Wildfire smoke, which contains harmful particles and toxic gases, is widely regarded as an acute threat to human health. As wildfires become more frequent and intense with worsening climate change, this knowledge has only become more widespread. This is especially true for North Americans who have recently lived through some of the most devastating wildfire seasons in living memory — like in 2020, for example, when so much smoke blanketed the California Bay Area that it blocked out the sun and changed the afternoon sky. in a dull shade of orange.
But wildfire smoke harms so much more than just human lungs. For example, poor air quality can also causing trees to close the pores from which they release oxygen. Bird watchers reported that landscapes teeming with birds are often eerily quiet during smoky days, highlighting that what is harmful to humans is also bad for the rest of the animal kingdom. And while empirical evidence of how bad past wildfire seasons have been for birds and other animals is still developing, an emerging field of research is beginning evidence discovered of the damage that extreme wildfire smoke deliver to animal and bird health too.
A new study found that during the same 2020 wildfire season that scorched millions of acres and blotted out the sun in San Francisco, hundreds of acoustic sensors in Washington state registered a significant decrease in bird activity in the weeks immediately following the heavily smoky days of early September. The studypublished in the October issue of the peer-reviewed journal Global Ecology & Conservation, is the first in North America to attempt to empirically confirm anecdotes long observed by birders and others. The researchers found that one environmental sound index used to monitor biodiversity dropped by more than 15 percent during the particularly smoky conditions.
“Because birds are so sensitive to air pollution, we expect birds to be particularly vulnerable to smoke,” said Olivia Sanderfoot, a lead author of the study and a postdoctoral fellow at the University of California, Los Angeles. “During the window in which our sites were affected by smoke, we saw the biodiversity index and the acoustic complexity index decrease, and it decreased after the event.”
To capture the soundscapes of rural Washington, researchers deployed more than 700 acoustic monitors about the size of a television remote control at approximately 240 locations. These sensors recorded audio continuously for about a month before the battery ran out, after which researchers replaced them. Much of the fieldwork to install and collect the monitors was carried out by Sarah Bassing, a co-author of the study. Bassing sometimes spent up to 10 hours in the field driving as far as possible on Forest Service roads before walking the remaining distance. Some of the monitor deployments required overnight backpacking trips, while others took all day to deploy just a single monitor.
“There were some sites where I had to almost crawl on my hands and knees to get to the site because the terrain was so steep,” Bassing said in an email. “The ground and stones kept slipping away from under my feet.”
The use of bioacoustic indices such as those deployed in last month’s study is hotly debated in the ecological research community. Some studies showed that these indices may not be good indicators of animal and bird activity, and their effectiveness may vary based on geographic location and time of year. Sanderfoot acknowledged these challenges, but she emphasized them existing research shows a correlation between bird activity and the indices and measurements her team used. In 2022, researchers compared bird survey data collected in California with acoustic measurements and found that the acoustic complexity index was “a useful, albeit rough, surrogate” for bird diversity. Since the study covers an ecological region that includes the parts of Washington where the wildfire smoke research was conducted, Sanderfoot said it would be reasonable to expect a similar correlation in their study area.
“We did our absolute best to avoid bias and to be honest about the caveats to our work,” she said. “But I still think that the strength of our findings suggests that the soundscapes were different [following the smoke events].”
A study conducted in Southeast Asia also reached a similar conclusion. Researchers collect acoustic data in central Singapore in 2015 and found a dramatic decrease in bird and insect activity during hazy days. “Our results suggest that large-scale air pollution crises may have hitherto underestimated and potentially far-reaching impacts on biodiversity, particularly in parts of the world prone to extensive forest fires,” the researchers concluded.
This study inspired Sanderfoot’s work. Building on her just-published research, Sanderfoot is too recruiting birdwatching enthusiasts on the West Coast to submit their observations of bird activity during wildfire season.
“We don’t know as much as I thought we would about this topic,” she said. “The knowledge gap limits conservation efforts in this space.”
