UM scientists ‘accidentally’ discover leafcutter ants produce natural greenhouse-gas hotspots
By Kasey Rahn
When Fiona Soper joined UM Professor Cory Cleveland’s lab as a postdoctoral researcher in 2016, she didn’t anticipate studying leafcutter ants. She found those along the way. Or rather, they found her.
Soper signed on for a project examining patterns of greenhouse gas emissions in tropical forests. Originally from Australia, Soper earned her doctorate at Cornell University, where she worked in arid ecosystems. She was excited to broaden her horizons with research on the Osa Peninsula, a remote swath of tropical rainforest in southeastern Costa Rica.
There she found trails of leafcutter ants carrying puzzle-pieces of leaves, a familiar sight in tropical forests. These insects, which live in colonies and build elaborate nests, cut leaves from the canopy, carry them underground and use the leaves to farm a fungus that feeds the colony. They also build refuse piles — sort of like small landfills — where they meticulously place their waste, including leaf leftovers and dead ants. In those landfills, microorganisms break down the organic matter, generating gases like carbon dioxide, methane and nitrous oxide (N2O), just like human landfills do.
In a study published in January 2019 in the Proceedings of the Royal Society B, Soper, Cleveland and their coauthors show a previously unsuspected role for leafcutter ants in tropical forests: creating natural emission hotspots for the greenhouse gas N2O. They linked leafcutter ant refuse piles to some of the largest natural N2O hotspots ever recorded in nature.
The discovery, though, came as a surprise to the team — a sort of accidental science they stumbled into through a combination of chance and curiosity. They were in the Osa working on another project when leafcutter ants moved in, decimating one of their carefully chosen study plots.
“When they build a colony, they’ll just completely clear out all of the understory,” Soper says. “They’ll strip off every single leaf. When we put up plastic flags to mark things in the field, they’ll even cut all of the flagging off our posts. We showed up to this field site, and it had just been razed to the ground. Not a green leaf in sight.”
A field assistant had taken measurements as the site was being destroyed. The results had shown some interesting findings about greenhouse gasses, but nothing too spectacular. Still, Soper was curious.
“We just kind of casually took some samples because we were there,” Soper says. “We got back to UM, ran the data, looked at the numbers and realized these waste heaps in particular were very important sources of nitrous oxide. It’s naturally produced in ecosystems but not normally at anything near these kinds of rates. That told us that we’d found something interesting and that we should continue digging.”
The researchers, including UM master’s student Alanna Shaw and former UM postdoc Ben Sullivan, surveyed 22 colony refuse dumps of the Atta colombica leafcutter ant in Costa Rica. They discovered the refuse piles provided ideal conditions for the microbes that make N2O, creating hotspots on the forest floor. On a landscape scale, piles can sometimes emit more N2O than the rest of the forest combined, generating point fluxes comparable to those produced by human-engineered systems like wastewater treatment tanks or dairy manure lagoons.
To be clear, the leafcutter ants are a natural part of the ecosystem and have nothing to do with climate change, which is caused by excess greenhouse gas emissions released into the atmosphere mostly from human activities. What the ants do is not excess.
“Ecosystems naturally produce a range of greenhouse gas emissions,” Soper says. “They always have. But they’ve been in relative equilibrium, so what’s being produced by ecosystems naturally is being consumed by other natural processes. Those cycles have always happened. The human contribution to climate change has been that we’ve vastly increased adding greenhouse gases into the atmosphere, but there’s no corresponding sink that takes them back out again.”
For a long time, scientists (including Cleveland and Soper) have tried to understand the mechanisms behind those natural cycles: how greenhouse gases are produced in ecosystems and how that varies by place or time.
“Normally, we’re interested in emissions from landscapes as a whole — the whole tropical rainforest for example — but these refuse piles are very focused sources,” Soper says. “You can think of it as being sort of a chimney. You might have a lot of smoke coming out of a chimney, but a chimney is a very small area. One chimney is very different from the whole neighborhood being on fire.”
Instead, these hotspots show a new way that sophisticated insect societies can engineer their ecosystems and affect the way nutrients are distributed across a landscape.
“Sometimes cool science happens by accident,” Cleveland says. “Nature is just amazing and gets more amazing to me every day. We just have to look for it.”
The study isn’t just about some ants in one forest. Instead, the researchers say it’s a new puzzle piece that provides insight into the ways these systems function — a broader picture that has defined most of Cleveland’s career.
A biogeochemist, he studies how terrestrial ecosystems function and how they are affected by people. He’s worked in tropical forests for 20 years, since he was a doctoral student studying how land-use changes affect soil nutrient cycling processes in Brazil.
He says research in tropical forests can be challenging, especially compared to more temperate climates.
Take the Osa for example, where, through a variety of projects, Cleveland studies how nutrients affect microbes and plants in tropical forests and, in turn, how those affect bigger phenomena from nutrient cycling to climate change. He’s worked on the Osa since 1998, bringing a steady stream of other Grizzlies along with him, from undergraduate students to postdoctoral researchers.
Depending on the airline, it’s two or three flights from Missoula to Costa Rica’s capital of San Jose, then a seven-hour drive to the field station — the last hour on a one-lane dirt road crossing four different rivers. The station often lacks electricity. And since the Osa can get as much rain in a day as Missoula gets in a year, it’s not uncommon to get stuck waiting for the rivers to recede following a storm.
Last trip, two researchers ended up with giardia.
“It sounds great. Tropical forests — that sounds amazing,” Cleveland says. “Let’s go to Costa Rica. But then when you get there, the magnitude of what you’ve actually signed up for hits you. I got stung by a scorpion while I was sleeping one time. It’s not what people imagine.”
For Cleveland, Soper and others, any challenges are well worth the chance to better understand how these ecosystems work. They’re vitally important, the researchers say, but scientists understand orders of magnitude less about tropical ecosystems than they do about temperate forests because historically they’ve been less thoroughly studied.
“In the tropics there are still very fundamental questions we’re only just starting to chip away at,” Soper says. “As a scientist, they’re a really exciting place to work because there are just so many globally important questions and just so few people asking them.”
Tropical forests aren’t that big, covering about 10% of the Earth’s area, Soper says, but they’re a carbon cycling powerhouse. One-third of the Earth’s carbon uptake and one-third of plant growth that occurs on Earth in a year takes place in tropical forests, so the ecosystem is vital to global cycles like the carbon cycle or the water cycle.
“Studying these sorts of globally important ecosystems and continuing to do on-the-ground field science is so important because you never know what you’re going to turn up,” Soper says.
The leafcutter ant study is just one small part in a longer string of research on the Osa by Cleveland and his lab. Once Cleveland built large sheds to divert rainfall from sections of forest to study what might happen if they receive less rain in the future.
Another project measured and mapped the forest canopy’s leaf nitrogen concentration using an aircraft-based hyperspectral imaging platform for the first time in this hyper-diverse ecosystem. The results were digital maps that, when downloaded on an iPad, allowed researchers to walk through the forest holding a map of what the nitrogen makeup of the canopy above them looked like in real time. As a postdoc, Soper studied whether those maps of trees could also tell researchers about the processes taking place in the soil below.
“Sampling is challenging,” she says. “It’s one thing to do it in Costa Rica, but another to do it at larger scales. Developing tools that can be done via satellite instead of boots on the ground really greatly increases our potential understanding of processes over big areas.”
Soper, now a faculty member at McGill University in Quebec, and Cleveland are collaborating on a new project that will bring researchers together from all over the world to UM. Working with a grant from the National Science Foundation, they’ve formed a research coordination network, which over the next five years will bring together boots-on-the-ground research scientists and modelers — scientists who run complex computer models that predict the effects of climate change — to try to represent biogeochemistry better in Earth system models.
“Right now, the models that are predicting climate change mostly ignore nutrients,” Cleveland says. “Our work shows how nutrients affect plants and soil microbes. How do we represent those important processes better in Earth system models?”
As Cleveland sees it, we can learn something from almost everything — whether from other international scientists or from something as seemingly simple as ants.
“This leafcutter ant study highlights the complexity of what’s going on in a fascinating way. On the ground, at the sort of scale of even a few hectares, all these processes are going on, and we still don’t even know about many of them,” Cleveland says. “You look at these landscapes and you think you understand them, and then you see something like this. It just makes you realize, ‘Oh my gosh, we have so much to learn about these ecosystems.’
“And there are things people can learn from studying ants.” •