A vast series of earth mounds on the eastern coast of South America may be living landscape fossils of a forgotten civilization’s agriculture.
People raised the mounds between 1,000 and 700 years ago in order to create cropland in terrain that is flooded for half the year, and parched for the other half. New insect ecosystems formed on the mounds, further enriching the soils and keeping them fertile for centuries, long after their human stewards had vanished. This lost agricultural system could be a model for modern farmers, according to a new study.
“Today these lands are used for cattle ranching or hunting. People think agriculture must not be possible in these areas,” said ecologist Doyle McKey of the University of Montpellier in France, co-author of a study published April 12 in the Proceedings of the National Academy of Sciences. “The common conception is that these areas are wastelands.”
McKey and a team of archaeologists, paleobiologists and soil scientists describe the earthworks, which run for 360 miles from the Berbice River to Cayenne, the modern-day capital of Guyana.
The study is part of a fast-growing body of research on the pre-Columbian world of the Amazon basin. Historians and anthropologists once thought it inhabited only by small bands of primitive hunters and gatherers, with interior jungles and coastal floodplains unable to support large-scale agriculture and complex societies. That picture no longer seems accurate.
Scientists have shown that now-vanished people transformed the Amazon, using biochar to nourish jungle soils, and moving floodplain soils to create irrigation channels and planting beds. McKey’s findings expand the range of known coastal agriculture and take an in-depth look at the beneficial ecological changes it created.
“Human engineering, if we do it cleverly, can work together with natural ecosystem engineering,” said McKey.
In addition to 100-foot-long, water-diverting berms, they identified expanses of mounds covering hundreds of acres. From the air, the mounds were too symmetrical to be natural. On the ground, soil samples returned fossilized evidence of maize, squash and manioc.
The mounds appear to have been constructed from layers of surrounding topsoil, which was shoveled out and layered like cakes. That formed the basis of the mounds, which put crops above the flood line but that was only one part of the agricultural trick.
Species of ants and termites settled in the mounds, where their colonies wouldn’t flood. Their burrowing aerated the soil, and plant matter foraged from surrounding areas enriched it further. As a result, the mounds acted like sponges for rainfall, and outsourced insect labor made them rich in key fertilizer nutrients of nitrogen, potassium and calcium. The root systems of perennial plants kept the mound structures intact, and likely did so when mounds were rotated out of production.
McKey is reluctant to speculate on how many people were supported by mound agriculture. A conservative guess based on crop yields from modern raised-bed farming experiments put the figure at one person for every two acres of farmland. That’s a very rough estimate, but enough to suggest that the farmers were not just small, family-based tribes.
More important than exact numbers is the evidence of agricultural success in a region that’s not considered suitable for modern agriculture. McKey thinks today’s farmers could learn from ancient tricks, and supplement them with modern tools.
As for the original inhabitants, little is known. They belonged to so-called Arauquinoid cultures, which emerged 1,500 years ago and vanished shortly before the arrival of Europeans. Whether they left descendants is unknown. They’re known only from a single wooden shove, some ceramic fragments and their farms.
“When people modified these ecosystems long ago, they changed the way the ecosystems work. We can use that knowledge,” said McKey.