Faint Sounds, Loud Impact

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

  • 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
  • This sound map created by the National Park Service depicts natural sound conditions, before human influence.
    Image courtesy National Park Service
  • This sound map created by the National Park Service depicts current sound conditions, including human influence.
    Image courtesy National Park Service

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.

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