Faint Sounds, Loud Impact

At the edge of our industrialized wildlands, Kurt Fristrup is listening.

Studying Sound

A team of bio-acousticians from the National Park Service's Natural Sounds and Night Skies Division records the sounds of Yosemite National Park during a snowstorm.

Photo courtesy National Park Service

Content Tools

Before he joined the National Park Service in 2005, Kurt Fristrup was the assistant director of the bioacoustics research program at Cornell University’s Lab of Ornithology. Bioacoustics is the study of sound as it relates to animals, including humans, but over the past several decades, scientists have found that sound also affects plants. Fristrup is now a senior scientist in the park service’s Natural Sounds and Night Skies Division, which has headquarters in Fort Collins, Colo., and in 2015 released a series of “sound maps” of the United States.

The basic idea behind managing sound is that it can be harmful to wildlife and disruptive for humans. What have we learned about how sound actually affects the ecosystem?

The thing that connects night skies and sound, and possibly some other disciplines, is what you might call sensory ecology. Our senses are what connect us to each other, to other organisms, and to the physical environment. At its foundation, the issue isn’t just whether we perceive noise and are irritated by it, but the extent to which noise or light pollution has altered the fundamental conditions that allow us to perceive each other and other things in the environment. When noise raises background sound levels, there are a lot of faint sounds that can no longer be heard, and there’s good evidence in ecology that those faint sounds really matter.

There’s been a lot of research showing that male songbirds change the pitch—the frequency and the amplitude—of their songs in noisy environments. And that is one legitimate concern, but there are also all the sounds that animals make without intending to, the sounds of movement, the sounds of breathing. These sounds are unintentional, they’re not repeated, and they’re generally a lot quieter than intentional vocalizations. Losing the capacity to hear those subtle sounds may very well be the most important issue with introducing noise.

Why is that? Because those types of sounds are what allow animals to avoid predators?

Exactly—to avoid predators, to find each other, to find food. There are some animals like owls that we know can find their food in total darkness just by sound. They can not only find it, they can locate it and catch it. There’s a BBC film clip of this fox up in Yellowstone creeping across the surface of the snow, leaping in the air, and diving headfirst into the snow. And he comes up with a gopher in his mouth. Accidental sounds are playing two roles there: The fox is walking along very, very quietly to make sure the gopher doesn’t hear him coming, and it is spatially locating the gopher well enough that it can leap in the air and dive down and hit the right spot.

Those kinds of things are happening around us all the time. It’s very likely that the unintended sounds of nature tie this entire food web together. When we start injecting—especially chronic noise, sounds that are there all the time, like a chronically busy street or a busy airport, those things have effects at very long ranges because these sounds are so subtle, it doesn’t take much noise to mask them.

What about plants? Plants don’t have ears, but is flora also affected by noise?

Yes. There’s a researcher out of Cal- Poly San Luis Obispo named Clint Francis who’s been able to show that because animals are affected, there are some fairly immediate effects on plants. For example, pinyon jays do not like noise, so they tend to vacate areas where there’s loud energy-production noise. Because pinyon jays are the primary disperser of pinyon pine seeds, there’s a lack of pinyon pine recruitment in noisy areas. So over time, the forest structure in noisy areas has become more sparse. On the other side of it, hummingbirds actually nest more successfully in noisy areas because their main nest predators are driven away. So bird-pollinated plants do better in noisy areas. And this is not speculated; it’s been measured.

What do we know about sound’s effect on humans? Does noise actually cause us harm?

Humans, just like wildlife, evolved very sensitive ears—not as sensitive as some, but we have quite good hearing, far better hearing than we need to talk to each other, for example, or to listen to music. So all those subtle cues of nature, at least historically, were really critical for us as well, and I would say when you’re in a park, they’re still critical. Your ears can clue you into the most interesting wildlife viewing opportunities. You can hear the subtle sounds that would tell you that there’s some animal nearby, which, if you’re listening to your favorite music on your earbuds, you’d miss entirely. You could also say there’s a safety issue there. For animals that might pose threats to people, like bears, or if you’re hiking and you’re crossing roads, being able to hear is a safety issue.

On the health side of things, there are two pieces to it. There’s a lot of literature showing that exposure to quiet, natural sound environments has a restorative effect on people. It helps relieve stress; it helps improve memory. Then there are costs. If noise levels get high enough, they can begin to cause elevated blood pressure levels, which, if you’re chronically exposed to higher noise levels, can lead to lifetime health effects.

In 2015, you released sound maps of the United States, based on recordings made in parks all across the country. What was the goal of this soundscape project?

We can’t manage parks like islands. We need to create tools that allow us to work with other federal, state, and local agencies to sort of manage resources on landscape scales and with a large, cooperative framework. So we wanted to be able to extend what we learned inside parks all the way across park boundaries so that we could have a conversation with others about resource conditions and what we might collectively do about it.

How did you create the maps?

The 600 sites we monitored have been expressly chosen because they capture some sort of unique set of environmental characteristics in each park setting. Well, there are now many data layers available for the United States that describe things like elevation or hydrology or light pollution or road density or population density. So you take all of these nonacoustic factors, you calculate some summaries of those factors around each monitoring site, and then you build a model that predicts the acoustic measurements. Then you can take that model and sort of paint the entire United States.

What did you learn?

Well, one interesting thing is that because we knew which factors were derived from human activities, we could actually look at a scenario where we set all the human factors to their minimum values and predict what sound levels would be like in the absence of human activities. The big pattern that emerges from the map of natural conditions is that water tends to increase natural sound levels. The more water, the more precipitation, the higher the stream density, the higher the natural sound levels become. The lowest natural sound levels in the United States are found in places that are dry and have very little water and very little vegetation, which is basically the Intermountain West.

That makes sense. People characterize a rain forest as being a naturally loud place.

Right, and some of the bottomland river forests of the southeast United States approach rain forest in the richness of their vegetation and the richness of their acoustic chorus. The other interesting thing about the maps is that noise is not confined to populated areas. When you look at our maps, you can see that noise is spread across the continent not only by the interstates and roadways but by energy developments and other human activities. If you were to look at a map of just population, you might assume that our impacts were confined to relatively densely populated areas, but when you look at both the spread of light pollution in terms of the light domes projected by cities and the spread of noise generated by transportation networks—what we might call the industrialized wildlands—you can see that we’re having effects far outside our communities.

What has the National Park Service done to actively promote and preserve natural soundscapes?

When the Denver International Airport was redesigning its approach and departure plans—the routes by which aircraft come in and out of an airport—it reached out to Rocky Mountain National Park and asked if there were things it could do that would reduce noise impacts to the park. And working with the park service, the airport actually found a way to consolidate what had been three separate inbound flight lanes from the West Coast into one, so that the traffic over the park was just in one stream. And it placed that stream more or less over Trail Ridge Road, which is the road that runs east to west through the park.

So essentially, we’ve managed to concentrate the aircraft noise along the road corridor where other transportation noise is already present. And that’s resulted in a significant reduction in aircraft noise, both down south in Wild Basin and up north in the Mummy Range. These changes didn’t place huge burdens—they wouldn’t have happened if they had huge impacts on fuel consumption or flight schedules. It was just a matter of factoring in natural resource concerns into the intelligent planning of air space utilization.

One thing it seems that light and sound pollution share is that humans sometimes register them physiologically but not always consciously.

People talk about shifting baselines all the time, and these are two areas where people’s experience can cause them to change their standards about what they expect. One concern is that if our communities continue to get brighter and noisier over time, as each new generation comes in, they won’t even realize what they’re missing. It’ll be all they’ve ever known. And it’s possible that when they come to parks, if we are successful in making them darker and quieter, they’ll become increasingly strange places.

It’s hard to know—now we’re out on the bleeding edge of speculation. What we might hope is that noisier and brighter communities would lead to higher value being placed on those places that remain quiet and dark because they’d be places for respite and restoration and appreciation. But it might not work that way. It’s possible that as people’s expectations change, what they’ve become accustomed to in their communities, they’ll increasingly tolerate in parks instead of expecting us to preserve these relatively pristine conditions.  


Timothy A. Schuler writes about landscape architecture, ecology, and urban design. He lives in Honolulu. Reprinted from Landscape Architecture Magazine (December 2015), the monthly magazine of the American Society of Landscape Architects.