In 1997, at age 19, Toby Kiers talked her way into the Smithsonian's renowned tropical research institute on Barro Colorado, an island in the middle of the Panama Canal. The scientists studied the many species of local bats, the monkeys they outfitted with radio or GPS collars, and the towering rainforest canopy.
There, during a year-long fellowship, Kiers learned about a type of fungi known as mycorrhizae that formed intimate associations with the tropical trees and grew in sprawling, underground networks. Found all over the world, mycorrhizae are microscopic. But if you stretched out all the mycorrhizae present in a hectare of grassland, end to end, they would be the length of many, many Amazon Rivers.
"It seemed like the most sort of frontier world at that time," she says, "because you just couldn't see it."
It became her life's work. The fungi would penetrate the roots of nearby plants, so some scientists suspected they were parasites. But over the following years, as Kiers earned her PhD and became an evolutionary biologist, she and other researchers showed that mycorrhizae were exchanging resources: They gave the plants phosphorus and nitrogen in exchange for sugars and fats that plants made from carbon in the air.
This led Kiers to the defining question of her career: Were plants and fungi canny economic actors? Was there a market price—a fluctuating exchange rate—between phosphorus and carbon?
Her goal is not to figure out whether fungi can trade and make economic decisions as well as us brainy humans. Instead, she suspects that by some measures fungi are better at economics than us—and some of the world's most powerful corporations seem to think she might be right.
Markets in Nature
Over the past few decades, scientists have come to increasingly appreciate plant intelligence. Plants communicate and alert each other to predators. They mount defenses, such as releasing toxic or unpleasant chemicals when animals munch on their greenery, and learn to ignore harmless stimuli. They may even form memories.
Some 70 to 90% of plants engage in symbiotic exchanges like those between mycorrhizae and their plant partners. Many scientists view this cooperation as akin to sharing. In 2016, ecologist Suzanne Simard memorably called a forest "a cooperative system."
To Kiers, though, the idea that plants and fungi shared resources like well-behaved kindergarteners seemed to underestimate them. Why wouldn't these amazing organisms, these survivors of millions of years of evolution and resource competition, try to cheat each other? Why not take their partners' resources and offer nothing in return?
Nature is full of literal parasites. So if plants and fungi had been exchanging resources for millions of years, they must have strategies to prevent freeloading. It seemed like an economic problem.
Around this time, primatologist Ronald Noë was drawing on economic ideas to study how monkeys trade food and grooming sessions like commodities. (Here's an ancient Planet Money episode about that from 2009.) With a colleague, Noë defined a line of research called biological market theory that documented additional examples:
- Biologists have long-noted supply and demand at work in "mating markets," where the number of eligible males and females dictate power dynamics. If many males compete over a small number of mates, they may offer more food or resources (a higher price) to win over a female.
- When one business holds a monopoly, it can charge customers higher prices. In various coral reefs, the cleaner wrasse fish makes a living by removing (and eating) dead skin and parasites from larger fish. But they occasionally take large, unwanted bites from their clients. Scientists have found that when larger fish are not mobile enough to choose between different "cleaner stations," the wrasse exploit their market power by taking those large bites.
- The concept of comparative advantage—that countries benefit when they specialize in what they are relatively better at producing—has helped biologists understand why plants evolve to specialize, even when they could efficiently produce the resources they get from their trading partner. (Comparative advantage refresher from Planet Money summer school here.)
This line of research inspired Kiers's work with fungi. "Could we apply those same [economic] principles to other organisms that didn't have a central nervous system?" Kiers asks. "That was the start of my journey into this."
Unlike humans, plants and fungi can't sign contracts. Without courts to enforce binding agreements, a primary way to ensure trade is beneficial is partner choice—trading with partners offering a good deal and ceasing to trade with those that don't. So Kiers went looking for evidence of partner choice in the plant and fungus kingdoms.
Free Trade, Circa 500 Million BC
Unlike with the wrasse fish, Kiers could not directly observe plants and fungi trade. She needed some cleverly designed experiments—and some quantum dots.
To tease out evidence of partner choice, she first studied a more simplified economic exchange than the mycorrhizae-plant marketplace. For her PhD research, she studied legume plants, like soybeans, that partner with microbes that reside on their roots. The legume provides sugars; the microbes give it fixed nitrogen in return.
Was the legume a discerning trading partner? If a microbe didn't provide nutrients, would the soybean still feed it sugar? To find out, Kiers and a colleague surrounded some of the microbes with air completely lacking in the nitrogen that they turned into tradeable nutrients. In response, they observed, the legume "sanctioned" the microbes. Many of these sanctioned microbes failed to reproduce, most likely due to the legume reducing their access to oxygen.
"That's soybeans," says Kiers. "How cool that they're able to do this." To Kiers, the soybeans "sanctioning" of freeloading microbes looked like partner choice.
Kiers then returned to her beloved fungi. Plants and fungi form what could be a "thick market": Mycorrhizae are (almost) continually growing and forming connections with new plant roots, so, unlike the legumes, plants and mycorrhizae generally each have multiple trading partners. Do plants and fungi exercise partner choice, too? Do they respond to supply and demand? Kiers's experiments suggest that they do.
In one experiment, for example, she tracked the exchange of resources between fungi and plants when some of the plants were in full shade and the others in full sun. The plants in full sun had more sugars to trade and, sure enough, the fungi traded more with them.
In further experiments, Kiers worked with chemist Matthew Whiteside and biophysicist Tom Shimizu to tag the resources traded by plants and fungi with quantum-dot nanoparticles of different colors, allowing them to track trading patterns. Then with biophysicists at AMOLF to build a robot that continually images the fungi's ever-growing trade networks.
With these tools, they have documented that fungi seem to save their resources during plentiful times and spend them during lean periods, to hoard their resources when the market price is too steep, and to respond quickly to changes in supply and demand.
It's hard to overstate the importance of this kind of trade. Because it very literally created the world as we know it. A billion years ago, there were no plants on land. Plants managed to expand from the oceans by trading with fungi and microbes, who could break rocks down into nutrients they needed.
"This led to a 90% reduction in CO2 levels," says Kiers. "We owe our atmosphere, we owe our forests, we owe our grasslands to this partnership."
(Mycorrhizae are still responsible for drawing down so much CO2 each year—the equivalent of ⅓ the emissions from fossil fuels—that Kiers co-founded an organization, SPUN, to "protect the underground" the same way we protect the Amazon Rainforest and biodiversity hotspots like the Galapagos.)
Specialization and trade is such a powerful, productivity-boosting strategy that plants that abstain from it are practically the exception that prove the rule.
Brains Are Overrated
When I ask Kiers how studying fungi's underground economy has changed her view of our human economy, she says it's made her extra skeptical of when a small group of people control all the resources and make all the decisions.
With mycorrhizae, she says, " what we're looking at is decentralized decision making." It's still a bit mysterious how fungi "decide" when and where to trade, and how they respond to supply and demand. But it's a bit less mysterious when you compare fungi's underground economy to our own.
Unlike fungi and plants, humans can mull whether, say, Facebook stock will go up. But much of our economy runs on simple decisions. If the price of eggs is low, we buy eggs. If the price of steel increases enough, auto companies may switch to making chassis with more aluminum.
Like fungi, we make simple decisions in response to supply and demand. But collectively our decisions dictate the flow of steel and eggs around the globe, sending resources where they will be more productively used or in-demand, all without any one person calculating who needs what.
Kiers is careful not to fully equate humans buying, selling, and trading with the bartering between plants and fungi. Not because their lack of a brain means they can't match our sophistication. Quite the opposite! She suspects that their markets, honed by millions of years of experience and evolution, are more advanced than our adorable, few-millennia old attempts at trade and long-distance supply chains.
"I really do think that there's a lot to be learned from the way that they build infrastructures … for developing [our] supply chains above ground," Kiers says.
Consider the case of Apple CEO Tim Cook, who previously managed the company's supply chains, ensuring all the materials needed to make iPhones came together at Apple factories. The long-term trend of our economic history has been replacing centralized decision making with decentralized markets; and fungi are, without brains or central coordination, building and operating huge, underground infrastructures and trade networks.
Kiers says her team is fielding calls from "big tech companies"—companies "like" Google DeepMind that are very interested in decentralized decision making. It's early days, but it could be that an important part of artificial intelligence is mimicking the intelligence of the plants and fungi that created the natural world's infrastructure and trade systems.
In the meantime, Kiers's research has helped me understand why it's so hard to keep houseplants alive: We've cut off their access to trade.
"I do think there's something to be said about intact networks," she says. "They really offer a lot of resilience."
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