French Guianan coastal savannas: A landscape shaped by humans and by nature

The researchers discovered that these mounds are agricultural raised fields, vestiges of a pre-Columbian agricultural system constructed over 900 years ago. Above all, the researchers showed that following the abandonment of this system, these well-drained islands in seasonally flooded environments were colonized by other organisms (animals and plants) that have maintained these small elevated structures up to the present day. This example of a landscape modeled by humans and then maintained by nature could help us design ecologically intensive agricultural systems.

These results will appear online on the website of the journal Proceedings of the National Academy of Sciences on 12 April 2010.

The coastal plain of the Guianas plateau, from Cayenne Island to Guyana, is dotted with thousands of small mounds, which at first sight appear to be rather commonplace mounds of earth. Were these small elevated structures created by natural processes? For local inhabitants, their origin is mysterious, and a great range of notions abounds. Some believe, for example, that these mounds could have resulted from the repeated passage of cattle in these marshy savannas. However, archaeological studies carried out in the late 1980's had already shown that these structures were created by humans.

How have these raised fields-built by pre-Columbian Amerindians and abandoned around 1250 AD, before the arrival of Europeans-been able to persist to the present day? They should have disappeared as a result of the erosion, rainfall, fire, and the shifting of vegetation. Since 2007, an interdisciplinary research team has tried to answer this question. Led by Doyle McKey, ecologist at the Centre for Functional and Evolutionary Ecology (CNRS / University of Montpellier 1, 2 and 3/ENSA Montpellier/ CIRAD/Ecole pratique des hautes études de Paris) and Stéphen Rostain, archaeologist in the research unit Archaeology of the Americas (CNRS / University of Paris 1), the team includes over 20 specialists in diverse fields belonging to several institutions from France and other European countries. Relying on expertise in a broad range of fields (archaeology, archeobotany, paleoecology, soil sciences, ecology and aerial imagery), the scientists have tried to understand how the past actions of humans on these landscapes has affected the contemporary functioning of the ecosystem.

Raised fields that were built and exploited between 650 and 1250 AD

This study confirms first of all that pre-Columbian farmers built the vast complexes of raised fields found in Guianan savannas. This coastal fringe, considered inhospitable, is subjected to seasonal flooding (an alternation of periods of rain and drought). The Amerindians constructed mounds to make well-drained soil, permitting intensive sedentary agriculture. They thus efficiently applied agricultural engineering to exploit lands that are today considered unsuitable for farming. The researchers succeeded in precisely dating some of these fields: one of the sites dates back to the 12th century, while the second is yet another century older. The analysis of two types of plant microfossils -- phytoliths found in the mounds and starch grains, found on fragments of ceramic cooking utensils discovered in ancient Amerindian villages -- showed that these farmers cultivated at least three plants: maize (which, astonishingly, is absent from the panel of plants cultivated in the region today), manioc (cassava) and squash. By constructing these well-drained islands, the Amerindians produced heterogeneity between the well-drained and elevated parts of these landscapes: the biogeochemical composition of the top 50 cm of soils of the two zones is still different today.

Landscapes co-constructed by Humans and Nature

Once abandoned, these fields were taken over by nature. Ants, termites, earthworms, plants and other organisms preferentially colonized these well-drained structures. These "ecosystem engineers" generated self-organized processes. These organisms transport organic matter and mineral soil to mounds and modify the structure and composition of mound soils. Owing to their effect on soil porosity, the infiltration capacity of rain water is nine times greater on the mounds than on the seasonally flooded plain, reducing the susceptibility of mounds to erosion. These biogeochemical mechanisms have thus permitted the maintenance of these elevated structures, where the concentration of resources initially created by humans is conserved.

These self-organized ecosystems are the depositories of the ecological legacy of the pre-Columbian inhabitants of Amazonia. The agricultural technique of raised fields, today largely forgotten (1), could be a source of ideas for designing ecologically intensive agricultural systems. These results show how some ecosystems have been conserved over centuries and help us better understand the history of Amazonian biodiversity.

This work was supported with funding from CNRS and the French Ministry of Culture and Communication. It also benefited from the cooperation of the Guianan Space Center, which allowed access to sites on their property.

Notes

(1) Similar techniques are used today by certain African populations and populations of African origin (for example, a few Haitian farmers recently settled near Kourou).