Leafcutter ants run some of the biggest farms on Earth, and just like ours, they're pumping out greenhouse gases.
The world's oldest industrial farmers
These are leafcutter ants, Atta. Across Central and South America, they strip leaves from the forest, carry them underground, and use them to grow fungus, which is what they actually eat. A single nest can go more than 6 metres deep, stretch as wide as a tennis court, and hold millions of workers. By some estimates, leafcutters can remove up to 8% of living forest leaves each year.
That makes them one of the most powerful plant-eaters in the tropics. And like any big farm, all that production leaves something behind.
Chimneys in the forest floor
A leafcutter nest is full of life that breathes. The fungus gardens and millions of ants all release carbon dioxide, and so do the leaves and trash rotting inside.
Angel Fernandez-Bou and his colleagues at the University of California, Merced spent 30 months measuring nine leafcutter nests at La Selva Biological Station in Costa Rica. They found that the nest vents released up to 100,000 times more carbon dioxide than the soil around them.
Surprisingly, the soil inside the nests often held less carbon dioxide than normal soil. That's because the nests work like chimneys. Their tunnels pull gas out of the ground and vent it into the air. In a later study, the team found this happens mostly at night, when the nest is warmer than the air outside and warm gas rises out of the vents, a process called free convection.
In that study, leafcutter nest vents made up about 0.2% to 1% of all the carbon dioxide coming out of the rainforest soil. That's from a few holes in the ground.
Methane hotspots
Carbon dioxide isn't the only gas. A 2021 study measured three nests of Atta cephalotes with a portable gas analyzer and found that soil with nest holes could release far more methane than the soil around it, with some holes giving off huge amounts. In normal forest soil, methane emissions stayed low.
That matters because tropical forest soils usually soak up methane from the air. Leafcutter nests may turn small patches of that sink into sources. The researchers say more work is needed to understand why.
Landfills that leak laughing gas
Leafcutters are very clean. They don't leave trash lying around their gardens. Instead, they carry dead fungus, old leaf bits and the bodies of dead ants to refuse dumps, away from the colony.
Fiona Soper and her colleagues measured 22 of these dumps from Atta colombica colonies across 4 square kilometres in Costa Rica. Every single one was a hotspot for nitrous oxide, sometimes called laughing gas. It's a powerful greenhouse gas, and tropical forests are already one of the biggest natural sources of it.
The researchers compared the dumps to our own landfills. In both, microbes break down the trash and release gases like carbon dioxide, methane and nitrous oxide. On their own, the dumps don't produce a huge amount. But leafcutter ants are everywhere, and the researchers suggest they may be an unrecognized source of greenhouse gas across the tropics.
The cost of farming
Leafcutter ants have farmed for millions of years. They aren't ruining the climate. The forest has lived with them for a very long time. But they show something we know from our own farms: when you feed millions, the waste has to go somewhere. More crops means more trash, and more trash means more gas.
Even the world's oldest farmers haven't solved that.
For the bigger story of how ants learned to farm, read Ants Invented Farming 66 Million Years Before Us, and meet the ants themselves in our leafcutter ant story.
Further reading
- Fernandez-Bou, A. S. et al. (2019). The role of the ecosystem engineer, the leaf-cutter ant Atta cephalotes, on soil CO2 dynamics in a wet tropical rainforest. Journal of Geophysical Research: Biogeosciences.
- Fernandez-Bou, A. S. et al. (2020). Diel pattern driven by free convection controls leaf-cutter ant nest ventilation and greenhouse gas emissions in a Neotropical rain forest. Oecologia.
- Leaf-cutting ant (Atta cephalotes) nests may be hotspots of methane and carbon dioxide emissions in tropical forests (2021). Pedobiologia.
- Soper, F. M. et al. (2019). Leaf-cutter ants engineer large nitrous oxide hot spots in tropical forests. Proceedings of the Royal Society B.
- Swanson, A. C. et al. (2019). Welcome to the Atta world: a framework for understanding the effects of leaf-cutter ants on ecosystem functions. Functional Ecology.
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