Mounds on the coastal plain of the Guianas plateau. Photo by Stéphen Rostain.

Exeter archaeologist helps uncover 900 year old agricultural secrets

The coastal savannas of French Guiana, dotted with thousands of small mounds, have given up some of their secrets, thanks to an interdisciplinary European collaborative research project.

Dr José Iriarte of the University of Exeter’s Department of Archaeology (School of Humanities and Social Sciences) is a member of the team, which includes over 20 specialists in diverse fields belonging to several institutions from France and other European countries.

The research was led by Doyle McKey, ecologist at the Centre for Functional and Evolutionary Ecology (CNRS / University of Montpellier) and Stéphen Rostain, archaeologist in the research unit Archaeology of the Americas (CNRS / Université Paris 1).

The researchers discovered that these mounds are agricultural raised fields, vestiges of a pre-Columbian agricultural system constructed over 900 years ago. The pre-Columbian farmers cultivated three plants (with evidence for others likely still to be discovered): maize, manioc (also known as cassava), and squash. The researchers showed that following the abandonment of this system, these well-drained islands in seasonally flooded environments were colonised by other organisms (animals and plants) that have maintained these small elevated structures up to the present day. This example of a landscape modelled by humans and then maintained by nature could help us imagine how to design ecologically intensive agricultural systems.

These results are published on the website of the journal PNAS (Proceedings of the National Academy of Sciences of the USA) during the week of 12 April 2010.

The seasonally flooded savannas of the coastal plain of the Guianas plateau, from near Cayenne to Guyana, are 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. Some believe that these mounds could have resulted from the repeated passage of cattle in these marshy savannas. However, archaeological studies carried out in the late 1980s had already shown that these structures were created by humans, and provided strong evidence for their pre-Columbian origin.

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 occasioned by heavy tropical rainfall and by fires that frequently reduce the protective cover of vegetation. Since 2007, an interdisciplinary research team has tried to answer this question.

Relying on expertise in a broad range of fields (archaeology, archaeobotany, 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. Until now, the interactions between natural and cultural processes, and how they could affect these landscapes, have escaped the attention of researchers.

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 subject to rainy-season flooding and dry-season drought and fires. 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 was made in the XII Century another to the XI Century.

Once abandoned, the fields were taken over by nature.  Ants, termites, earthworms, plants and other organisms preferentially colonised these well-drained structures, maintaining them.

The agricultural technique of raised fields, today largely forgotten, could be a source of ideas for designing ecologically intensive agricultural ecosystems. These results show how some ecosystems have been conserved over centuries and help us better understand the history of Amazonian biodiversity.

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

Date: 13 April 2010

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