by Rebecca Silberman
figures by Aparna Nathan
It only took a few weeks of the coronavirus pandemic to send the American farm industry reeling. The supply chains that connect soy fields and chicken coops and greenhouses with American tables were not built for a world that lacks steady restaurant traffic or school lunches, where instead consumers are emptying supermarket shelves. There is at once a surplus and a deficit, a profound mismatch between production and demand. Healthy crops are being churned back into the soil. Dairies are dumping their milk. There’s a waiting list for livestock entering meat processing plants.
Every decision a farmer makes requires them to navigate tradeoffs: sustainability or affordability, crop diversity or efficiency, short-term rewards or long-term investment? Watching American agriculture buckle under the weight of the coronavirus crisis and the decades of collapsing crop prices, foreclosures, environmental destruction, and disenfranchisement which characterized the crises that preceded it, begs the question: have we made the wrong choices, boosting production at the expense of resilience?
Perhaps we shouldn’t be surprised that humans haven’t figured it all out yet; compared to other species, we haven’t been farming for very long. While humans have been cultivating crops for around 12 thousand years, ants have been farming for as long as 60 million years. Among non-human farmers, ants’ agricultural practices are unrivaled in their scale and sophistication. No wonder over millions of people are escaping their worries by roleplaying online as a colony of ants.
But are ant colonies really the perfectly honed, unflappable agricultural machines they appear to be? And even if they are, is this because ants are simply exempt from the fraught choices human farmers face daily? A group of researchers investigated whether farming ants face agricultural tradeoffs and, if they do, how they have evolved to respond to these difficult decisions. Their discoveries offer both warnings and encouragement for our species’ farms.
Ants’ farming practices are varied, precise, and wildly successful
The most famous insect farmers are the leafcutter ants, which are the dominant herbivores in many tropical forests. Certain leafcutter ant species consume 12-17% of all leaf growth in their forests. These leaves are ferried in dramatic supply lines back to the ants’ nest where they are chewed up and used as fodder for growing fungus, which the colony consumes.
This is a symbiotic, or mutually beneficial relationship; the ants get a consistent food source and the fungus gets to thrive. These cultivated fungi depend on the ants and are not found outside of ants’ nests. Similarly, you won’t find a row of Brussels sprouts growing in the wild, unaided by thousands of years of seed selection and cultivation by human farmers. It’s hard to overstate how successful leafcutter ants’ farming practices are. A single colony’s garden supports all eight million of its residents and the global population of leafcutter ants make up a third of total insect biomass on the planet.
Leafcutter ants aren’t the only farming ants. The ant Philidris nagasau cultivates epiphytic plants—plants that grow on trees, rather than growing from the ground. P. nagasau are found in Fijian rainforests and construct their treehouse gardens in bulbous structures protruding from trees, which have earned the elegant nickname “trees’ testicles.”
P. nagasau is the star of this recent study into agricultural tradeoffs. The ants collect the seeds of plants in the genus Squamellaria and carry the seeds up the trunks of trees, planting them under the host’s bark. As the plants grow, they create “domatia” or chambers that house the ants. Housing is just one benefit that ants receive from this symbiotic partnership. Since there’s no soil, the ants fertilize their gardens by defecating on highly absorptive regions of the plants. In return for their stewardship, the ants feed on the sugar- and amino-acid rich nectar which is found in the plants’ flowers.
Farming ants face agricultural trade-offs
Researchers observed that P. nagasau’s crop is 7.5 times more productive—it yields more nectar-filled flowers—when growing in high sun conditions than in shade. These highly productive plants are a beacon to pests, which are much more numerous around high sun gardens than shaded gardens. Remarkably, the presence of pests does not correlate with damage. While 90% of farmed plants growing in the shade showed damage from pests, less than 25% of high sun plants were damaged. Researchers observed a huge increase in ants patrolling these high sun gardens to protect them from pests.
Surprisingly, researchers found that these highly productive sun-loving plants have lower nitrogen content than plants that prefer shade. Ultimately, researchers attributed this difference to the quality of the ants’ diets since P. nagasau are the only source of fertilizing nitrogen for their gardens. Colonies living in high-sun areas exclusively eat the nectar produced by their crops while shade-dwelling colonies—whose plants produce far fewer flowers and who spend far less time and energy defending their plants from pests—also hunt and consume other insects. As a result, the excrements of shade-dwelling colonies contain more nitrogen than the high-sun colonies.
These observations identify clear tradeoffs: a colony can either dedicate its workforce to guarding against pests or hunting for prey. Ants cannot simultaneously maximize all possible benefits. On the one hand, ants can farm in high-sun areas and produce much more nectar, but the colony then has to defend these plants and eat a less rich diet. On the other hand, ants can colonize shady branches, sacrificing their nectar yield but freeing their workforce to hunt, gaining a more complex diet and making better fertilizer for their plants.
If these tradeoffs sound familiar, it’s because they’re not so different than the choices human farmers face. Human farmers have to choose between growing a single, profitable crop or cultivating a more environmentally sustainable mix of crops, which would mean growing and selling less of their most lucrative product. Furthermore, human farmers can use synthetic pesticides to decrease crop destruction by pests, or they can forgo synthetic pesticides to avoid exposing themselves and their customers to these chemicals. Farming without synthetic pesticides can allow farmers to label their crops “organic” and sell them at higher prices, but produces lower crop yields. Like ants, human farmers cannot optimize every aspect of their practices. The key question is whether ants make the same choice that the vast majority of American farmers do when faced with these tradeoffs, prioritizing crop yield above all else.
Ants prioritize crop yield
Researchers observed that the plant species that P. nagasau farm occur significantly higher up trees, in significantly sunnier spots than closely related un-farmed plant species. This finding suggests that ants prioritize crop yield. Furthermore, almost 70% of colonies maintained all of their farms at a single light level, indicating that ants do not balance their cultivation strategies.
Prioritizing crop yield is not a momentary decision, but a consequence of evolution. P. nagasau have been refining their farming practices for around 20 million years. During this time, their farmed plant species have evolved alongside them. Under the ants’ influence, four separate lineages of plants developed the ability to produce nectar-bearing flowers to entice their farmers to keep investing in the planting and care of these species. Plants evolved to produce “food rewards,” and in turn the ants evolved to maximize the production of these nectar bearing flowers. Both species win, for now at least.
Neither P. nagasau nor their farmed plants appear to suffer from limiting their diets to nectar or reducing the quality of their fertilizer. But our planet is warming. Natural habitats are being degraded, fragmented, and destroyed. Whole species are dying. Humans are already facing one agricultural crisis—with more to follow as climate change intensifies—and it’s not hard to imagine disaster coming for P. nagasau. Perhaps the apparent fragility of these non-human farmers’ practices should serve as a warning to our own species. In abundant times, it’s hard to blame either ants or humans for prioritizing crop yield, even if some of our strategies prove wasteful, risky, and destructive; we’re just following our evolutionary cues. But as our species and our country faces the consequences of our tradeoffs, it’s time to reevaluate, and to evolve.
Rebecca Silberman is a Ph.D. candidate in the Biology department at MIT studying the role of chromosome imbalances in cancer.
Aparna Nathan is a third-year Ph.D. student in the Bioinformatics and Integrative Genomics Ph.D. program at Harvard University. You can find her on Twitter as @aparnanathan.