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I don't know about you, but I am pretty into saving the rainforest. Like, I think people have made a pretty good case for doing so. The Amazon is the largest tropical rainforest and one of the most diverse, biodiverse places on Earth. Home to one in ten species. The rainforest is considered the lungs of the world. It also acts as a major carbon sink. That means it basically absorbs carbon dioxide emissions.
I personally would like the planet to have lungs, and I think it's pretty clear by now that biodiversity is a good thing. Like, if all the plants and animals die, so do we. So, you know, save the rainforest. Except scientists still haven't figured out exactly what it means to save a rainforest. Even ecologists like Jörg Müller. It's easy to track that the trees are regrowing. This you can nowadays follow from satellites easily.
It's much more difficult to see if the animals are coming back and how successful is it in terms of biodiversity in your different settings, in your different contexts. Part of the trouble is that we don't actually know how many different animal species are supposed to be in most patches of rainforest to begin with. We know rainforests hold millions of species, thousands of birds and frogs, mammals and insects that don't exist anywhere else.
But that complex tangle is so under-understood that we don't really have a great baseline of what a healthy rainforest looks like. It's almost like as if we said, oh, we want to make sure that patients are healthy, but we don't really know what does it mean to be in good health. This is Zuzana Buhivalova. She's an ecologist who's collaborated with YORG.
And she says that treating the rainforest is like treating sick humans. If we don't have baseline information about what healthy humans look like... How can we then design drugs? How can we design, I don't know, physical therapy if we don't know what is the normal range of movement? What is the normal daily life of a healthy person? So that's what Jörg Muller and Susanna Bojewalowa are trying to figure out. Like...
What does a healthy rainforest look like? What does a healing rainforest look like? And which remedies or interventions are actually helping? It is a big problem. And to solve it, Jörg, Susanna, and researchers around the globe turned to a super basic tool, sound.
The metaphor that I like the best, probably, is that we are using a stethoscope. We're trying to listen to the vital sounds of the forest to understand, okay, what does the normal sound sound like? And then we can detect the problems. This is Unexplainable. I'm Burt Pinkerton. And today on the show, how to build a stethoscope or the rainforest.
*water splashing*
Jörg Moeller's interest in sound as an ecological tool started a little closer to home. Literally. It started in the oldest national park in Germany, which he lives near. And I have the opportunity to go out and enjoy the morning or to see a lynx in my garden or to have a river otter in my fishing pond. Something like that is my life. And Jörg doesn't just live next to this charming forest. He studies it.
He does stuff like bird counts, where he goes to various spots in the forest and just counts the birds there every year. It's this well-established way to track biodiversity.
But a few years ago, he noticed a problem with this method. I was running around in the forest by myself, making counts of birds in the morning. And already then I recognized it's impossible to be at the same time at every plot. Birds are more active at different times of the day. So anyone who has been woken up by a bunch of birds knows that they go kind of nuts at dawn, specifically. And this
worried Jörg because he knew that if he went around doing bird counts at 20 different plots... Some are before dawn, others are after dawn. You cannot be everywhere at the same time. Which might throw off his data. Like, maybe he'd count more birds in one part of the forest than another, not because there were actually more birds there, but because he'd visited that first spot at dawn and the other one a little later in the day.
So around four years ago, he says he started leaving a sound recorder at each bird counting spot. He left them running and then took short snippets of sound from each one and compared them. The same snips from every plot before dawn, after dawn, and so on. And then you have a selection which is the same from every plot, and then you listen to them and identify the species. This refinement had a lot of advantages. It standardized things, it was fairly cheap,
Also, you didn't have humans blundering around disturbing animals. So just overall, it made these counts easier and more accurate. Seemed like a pretty good stethoscope.
And Jorg started to wonder if a stethoscope like this could be used in other contexts. So he posed the question to Susana Buhivarova, the ecologist we heard from earlier. And she specializes in studying animal sounds. And she's something like the rising star, young rising star in sound ecology. Susana had actually been trying to use animal sounds to measure ecosystems for a while. She'd even done some research using sound to diagnose rainforests.
And the science seemed really promising. So Jorg and Susanna decided to collaborate on a project focused not on German forests, but on tropical rainforests. The plan was to understand, are forests recovering? Are they recovering not just in terms of the trees that are growing back, but are they also recovering in the animals that are inside those trees?
This is the puzzle that we started with. Jörg and Susanna basically knew that they could answer this question fairly well in a place like Germany. It was relatively easy because there aren't that many species of birds and insects in German forests.
But in tropical rainforests, there are hundreds of animals making noise. And a lot of them are either understudied or unknown. If you are standing there in the tropical forest and you are a beginner, it's very hard to identify all these different species.
So in the tropical forests, the first idea was not to identify any species. Jörg and Susanna needed a different approach. They turned to one that Susanna had started exploring in her sound ecology research. We use this approach called calculating soundscape indices or acoustic indices. And this allows us to take every single minute of the recording
and calculate something about it. We can start monitoring how busy the soundscape is, how complicated it is, how is it changing over time. Basically, the theory goes that in a healthy ecosystem, you have a lot of different kinds of animals, and they all make different sounds, like high-pitched sounds and low-pitched grumbles. I always like to think of this as if you were listening to an orchestra.
If you have lots of different instruments like violins and cellos and flutes, they will all be playing sounds at different frequencies. Whereas if you have just a few species that aren't occupying all the kind of sonic niches of an environment, it's less of an orchestra and more like a lonely two or three violins.
And that violin playing... It will stay within a certain frequency range or pitch. So the plan was to take a bunch of recordings from recovering rainforests and then measure the orchestras in these areas and sort of see how rich they were. So we can, for example, calculate, okay, in one minute...
how many different frequencies or pitches are occupied. And we call this sound saturation. So if they use a wide range of the space, then we say it's a saturated sound space, not in a spatial sense, but in a frequency sense. Researchers have shown that as rainforests are cut down, their soundscapes become less saturated, less rich. Jörg and Susanna thought that recovering rainforests might rebuild those soundscapes.
and that this could be one way to measure sort of how much biodiversity was healing in an area. So to test this out, Jorg and some of his fellow researchers headed to a lowland region of Ecuador. It used to be rich rainforest, and then settlers came and cleared it for cacao plantations and pastures. An NGO has been trying to restore rainforest here for several years, working with the local community to regrow rainforests.
And they want to know if it's working. So as you can imagine, for a normal forest to recover, to regrow back, it takes decades. So that's why we wanted to look pretty early after the abandonment of these fields. And then a couple of years later, and then basically decades later. So they went in with these little pale gray sound boxes, pretty small. They put out...
these sound recorders into several places in each of these regeneration stages. So in the pasture areas you have very dense cover of grass with a few trees or very sparse trees. If you close your eyes and you go to these pasture areas, then you will recognize very heavy song of birds.
So they are very similar. Many of them are small seed eaters, like sparrows, buntings. And they love this open field and forage there. So they put soundboxes in these newly abandoned plantations and pastures. And then they put them in plantations and pastures that have been regrowing for a while. You see a lot of bushes. You see some small trees. But it's not this
forest that you can easily walk through. There'll be a lot of undergrowth, which will make it tricky to walk through. And finally, they put sound boxes in the parts of the rainforest that had never been cut down, which gave them a baseline, like a sense of what a forest should sound like. You are surrounded by a complex 3D forest with very huge trees, very dense understory. Huge diversity of
orchids or lianas. So you feel like you're in this bunch of vegetation, there are huge leaves. So it's a totally different world and it's very dark inside the forest. Visibility can be pretty low. But you really feel, as a human in an old-growth forest, you really feel kind of buffered from the rest of the world. They let those boxes sit, they collected the sounds that you heard, kind of applying their stethoscope to the forest.
And then they collected those boxes, got the sound files from them. And what they found was what they expected. In the newly abandoned pastures and plantations, the soundscape was less rich, less full. But in the patches that had been recovering for a few years, the sounds got more complex with kind of creatures making noises at more frequencies, which is great, right? The recordings were helpful-ish, right?
But Zuzana still has questions. We still don't know enough how closely they actually reflect the biodiversity of the species. Zuzana and Jörg, they wanted to know which species were in the forest. Like, were these native species or foreign animals that had come in from elsewhere?
Their stethoscope was not precise enough to tell them. Like, how could you pick out each individual frog, say, when there were hundreds of species of frogs, some of which people didn't even have recordings of? It was impossible. So they thought, until Jörg went for a walk.
Yeah, it's a very nice story. I was walking with my dog at home in the evening. On this walk, he caught up a friend, a former PhD student who loves to birdwatch, and he told him about the audio files that he was collecting. And he said, "Yeah, I was just recently in Ecuador, and I had this field guide there, and he showed me more than 700 species in three weeks."
And this is only possible if you know very well the voices. And then I asked him, do you think they would be willing to identify my audiophiles? And he said, yeah, why not? After the break, a superhero comes to the rescue.
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Hey, unexplainable listeners. Sue Bird here. And I'm Megan Rapinoe. Women's sports are reaching new heights these days, and there's so much to talk about and so much to explain. You mean, like, why do female athletes make less money on average than male athletes?
Great question. So, Sue and I are launching a podcast where we're going to deep dive into all things sports, and then some. We're calling it A Touch More. Because women's sports is everything. Pop culture, economics, politics, you name it. And there's no better folks than us to talk about what happens on the court or on the field.
and everywhere else too. And we're going to share a little bit about our lives together as well. Not just the cool stuff like Met Galas and All-Star Games, but our day-to-day lives as well. You say that like our day-to-day lives aren't glamorous. True. Whether it's breaking down the biggest games or discussing the latest headlines, we'll be bringing a touch more insight into the world of sports and beyond. Follow A Touch More wherever you get your podcasts. New episodes drop every Wednesday. Speak for the trees.
Alejandro Arteaga has been wandering through South American forests ever since he was a little kid. We used to live in Venezuela in a cloud forest, and that's where I grew up. Very close in contact with nature. Just, you know, getting my boots dirty and walking on the forest trails with my brother. That was my pastime. On those explorations, Alejandro would find animals and learn their names.
First insects, then birds, fish, and eventually I capitulated into amphibians and reptiles, which is the main group of animals that I study now. He identified his first new species as a teenager. It was this little brown frog that he distinguished from all the other little brown frogs because it was making weird noises.
And over the years, he's built up a rich vocabulary of amphibian croaks and honks. Like, for example, the call of the smoky jungle frog. A frog that lives in holes and you almost never see it, but you hear it in the night of the jungle. It's like, whoop, whoop, all night. That goes on.
on and on throughout the night. And you know the species is there instantly. There's also a tree frog, for example, that's hard to see, apparently, but easy to hear. And you recognize them by this particular sound that they do. It's like... Nature's squeakiest door, basically. And Alejandro estimates that he has around 100 to 200 of these kinds of calls just kind of bouncing around in his head.
He's also spent a lot of time in Ecuador specifically. He even wrote a catalog of Ecuadorian reptiles. So he was kind of the perfect person for Jörg Muller and his team to reach out to when they were looking for help identifying the amphibians in their sound recordings. There are many, many experts of frog vocalizations. I say I'm one of maybe three. So maybe the choice was not so difficult.
Either way, Alejandro agreed to help Jörg and his team out, and they sent him lots and lots and lots of recordings. These like two-minute sound files drawn from the pastures and the cacao plantations and the old growth forests. And his job was to listen to the files and mark the species he was hearing at various time codes. I like the old growth better. It's more diverse and cooler species. Here we go. Well, this is not going to work.
Because it's in the day. Oh, it's in the day. Got it, got it, got it. Let me see if there's anything from the night. Early morning sounds. Okay, that's a poison frog. Wait, wait, wait. Where? Okay, that's a poison frog, you said? So the scientific name is Epipedobates boulangeri.
The dorsum is like reddish-brownish. And what is peculiar about this frog is that it carries the tadpoles on its back. You can see them jumping around the wet leaf litter during the day. Alejandro sat and listened to lots and lots of audio this way, and he loved it. It's great because it feels like you are back in the forest.
Which for me is where I belong. So, oh my God, the sound, the sounds again. But still. It's a pretty intense process. And while Alejandro was listening for frogs, other experts were listening for birds too. In the end, they ID'd around 200 species in all. And when they looked at the species in the plantations and the species in the recovering forests…
The news was good. Clearly, the regenerating forests are recovering species that are only really found in old-growth forests, which is a really good sign. A good sign for the ecosystem and for the ecologists, right? Like, their new tool for monitoring biodiversity, this Alejandro-powered version of the stethoscope, it was working. But it was still not ideal.
In fact, in some ways, it still had some of the original problems Jörg encountered when he was wandering through the trees all day taking counts of birds. Because it still took a lot of time and a lot of highly skilled labor, and it was still not super standardized. So Jörg and Susanna wondered, could we make this still just a little bit better? And they had an idea. The idea that we can train a machine
to recognize these sounds. Basically, if you say to an AI that learned stuff, "Hey, this is the sound of a poison frog, and this is also the sound of a poison frog, and here is another sound of a poison frog," eventually, the AI is able to figure out what all those recordings have in common, and it can filter out the background noise to learn to recognize just the poison frog's trill.
Maybe AI could get so good that you wouldn't need Alejandro to sit and listen to hours of audio hunting for poison frogs, right? You could just have an AI do it. There was a group called Rainforest Connection that was already pulling together an AI model for rainforest species. So Jorg reached out and essentially he said, "We want to make a proof of your existing model." "Why don't we run our files through your model,
and see how your model compares to our human experts. And they learned that the AI couldn't recognize as many species as the experts, but
For the ones it could recognize? In general, it was pretty good. Their stethoscope still has some pretty fundamental limitations. Like, this one is tailored to Ecuador. And even with AI's help, it can't track animals that don't make sound. And lots of insects, for example, are pretty quiet. But still, these researchers, they think that this AI-powered version of the stethoscope changed the game on rainforest preservation.
When the medical stethoscope was invented in the 1800s, it helped kick off big changes in how humans took care of each other. Like, the stethoscope alone couldn't heal people, but it helped doctors peer inside bodies without cutting them open. It helped us understand the most fundamental things about ourselves, things we hadn't been able to measure. And that led to better treatments and new cures. Susanna and Jorg's natural stethoscope helped
It won't treat deforestation or fix fire damage, but it will let researchers listen to the heart of some of our planet's most important ecosystems and hopefully help them find better ways to heal them. If you want to read more about Zuzana's work on sound and ecosystems, look up the Sound Forest Lab. You can also find Jörg and Zuzana's paper on soundscapes in the journal Nature.
This episode was reported and produced by me, Bird Pinkerton. It was edited by Jorge Just, with help from Brian Resnick and Meredith Hodnot, who also runs our show. Noam Hassenfeld did the music. Christian Ayala did the mixing and the sound design, along with Erika Huang. Angelia Mercado took the facts. And Manning Nguyen still owes me vacation stories.
Special thanks also to Lali Morelos-Juarez for taking the time to speak with me for this episode, to Liam Brooks for his help, and to Benji Jones for suggesting that we look at this story to begin with. And thanks to Regina Nuzzo, a great statistics professor, for her help.
This podcast and all of Vox is free, in part because of gifts from our readers and listeners. You can go to vox.com slash give to give today. And if you would, it would be very, very much appreciated. But if you can't, that's also fine. And I would also really appreciate it if you took the time to leave us a review or to write us an email with your thoughts and questions.
We are at unexplainable at Vox.com and it brings me a lot of joy to hear from you all. Unexplainable is part of the Vox Media Podcast Network and we will be back next week.
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