Thursday, 15 April 2010

Paper of the Week: Humans and Biodiversity

A new study interestingly implies that human activities may not always be bad for biodiversity.

Long before the colonizers arrived in South America, indigenous farmers, belonging to the Arauquinoid cultures, had already interfered with the Amazonian biodiversity. Their novel agricultural engineering methods had changed the savannah ecosystem, resulting in increased biodiversity. Thus states the solid paper, on 'Pre-Columbian agricultural landscapes, ecosystem engineers, and self-organized patchiness in Amazonia', published in Proceedings of the National Academy of Sciences (online before print, April 12th 2010), by Doyle McKey (Université de Montpellier II, France), Stéphen Rostain, José Iriarte, Bruno Glaser, Jago Jonathan Birk, Irene Holst, and Delphine Renard.

The savannahs of coastal Guyana tend to flood during the rains and are dry during the summer. However, strange complexes of mounds are seen in the terrain of these plains, running for 360 miles from Berbice River to Cayenne. Due to their perfect symmetry, the mounds were deduced to be man-made. The mounds drained well during the rains and floods (their drainage capacity was nine times as high as the seasonally flooded savannah). The authors deduce that these are large raised beds/fields, made out of the surrounding topsoil, for cultivating crops (a theory further substantiated by soil samples containing microfossils of maize, cassava, and squash), constructed by the pre-Columbian farmers, around 1000-700 years ago. The interesting point is that this farming was practiced in wastelands considered to be unsuitable for agriculture- a feat achieved due to their effective agricultural engineering.

When these fields were abandoned, the mounds were colonised by flora and fauna, thus creating a new ecosystem. These 'ecosystem engineers' (viz., ants such as Acromyrmex octospinosus and Ectatomma brunneum, termites such as Nasutitermitinae, and earthworms) built their nests on the raised beds so that the colonies wouldn't be flooded. Their burrowing aerated it further, helping in accumulating sufficient rainfall. Moreover, the mounds were fertilised as a result of them congregating organic matter into their nests and accumulating minerals such as nitrogen, potassium, and calcium. As a result, the perennial plants on the mounds flourished and their strong roots prevented the erosion of the mound. All of these alterations initiated by humans have resulted in a higher biodiversity than seen in the normal savannahs.

This study would give additional impetus to the debate over whether most of the Amazon rainforest and savannahs (commonly considered to be pristine) are sites of significant human occupation, especially during the pre-Columbian times. The authors suggest that this agricultural system could be a model for modern farming, especially considering the beneficial ecological changes.

Although this is a perfect example of a terrain modified by humans and maintained by Nature, it must be noted the increase in biodiversity was a result of 400-800 years of no/minimal human intervention. Secondly, the ‘punja’ technique of rice/paddy cultivation has a very similar methodology and is followed in parts of Kerala.


McKey D, Rostain S, Iriarte J, Glaser B, Birk JJ, Holst I, & Renard D (2010). Pre-Columbian agricultural landscapes, ecosystem engineers, and self-organized patchiness in Amazonia. Proceedings of the National Academy of Sciences of the United States of America, 107 (17), 7823-8 PMID: 20385814


David said...

Very interesting article, especially the idea that ants and others maintain the raised beds for their own purposes. I guess I assumed that man made structures are reclaimed by, or at least covered by, the original vegetation - I can say from personal experience how even an object as large as a Mayan pyramid can be completely "invisible" until shrubs and trees have been cleared away.

I wonder how many other structures are hidden in forests around the world?

Sarah Stephen said...

Valid question- it would be so exciting to stumble across a few!

I've observed how even our maintained grounds can transform into a mini-jungle when left untended for a month or two! After 600-1000 years of sustained neglect, I assume that it might be the next biodiversity hotspot!

Ants are interesting- no doubt that they wowed Solomon. The ants in our premises make their winter (rainy months) nests in our house & compound walls. Their summer nests are outdoors in the bunds created for our coconut trees. Another example of human engineering being used by nature, I guess.


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