HBO’s series The Last of Us may have been the first time many viewers encountered the nightmare-inducing fungus of the genus Cordyceps. Or perhaps it was in that spectacular sequence in Sir David Attenborough’s Planet Earth, where viewers watched an ant, infected with Ophiocordyceps unilateralis, seek higher ground, die, and then sprout a stalk from its head that will release spores to infect the next hosts.

As a biological agent of zombification, it’s a good one: usually zombie outbreaks are attributed to viruses, as in The Walking Dead, or 28 Days Later, but certain species of Cordyceps have another thing going for them: they’re worth more than their weight in gold.

Ophiocordyceps sinensis, formerly Cordyceps sinensis, is a fungus that does much the same to Himalayan caterpillars (moth larvae) of the Thitarodes genus as O. unilateralis does to ants. The larval caterpillar form of ghost moths are infected while living underground in late summer. The caterpillar-fungus pair over winter and, once the ground thaws, are ready to infect a new generation—unless they’re harvested for medicinal use.

Known in Tibetan as yartsa gunbu and in Chinese as dong chong xia cao, the caterpillar fungus has been a centuries-old prescription for multiple illnesses, as well as an aphrodisiac and symbol of wealth. More recently, companies making “nootropics” or “smart drugs” have been including the fungus in supplements (and coffee) meant to enhance “focus” and other cognitive functions. Harvesters have provided the insect/fungal complexes to the market and are part of a regional economy dependent on infection and collection. For many harvesters in Nepal, Bhutan, India, and China/Tibet, O. sinensis is their primary source of income.

In May 2017, high-quality caterpillar fungus was worth as much as $140,000 per kilogram (≈ $64,000 a pound), about three times the price of gold at the time on the Beijing exchange (“high quality” meaning that the fungus has not yet sporulated, the form preferred by Chinese medicine practitioners.) The increasing demand and high prices for harvested fungi have encouraged more people to collect them, and as with any scarce commodity, crime (even murder) has grown up around it.

A November 2018 study in PNAS examined “[t]he demise of caterpillar harvesting” as the possible result of this over-harvesting as well as a less easily managed (and perhaps much greater threat): climate change.

The researchers used multiple data sources in their work: collector interviews, a review of relevant literature, regional climate data records, and official records of caterpillar fungus collection. By matching collection reports to long-term and interannual weather records, the ecologists could narrow the ranges of temperature, elevation, moisture, and soil conditions that favored higher (or lower) numbers of fungi. The interviews allowed them to pinpoint areas of increase or decrease and examine those for increased collection competition and/or climate-related changes.

The study found that production increases with higher elevation (within a specific range), cooler temperatures, and higher precipitation, all variables affected by a warmer climate. In Bhutan, “mean winter temperatures [have increased] by 3.5–4°C across most of [the] predicted habitat (+1.1°C per decade, on average).” When temperatures rise in mountainous regions, organisms that require lower temperatures will try to migrate up to avoid the “heat.” One North American example is the American pika, which prefers to live in cool, moist areas in mountainous regions in the western US. As their homes have warmed, some surveyed populations have disappeared and some have moved to higher ground. For those living closer to sea level, the US Fish and Wildlife service believes that the end will come: “low-elevation populations of pikas are likely to disappear due to rising summer temperatures over the coming decades.”

Moving caterpillar fungus “up the mountains” is not a simple fix. If there’s permafrost or not enough vegetation, the conditions are too dry; if the area is too warm, it’s likely that production will decrease even more. The region’s residents, who depend on collection for their livelihoods, will be competing for fewer fungi, driving prices higher and possibly encouraging more speculators to begin collecting. Based on their research, the authors recommend affected regions developing economic alternatives to collection (more easily said than done), as both climate change and sustainable management of caterpillar fungus will reduce the amounts available to harvest.

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