There are thousands of parks, refuges and wilderness areas in the U.S. that are kept in something close to their natural state. But one form of pollution isn't respecting those boundaries: man-made noise.
New research based on recordings from 492 protected natural areas reveals that they're awash in noise pollution.
Researchers from Colorado State University spent years making the recordings by setting out microphones in natural areas across the country. They caught all sorts of wildlife sounds, such as rutting elk and howling wolves. But they were also after "background" sound — wind, rain, birdsong, flowing streams and rivers, even bubbling mudpots in Yellowstone National Park.
They compared the decibel level of this natural background with the intrusive noisiness from human activity. And they have discovered that in two-thirds of the places they studied, the median decibel level of man-made sound was double the normal background sound. These were sounds that came from within the area, such as road traffic, as well from as outside, such as passing jets or mining and logging equipment.
And in more than 20 percent of the areas studied, the unnatural sounds were 10 times as loud as the natural background. The result is "fairly shockingly high levels of noise pollution within protected areas," says Rachel Buxton, lead author of the study, which appears in the journal Science.
Buxton, an ornithologist, points out that adding just a few decibels of artificial background sound, like that of a jet plane, makes it harder to hear what's around you: a stream, wind through the trees or wildlife. Even with an increase in unnatural sound of 3 decibels, "if you could hear a sound before at 100 feet, now you can only hear it from 50 feet," she explains.
The National Park Service is well aware of the problem. It has a scientist dedicated to studying noise in parks: biologist Kurt Fristrup. He says people who are used to urban soundscapes might not know what they're missing.
"It can make for an immersive experience when you listen carefully enough," he says, "to hear all those sounds that the animals around you are also attending to."
For people, listening to natural sound may be optional. But animals have to listen — for danger from predators, for example. Or conversely, predators such as owls need to hear their prey, like the rustling of a mouse in the leaf litter. Hearing well means surviving, Fristrup says.
"For these animals, it's literally a matter of life and death if they miss these subtle sounds of nature: the sound of a footfall or the sound of another animal breathing," he says.
The researchers found some good news: Most national parks and larger wilderness areas are quieter than U.S. Forest Service or other federally protected lands or state parks. That's because those parks are generally larger and more remote. And not surprisingly, the protected areas farthest from urban centers are the quietest.
The team says there are solutions: keeping roads and road traffic to a minimum, for example, or controlling where and when overflights take place.
RACHEL MARTIN, HOST:
There are thousands of parks and wilderness areas in the U.S. that are kept in near pristine condition...
(SOUNDBITE OF BIRDS CHIRPING, VEHICLE PASSING)
MARTIN: ...Except for the noise, a form of pollution that has no boundaries. The sound of cars, planes and machinery is almost everywhere. NPR's Christopher Joyce reports on new research that evaluates the impact of all the sound interrupting the silence.
CHRISTOPHER JOYCE, BYLINE: Rachel Buxton knows this sound. She's a wildlife biologist who spends a lot of time in the Mountain West.
(SOUNDBITE OF ELK CALLING)
RACHEL BUXTON: And is a bit spooky the first time you hear it. But, I mean, there's nothing like it.
JOYCE: It's an elk calling in Rocky Mountain National Park in Colorado, not far from Colorado State University where Buxton does her research. Buxton's team has recorded sounds like these and also background sound - wind, rain, birdsong - in hundreds of protected areas. And they've discovered that in two-thirds of those places, the decibel level of human-made sound was double the normal background sound on average. Here's an example - a jet passing over the park where that elk call was recorded.
(SOUNDBITE OF JET ENGINE)
JOYCE: In many places, it was 10 times as loud as the background.
BUXTON: So fairly shockingly high levels of noise pollution within protected areas.
JOYCE: Buxton points out that adding just a few decibels of artificial background sounds, like that jet, means it's harder to hear what's around you, a stream or wind through the trees or wildlife.
BUXTON: So if you could hear a sound before at a hundred feet, now you can only hear it from 50 feet.
JOYCE: The National Park Service is well aware of the problem. They have a scientist dedicated to studying noise in parks, biologist Kurt Fristrup. I met with him last year in another Colorado park. He says people who are used to urban soundscapes may not know what they're missing.
KURT FRISTRUP: It can make for an immersive experience when you listen carefully enough to hear all those sounds that the animals around you are also attending to.
JOYCE: For people, listening may be optional, but animals have to listen for danger or for prey. Hearing well means surviving.
FRISTRUP: For these animals, it's literally a matter of life and death if they miss these subtle sounds of nature, the sound of a footfall or the sound of another animal breathing.
JOYCE: Writing in the journal Science, the researchers found some good news. National parks and larger wilderness areas are quieter than forest service or other federally protected lands. And not surprisingly, protected areas the farthest from urban centers are the quietest. Christopher Joyce, NPR News.
(SOUNDBITE OF THRUPENCE'S "FOREST ON THE SUN") Transcript provided by NPR, Copyright NPR.