poos – that doesn't bode well for areas of rampant deforestation. Not many other seed-spreading animals are as likely as bats to cross an area that's recently been clear-cut. The researchers offer some not-pretty implications for the future in their full study:
Problems associated with artificial light may become even more aggravated on a larger geographical scale, considering that light pollution is increasing rapidly at an annual rate of about 6% world-wide (Hölker et al. 2010). Since the degree of light pollution parallels population growth and economic development (e.g. Elvidge et al. 2001), it can further be expected that artificial light at night increases at exceptionally high rates in many tropical countries. For example, the outdoor lighting market in Latin America is estimated to nearly double between 2010 and 2020 (Baumgartner et al. 2011). Due to the exponential growth rate of human populations in many tropical countries (UNPF state of world population 2011), people will encroach further into formerly pristine habitats than ever before. Since this encroachment is probably accompanied by an intensified use of artificial light, it might have deleterious consequences for nocturnal seed dispersal and habitat connectivity.
To help the stealthy creatures continue their noble duty, the researchers suggest that governments crisscross rainforests with protected "darkness corridors." That way, the bats might be able to swoop through the trees on a kind of shadowy superhighway, maintaining the health of the tropics one little turd at a time.