A Listening Party for Nature

Scientists use many techniques to study natural habitats, but they rarely listen to nature.

To some biologists, sounds are more than an aural accompaniment to field trips: They convey information that traditional field surveys are hard-pressed to uncover. Now scientists are using sounds to tap into the pulse of ecosystems and monitor their health.

"The microphone is one of the most important biological sensors ever invented," said Michigan State University ecologist Stuart Gage. "It just hasn’t been properly used for interpreting ecosystem dynamics."

Scientists already study the sounds of individual animals, but such research focuses on particular species rather than entire ecosystems. By analyzing recordings made by microphones arrayed throughout habitats, Gage and other bioacousticians are looking for new representations of species balances and relationships.

The techniques and methods are still being refined, but audio tools could make it possible to easily conduct otherwise-difficult long-term studies of biodiversity, animal behavior, human environmental impacts and the effects of climate change on nature.



"I want to interpret the quality and character of the ecosystem," said
Gage. "I’m not particularly interested in species. I’m interested in the biodiversity, species timing, habitat disturbance and communications issues. It’s using sound as a metric to look at ecosystem dynamics."

Gage started his career as an ornithologist trained to identify birdcalls. Eventually he realized that bird song was only a part of nature’s symphony. "Acoustics were sending me a signal about the health of the environment and the type of ecosystem I was in," he said.

Sometimes these signals are otherwise hidden. In 1988, Gage and bioacoustics pioneer Bernie Krause — father of the term "biophony" and originator of the niche hypothesis, according to which nature’s audio spectrum is finely divided between species — recorded a patch of Sierra forest scheduled to be selectively logged.


We were told it would have no impact on organisms," said Krause. "To our eye and to the camera, it looks like it didn’t." But when they went back the next year and in 2003, "the sound had changed. It’s depressing to hear how little there is now. The diversity and density was gone, and it’s still gone today."

Gage and his students are now deploying solar-powered microphones in plots throughout the Muskegon River watershed. The results are sent wirelessly to a central computer, synchronized and analyzed.

"Rather than going into the field and using traditional, labor-intensive survey methods, you can deploy these in a fixed grid for a long time," said Bryan Pijanowski, a Purdue University forestry professor and Gage’s former student.



Gage and Pijanowski use the diversity of audio patterns as a surrogate for biodiversity, or the richness of life in an ecosystem — a vital indicator of environmental health, but difficult to measure, requiring meticulous and necessarily imperfect visual observations.

"We have very few long-term studies on these sorts of things, very little information, especially across large spatial scales," said
Pijanowski. "We’ve missed this whole dimension that we could measure that makes it easier to look at and monitor biodiversity across landscapes and long periods of time."

Even at smaller geographical and chronological scales, this is difficult. Almo Farina, an ecologist at Italy’s Urbino University, uses sound recordings to study birds; traditional methods, he said, "are strongly biased by the human capacity to recognize birds, to maintain attention for a long time, and by human intrusion.


Images: A recorder set up by Stuart Gage and Bernie Krause in Sequoia National Park; Bryan Pijanowski; proposed sensor grid in the Muskegon River watershed, courtesy of Stuart Gage; a visualization of robin calls in a one-hectare plot of beech forest in the Appenines, from Almo Farina; sonograms from Borneo, St. Martin and the Amazon Basin from Bernie Krause’s "Loss of Natural Soundscapes Within the Americas."

Farina’s own recordings — made by a grid of field sensors and a recorder trawled from a cable car across an Appenine mountainside — let him explore the relation between bird activities and the surrounding soundscape.

"I want to understand how acoustic activity in one site is connected with the song activity of another site," he said. "Some birds sing only after another species becomes silent, and vice versa. When you find acoustic overlap, this means that the community could be affected by some habitat disturbance."

Nature Conservancy ecologist Patrick Gonzalez said that bioacoustics sounds promising, though "it would require extensive field surveys to validate."

"What we’re talking about here is so new and out of the box that most ecologists look at me and say, ‘Huh?’" said Pijanowski.

Some ecologists want to construct a nationwide natural audio surveillance system, said Pijanowski. He’s already deployed sensors around his home in Tippecanoe County, Indiana.

"It’s a canary in a cage tool," he said. "Those signal potential disturbances that might alert a natural resource manager. They could monitor ecosystems in real-time."