Welcome back.

This week, guest writer and science journalist, Pragathi Ravi, connects the fungi-farming behaviors of leaf cutter ants to the sustainable agriculture and land stewardship practices of indigenous peoples in India and beyond.

Perhaps, the best method to conserve biodiversity isn’t to set land aside untouched and removed from human beings, but to live in relationship with the land and other species — something leaf cutter ants have been doing for nearly 60 million years.

Enjoy.

— Willow

As a child, I remember being fascinated by a parade of tiny red ants carrying crumbs of food, perhaps dusted off from our clothes after a meal, to their den.

They would ferry their foraged provisions through two fine lines, in a formation that rivalled military precision. This enraptured me as a five-year-old, who would spend hours in my home in India, fixated on this system that functioned like a well-oiled machine. 

A few years ago, I remember seeing a video shot in South America of another colony of ants carrying snipped pieces of leaves over tree bark, which kindled my childhood fascination. Here were a bunch of leafcutter ants, which do not directly feed on the leaves but take them back to their den to farm a fungus they consume. Again, fascinating. But as an adult now, I wanted to dig deeper into the ‘why and how’ rather than be content to watch this parade unfold. 

While humans have taken pride in heralding the beginning of modernization by pioneering farming, these pesky groups of ants have been doing it for far longer— almost 60 million years.

Scientists have dated this to the impact of the end-Cretaceous asteroid, which is believed to have wiped out non-flying dinosaurs, temporarily interrupted photosynthesis, triggered a global mass extinction, and aided the proliferation of fungi. 

Since then, certain ant species, including leafcutter ants, started farming fungus as food. In what evolved into a mutually beneficial relationship that has stood the test of time, leafcutter ants fed their fungus crop with the leaves they harvested. Interestingly, it has also been established that all of the fungi that the ants cultivate appear to have descended from a single, ancestral fungi cultivar. 

Evolutionary biologist Michael Poulson has studied these fungus-farming ants for a while now. He was trying to understand symbiotic interactions— either beneficial or antagonistic— which led him to spend over a decade teasing out the farming symbiosis that underlies the industrial-scale agriculture that the ants practice. 

I say industrial-scale because researchers have found that ants inhabit a complex society, where millions of ants perform clear-cut roles in maintaining the massive fungal chamber that supports their sustenance. 

These ants cut up fresh leaves, and they bring them back to their den containing the fungal gardens to feed the fungus.

“This is a cleaning process that reduces other microbes from entering the gardens. They then chew it to kind of a pulp, which leads to the initial mechanical breakdown of the leaf material,” Poulson adds.

They also sometimes mix this pulp with fecal fluids because the ants ingest fungus material, enzymes from the ants pass through the ant’s gut, and remain active. This primarily helps them decompose hardier plant components such as cellulose, which the ants themselves cannot digest directly. The fungi break down these plant compounds into fungal biomass, which the ants then feast upon.

“What is unique for the fungus farming ants is that you have these external fungus gardens [that take over the] decomposition of the plant biomass, so it's almost like an external gut in a way,” Poulson states.

What makes this system sustainable is that although the ants take huge amounts of leaves, this rarely kills large trees.

The vegetation regrows, making it unlikely for an entire population of a given plant species to succumb or be overrun by the ants, even if an individual plant might not make it.

“The system does not collapse, because it’s part of a balanced ecosystem,” Poulson points out.