The Amazon rainforest in Yasuni National Park in Ecuador. Photo by: Jeremy Hance.
The Amazon is the largest tropical forest on the planet, covering about 6.5 million square kilometers, although much has been lost (around 18-20 percent) in recent decades. The great forest also, very likely, contains the highest biodiversity of species on land; for example a single hectare in Yasuni National Park contains more tree species than all of the U.S. and Canada combined. Yet new research published in the Proceedings of the National Academy of Sciences (PNAS) finds that quite recently—just 500 years ago—a significant portion of the southern Amazon was not the tall-canopied forest it is today, but savannah.
“These results were very surprising. We went to Bolivia hoping to find evidence of the kinds of crops being grown by ancient Amerindian groups, and to try to find how much impact they had on the ancient forest. What we found was that they were having virtually no effect on the forest, in terms of past deforestation, because it didn’t exist there until much later,” said lead author John Carson with the University of Reading.
For centuries scientists believed the Amazon rainforest had been largely unsculpted by man, only inhabited by small bands of hunter-gatherers with a light environmental footprint. However, that view has been challenged in recent years. Deforestation across the southern band of the Amazon has revealed clear marks of human societies: large geometric earthworks, whose purpose remains unknown, but whose construction was definitely the work of people.
Forest loss trends in the Amazon. Click image to enlarge.
Deforestation in Amazonian countries from 2001-2012. While deforestation has fallen in Brazil, which contains most of the Amazon, it has risen in other countries such as Peru. Click image to enlarge. Related charts
The discovery of hundreds of these geoglyphs—in the form of ring ditches—has led a number of researchers to argue that the Amazon was, in fact, densely inhabited prior to the arrival of Christopher Columbus with his Old World diseases. This theory is also supported by many of the historical accounts of the Amazon by early travelers, some of whom describe large, complex civilizations in the forest and dangerous warriors. In fact, it was a description of a particularly fierce band of female warriors that gave the forest its name. Proponents of a more civilized Amazon further argued that these large populations hugely impacted the ecosystem through millennia of large-scale slash-and-burn farming. Instead of a wilderness, they say, the Amazon was more like a garden; only after Columbus brought genocide with him, did the forest become what it is today.
However, the new research rejects both these theories at their most extreme.
“Our unexpected findings reveal a surprising third scenario, in which earthwork builders took advantage of a naturally open savanna landscape, which existed under drier-than-present climatic conditions before around 2,000 years ago,” the paper reads. “This finding suggests lower environmental impact, less labor, and possibly a smaller population than previously assumed.”
Carson and his colleagues took sediment cores from two lakes in northeast Bolivia—Laguna Oricore and Laguna Granja—near ancient earthen mounds. Reading the evidence from these sediments—such as pollen and charcoal—the researchers were able to reconstruct a picture of the ecosystem over some 6,000 years.
They found that during most of that period the Amazon rainforest was actually smaller than it is today. Then about 2,000 years ago, climatic and precipitation changes brought the Amazon rainforest southward, engulfing a region that before then had been largely wooded savannah.
Indigenous Kaiapo boy in the Brazilian Amazon. The Amazon has been inhabited by people for thousands of years, but debate remains over the size and impact of this population. Photo by: Rhett A. Butler.
“Rather than cutting or burning down huge swathes of jungle, the early Amazonian people simply took advantage of a naturally more open landscape,” said Carson. “Still, the scale of the earthworks that where built on these sites suggests that the land was capable of supporting relatively large populations. Our analysis shows that they were growing maize and other food crops. They also likely caught fish, and there’s evidence from other parts of the Bolivian Amazon for people farming Muscovy ducks and Amazonian river turtles.”
In some places the expansion of the rainforest around the time of Christ may have been halted—or at least fragmented—by farming communities already living in the savannah. As the trees marched in—like Birnam Wood or Tolkien’s Ents—the farmers would have learned to clear large areas in order to survive. They probably did this for centuries before the arrival of Columbus and the subsequent collapse of their communities.
“Humans did have significant environmental impact on a more local scale by suppressing climate driven forest expansion around,” reads the paper, which argues that other geoglyphs across the southern Amazon were likely created in similar savannah conditions and not in primary-growth rainforest.
However, it must be noted that the evidence collected by Carson and his team can in no way be extrapolated across the vast and diverse Amazon ecosystem. Instead, it shows that portions of the Amazon—in this case the very southern edge—were savannah during the historical past. The region in question is known as the seasonal southern Amazonian rainforests (SSAR) and covers around one million square kilometers, or less than one-fifth of the Amazon Basin.
In contrast, research in other parts of the Amazon has found the forest was relatively stable and that the human touch may have been light, though even that contention remains controversial.
“Paleoecological studies conducted in aseasonal western and central Amazonia suggest stability of the rainforest biome throughout the Holocene [over 11,000 years ago] and appear to show little evidence for significant deforestation or biomass burning by their pre-Columbian inhabitants,” reads the paper.
A sharp line separates the Amazon rainforest from cattle pasture in Brazil. The majority of deforestation in the Amazon has been for raising cattle. Photo by: Rhett A. Butler.
There is more at stake in these debates than simply recreating a lost world. By looking at the past, scientists are trying to determine how the future will impact the Amazon rainforest. Currently, the Amazon is under siege. Cattle ranching, industrial agriculture, logging, mining, fossil fuel exploitation, hydroelectric dams, roads, and growing human populations have all taken their toll on the world’s greatest rainforest. In Brazil, alone, over 750,000 square kilometers has been destroyed since the 1970s, an area larger than Texas. While deforestation has slowed significantly in Brazil over the past decades, it’s still ongoing and has sped up in several other Amazonian countries. Overall, no one knows how much has been lost, but its likely around a fifth of the total Amazon. And every year, a little less of the world’s greatest rainforest is left.
So, how much deforestation can the rainforest stand before the ecosystem begins to unravel? And perhaps an even more important question for future generations: how will global warming impact the Amazon?
“Our findings have serious implications for understanding past climate change, and how the Amazon basin might react to more modern forest clearance. It suggests that Amazonia was neither pristine wilderness, nor has it shown resilience to large-scale deforestation by humans in the past,” said Carson.
This means, ongoing deforestation may pose a major risk to the ecosystem. Moreover, since past climate change has clearly impacted the forest in the past, there’s no reason to expect it won’t do so again.
The new paper is unlikely to end the debate over the make-up of the pre-Columbian Amazon, but instead provides an increasingly complex picture of the world’s greatest rainforest.
The Amazon rainforest is likely the world’s most biodiverse ecosystem. Insects make up the bulk of species, like this butterfly, but the majority of them haven’t even been named. Photo by: Rhett A. Butler.
Citations:
- John Francis Carson, Bronwen S. Whitney, Francis E. Mayle, José Iriarte, Heiko Prümers, J. Daniel Soto,
and Jennifer Watling. (2014) Environmental impact of geometric earthwork
construction in pre-Columbian Amazonia. PNAS. www.pnas.org/cgi/doi/10.1073/pnas.1321770111
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