What does it take to map something you have never seen?
In this episode, Matt Forrest sits down with Justin Stewart, quantitative ecologist at SPUN, the Society for the Protection of Underground Networks, to unpack how his team built the first global map of arbuscular mycorrhizal fungi, the soil organisms living in symbiosis with most plants on Earth.
Justin walks through the full pipeline, from robotic microscopy of threads two and a half microns wide up to a planetary prediction at one square kilometer resolution. The team assembled 16,000 soil samples from more than 300 studies across 11 languages, trained a random forest on 34 satellite-derived layers in Google Earth Engine, and
summed the result: roughly 110 quadrillion kilometers of fungal thread, about 300 megatons of carbon, six times the biomass of every human alive.
The findings were not what anyone expected. The densest networks are under grasslands, not forests. Croplands run about 50 percent sparser. And 90 percent of the biodiversity hotspots for these fungi sit outside any protected area.
In this episode, we cover:
- Why mapping a symbiosis is harder than mapping an organism
- Robotic imaging in the Amolf biophysics lab, and tracking half a million nodes at once
- Building a global database from literature in 11 languages
- Random forest niche modeling and uncertainty analysis in Google Earth Engine
- The effective radius algorithm that turns network length into biomass
- Why grasslands, flooded wetlands, and the Tibetan plateau light up on the map
- Tilling, fertilizer, and how a 450 million year old symbiosis gets broken
- Rights of nature, eDNA, and why predictive maps are not yet admissible evidence
LINKS:
Justin Stewart
SPUN profile:
https://www.spun.earth/team-members/justin-stewartLinkedIn:
https://www.linkedin.com/in/justin-stewart-70512645/Google Scholar:
https://scholar.google.com/citations?hl=en&user=1aqTJIUAAAAJSPUN
Website:
https://www.spun.earthUnderground Atlas:
https://a-hidden-infrastructure.spun.earth/story/a-hidden-infrastructureLEARN MORE
The paper: Science, June 11, 2026. DOI 10.1126/science.adu4373
Mycorrhizal Biodiversity Map:
https://a-hidden-infrastructure.spun.earth/story/a-hidden-infrastructureCHAPTERS:
00:00:00 – The largest living network on Earth
00:02:15 – Welcome and intro
00:03:38 – What the map shows: a partnership, not an organism
00:05:48 – Robots, petri dishes, and carbon moving at 400 kilometers an hour
00:07:47 – The question that started it: how much is out there?
00:09:25 – 110 quadrillion kilometers and six times human biomass
00:11:15 – What a quantitative ecologist actually does
00:13:53 – Trade, lipids, and 30 percent of a plant's nitrogen
00:16:31 – What people get wrong about soil
00:18:27 – The pipeline: 34 satellite layers, random forest, Earth Engine
00:21:02 – From length to weight: effective radius and the robot named Prince
00:25:41 – Croplands and a broken 450 million year symbiosis
00:31:40 – Rights of nature and getting predictions accepted as evidence
00:37:10 – Where to find SPUN and Justin
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