How acoustic ecology offers insight into forest health

One recent morning, awake far too early and not by choice, I heard the day’s first cicada. As a chorus, it’s a familiar late-summer sound to me, but the solo was startling and took me several seconds to recognize. Soon others joined in, and the songs merged to the white-noise background buzz I knew. A tūī (Prosthemadera novaeseelandiae, an endemic New Zealand songbird) began to chuckle and chime; the earliest commuter on my street started their car. 

As the soundscape became more complex, each of its components gave me snippets of information. They told me about the time of day, the turn of the season, the presence of certain flowers (which attract tūī to my garden), my neighbor’s schedule and more. 

While many of us live in sight-focused cultures, sound is critical to how we experience and assess the environments around us. Often, we do this quite subconsciously – we might feel soothed by a steady thrum of raindrops on the roof or stressed by the sounds of accelerating vehicles and sirens, without necessarily noticing that we are doing so.

Indigenous cultures often use sound to gauge the health of their ecosystems in a holistic manner. The Tūhoe Tuawhenua, a Māori tribal group based in the Te Urewera region of New Zealand’s North Island, has developed its own culturally-appropriate biodiversity indicators for forest health, which include audible signs such as the quality of the river’s waiata (song) and the thundering noise of large flocks of kererū (native woodpigeon, Hemiphaga novaeseelandiae) beating their wings as they pass overhead.  

Now, acoustic ecologists are using sound recording equipment and software algorithms to gain this kind of information on forest health in a more comparable and scalable fashion, which can then be used by local people, NGOs, and governments to track the impacts of different interventions and developments. “Working to protect tropical forest biodiversity, I was always really frustrated with not being able to measure that biodiversity as comprehensively as I wanted to,” says acoustic ecologist Zuzana Buřivalová, who directs the Sound Forest Lab at the University of Wisconsin-Madison. “Recording soundscapes seemed a perfect way to basically be in many places at the same time.” 

The process is relatively simple: the scientists strap brick-sized bio-acoustic recorders onto trees at regular intervals. The recorders are pre-programmed to record at selected intervals, such as at certain times of the day. When the recording period is complete, the researchers gather and analyze the data, using machine learning and algorithms to recognize different species where possible, and calculate the complexity and diversity of the soundscape as a whole: generally, a more diverse soundscape suggests greater biodiversity. There are plenty of species that the soundscapes miss, such as plants and non-noisy insect varieties, but they do provide a fast, cheap, and wide-reaching means of getting a general sense of biodiversity levels. 

Buřivalová’s research team have found that in Indonesia and Papua New Guinea, human interventions like selective logging are making forests quieter at specific times of the day. Now, they’re trying to work out whether these patterns hold true in other tropical forests, such as in Gabon, the Congo Basin, Mexico, Sierra Leone and Liberia. “We want to see if we can generate a universally-valid way to analyze the data that would let people who, for instance, want to source timber sustainably, to compare the impact of various logging companies on biodiversity and forest health in the areas in which they work,” says Buřivalová.

Because she spends so much time listening to forests, I imagined that Buřivalová’s chosen career might have honed her own hearing for the better. But she thinks it’s more the other way around. “I’ve always been this way: I can hear a lot more than other people seem to,” she says. “I’m lucky that I’ve been able to put that ability to use.”