The eureka moment for Bernie Krause, a bioacoustics expert, came when he was on the Masai Mara National Reserve in Kenya recording the natural ambient sounds of birds, animals, insects, reptiles and amphibians for the California Academy of Sciences. As a former player of the Moog synthesizer for George Harrison, the Doors and other 1960s rock musicians, he had made a spectrograph of a natural soundscape and realized that “it looked like a musical score,” he recalls. “Each animal had its own niche, its own acoustic territory, much like instruments in an orchestra.”

How well these natural musicians played together, Krause concludes, says good deal about the health of the environment. He argues that many animals evolved to vocalize in available niches so they can be heard by mates and others of their kind, but noise from human activity—from airplanes flying overhead to rumbling tires on a nearby road—threatens an animal’s reproductive success.

Since the late 1960s Krause has collected over 3,500 hours of soundscapes from Africa, Central America, the Amazon and the U.S. He finds at least 40 percent of those natural symphonies have become so radically altered that many members of those orchestras must be locally extinct. “Forests and wetlands have been logged or drained, the land paved over, and human noise included, making the soundscape unrecognizable,” says Krause, who heads Wild Sanctuary in Glen Ellen, Calif., an archive of natural sounds. Lately he has traveled to Katmai National Park and the Arctic National Wildlife Refuge to look for unpolluted sound and still had to get away from roads to find it.

Thomas S. Schulenberg, a neotropical bird specialist at Cornell University and one of the authors of The Birds of Peru, agrees that sound is a useful tool for assessing the natural environment. Schulenberg traveled to Vilcabamba, a wilderness of wet cloud forest in eastern Peru, which Conservation International wanted to access for possible protection. Although the ornithologist carried a pair of binoculars, he showed up to their dawn chorus with a directional microphone and recorder. As Schulenberg puts it: “You can hear many times more birds than you can see.”

Schulenberg believes animals can adapt to some noise pollution, but there are limits, especially if the noise becomes a permanent feature of the environment. Writing in the Journal of Animal Ecology, biologist Henrik Brumm of the Free University of Berlin found that male territorial nightingales in Berlin had to sing five times as loud in an area of heavy traffic. “Does that have effects on the musculature they need to sing?” Schulenberg wonders. “Can they sing even louder, or are they going to eventually hit a wall and be washed out by human noise?”

The U.S. National Park Service, under its Natural Sounds Program, wrestles with similar questions. Karen K. Trevino, the program director, cites studies showing that when exposed to the sounds of planes and helicopters, bighorn sheep forage less efficiently, mountain goats flee and caribou do not successfully reproduce as frequently. Senior acoustic specialist Kurt Fristrup of the National Park Service notes that human sounds cause problems other than acute annoyances. Namely, they can “mask some of the quieter yet important sounds of nature like footfalls and breathing—the cues that predators listen for to catch prey and that prey use to escape predators,” he says.

According to Krause, sound can also help determine how habitat destruction alters species populations. He did a 15-year study in Lincoln Meadow in the Sierra Nevada Mountains, a region that was selectively logged and of which loggers insisted there would be no change. Photographs showed little change, Krause found, but audio revealed a drastic drop in species diversity and density. Says Krause: “The transformation from a robust natural symphony to almost silent was quite alarming.”