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TRANSCRIPT
Dieckman: The music you’re listening to right now features drums by Lisa Schonberg and additional percussion by…ants.
When Schonberg first went down to the Amazon to record ant sounds for a music composition and ecology project, she connected with Fabricio Baccaro, a community ecologist at Amazonas Federal University in Manaus.
Baccaro: She wrote me and said, “I’m going to be recording ants, and blah blah blah.” And I said, “Whoa. Interesting. Ants don’t make sounds.” And she: “Yes! They make a lot.” And she showed me when she was here, and we went out here at the university. She plugged the mic there and I listened and said, “Whoa.” So I was hooked by this, when she showed me that they can use sounds for communication.
Reading List
Listening to Tropical Forest Soils (Ecological Indicators Paper)
Lisa Schonburg’s Amplifying the Tropical Ants project
Dieckman: And it’s not just ants that make noise. Lots of insects within the soil are making all sorts of noise to communicate with each other and while moving around.
Baccaro, who studies invertebrate biodiversity within the Amazonian soil, was intrigued.
He had been counting critters in the ground for years and was interested in finding a less invasive and more effective way to do it.
Usually, he removes a cube of soil from the ground and puts it under a heat lamp. The critters all flee the drying soil, and Baccaro identifies which species are present. It’s time- and labor-intensive. And some insects, like large ants, will escape before soil can be sampled, making the count not truly representative.
Then, Baccaro met Oliver Metcalf, an ecologist at the University of Lancaster who came to the Amazon as a graduate student planning to study bird biodiversity using sound.
Metcalf: But as soon as you start working in Amazonia and try to listen for birds, you realize that the vast majority of the sounds you hear are insects and other unknown sounds rather than birds, and that just to focus on birds is to miss out on 99% of what’s happening in the acoustic realm.
Dieckman: Researchers use acoustics to study various species. For example, they might use sound to detect endangered bat species, or study how tortoises socialize. With ecological acoustics, scientists study entire environments based on their sounds.
Metcalf, Baccaro, and their colleagues wondered whether they could hear a difference between soil that has been burned and soil that has never been burned.
This was partly for logistical reasons. They already knew there’s a difference between burned and unburned soils, of course. But this study would let them see if soil’s background noise reflected that. They also hoped it would show whether the two types of soil had different levels of biodiversity beneath the surface. Metcalf explained why it’s also important to expand scientists’ understanding of how the Amazon is affected by wildfires.
Metcalf: It’s not like Australian or Californian forests that are adapted to fire and it’s part of their natural cycle even though it’s happening more often now. These forests are completely unadapted. So even though the fire is only 2 or 3 inches high as it goes through—it looks really small, it looks like I can stamp it out—it’s killing the biggest trees as it goes through because they’re completely unadapted.
Dieckman: So the researchers got down to business. Or, down to soil. They chose 25 recording sites spread throughout a forest. Some mornings there were 3-hour drives out to the sites. Other mornings there were 10-kilometer walks. Using a machete to make a hole in the soil, they’d bury a microphone underground.
Carlos Abrahams, an ecologist and director of ecoacoustics at Baker Consultants, who wasn’t involved in this work, said he can think of only a handful of studies that have used ecological acoustics to study soil, and it had never been done in the Amazon.
Here’s Metcalf again.
Metcalf: You’d make this little hole and then you’d move away fifty meters far enough that you’re sure you’re not making any sound and sit there for half an hour getting bitten by mosquitoes and then go back and collect it. And to be honest, it was a little bit depressing, what I was doing. I didn’t have time to look at the data and it felt a bit like, “I’m not going to get anything from this. I’m getting bitten and it’s a lot of work.” And we didn’t have any way to protect it from the rain, so every time we got to a transect and it was raining, I just carried all this equipment and I couldn’t do any recording and ahh this is so much work, it’s just a waste of time. And then I got back and listened to it I was like, “Wow. There’s actually things happening here and there’s patterns and it’s incredible. This data is actually really interesting, and I was really glad that we’d stuck with it.
Dieckman: The team used six different indices to monitor biodiversity. Metcalf explained that these were originally developed to measure and monitor bird sounds, but they might be useful for bugs too. They’re also fairly simplistic. They aren’t designed to identify different species. The most basic index, for instance, simply recognizes and counts every time sound happens. Another index conveys complexity: The more different kinds of sounds there are, the higher the value.
The researchers expected biodiversity to be lower in burned soil than in unburned soil.
Sounds in soil tend to be at very low frequencies, so here, Metcalf increased the frequency by 900%. Sounds at 100 Hz are now at 1 kHz, for example. It’s just to make the clips more audible. Here’s a few seconds of burned soil.
And a few seconds of unburned soil.
Dieckman: But the findings weren’t what the team expected. They found that the values for all of the indices were higher for burned soil than unburned soil. For two indices, the values were much higher. Basically, burned soils were noisier, which seemed to suggest that they are actually more biodiverse than unburned soils.
Metcalf has three possible explanations.
The first is what he called “the bumbling about hypothesis.” The indices they used were developed to test bird soundscapes, remember. When birds coo, or chirp, it’s often because they want to communicate. But in the soil, most of the sounds might come from insects simply moving around. So, the indices may not respond to these sounds in the same way.
Option 2 is that the fire may cause a chemical change to the soil that allows sound to travel though it more easily. The third option, which Metcalf thinks is the most likely, is that while there may be less diversity in burned soil, there are actually more insects.
Metcalf: When you start impacting those forests, what happens very quickly is that you lose your rarest species and a whole bunch of generalist species become very, very common. So, although your species richness might decline, your overall abundance might actually increase because these generalist species which couldn’t survive alongside all these very rare specialist species suddenly have this competitive advantage and become incredibly common. And I think there might be an element of this happening in our soils. That there’s one or two species that suddenly lose this competitive competition and become much more abundant.
You can sort of notice it when you walk through the burned forest. You get a lot more chiggers and ticks over you and that sort of thing they’ve become hyperabundant in that environment. Yeah, I think that might be what’s happening, but we need to do more research to find out exactly what’s going on.
Dieckman: Overall, the researchers found that these acoustic indices probably aren’t especially useful for studying biodiversity. But they could correlate with functionality, or knowing what different species are doing.
Baccaro: If you are eating, if you are walking, you make sounds. So, if we have more ants walking, more earthworms eating the soil, it’s reasonable to expect that to have more activity, more functions, going in that place. So this is the reason I strongly believe that soil acoustics can be a useful and powerful tool to monitor soil functions.
Dieckman: Abrahams, who, again, wasn’t involved with this work, said the results mirror those of other ecoacoustics studies that have compared two types of soil, like organic vs nonorganic farms and original vs restored woodland.
Abrahams: So, it’s interesting and gratifying and useful that we are getting similarly sensible results out of this Amazon paper compared to previous work. I think it fits. It gives some expected results, and it gives some interesting little differences as well. And that’s the sort of the sort of paper that you want isn’t it that broadly agrees but shows you new avenues or new things to think about, depending on the recording context.
Dieckman: Moving forward, the team would like to combine the study of soil soundscapes with lidar data, and develop a library of soil sounds. One major question they’d like to answer is how far their microphones reach. Though they have some ideas, they’re not sure whether the sounds they captured are all within 10 centimeters or 5 meters of the microphones.
Metcalf: I’d like to know more, and we will research this: whether the sounds are coming from the leaf litter or in the soil itself, and how much of it is organic, and plants growing, actually. Someone played me a recording of roots growing and it makes a lot of tapping and popping noises that almost sound animal-like. ‘Cause obviously we can listen through and we can—you can—hear things that you know like, “That’s definitely an animal. That can’t be a plant.” But there’s a whole bunch of sounds that you’re just like, “I don’t know what that is. It’d be really cool to know what that is.”
Dieckman: The researchers published their study in the journal Ecological Indicators. Thank you to Oliver Metcalf, Fabricio Baccaro, and Carlos Abrahams for speaking with me. Also thanks to Lisa Schonberg for sharing her music. I’m Emily Dieckman. Find a reading list for this article at Eos.org, your source for Earth and space science news.
—Emily Dieckman (@emfurd), Associate Editor