The Hydraulic City

Buckley et al, 2010 present a good argument for the role of climate in the demise of the Khmer empire. Using Vietnamese tree rings that span the past 750 years, they show how severe inter-annual and inter-decadal drought periods intersected by intense monsoonal rainfall acted as a stressor leading to this societies’ collapse.

Angkor, the capital of the Khmer empire, if often referred to as the ‘hydraulic city’. This is because of the extensive canal, embankment and reservoir networks that supplied the city and its sprawling suburbs with water and irrigation for agriculture. These networks once supported a complex agricultural system.

A diagram of the water network in Angkor. Source: Blogspot

As the city expanded however, these networks became increasingly complex, convoluted and more difficult to manage, making them more vulnerable to large scale climatic events. Buckley et al argue that the tree rings show a severe drought at the time associated with the collapse of Angkor, which coincides with the transition from Medieval Climate Anomaly to the Little Ice Age. Drought periods were then disrupted by period of intense rainfall. This combination of drought and intense rainfall exacerbated erosion in the northern catchment. This sediment was then washed into the water system in the south which resulted in blocked water networks and the in-filling of canals. Buckley et al suggest that a single severe flood event could fill an entire canal section. In Angkor today, evidence of modifications in attempt to solve incurable failures in the water network can be found. 

Many academics have developed these arguments more recently and the argument remains very much the same. There is a vast amount of evidence from paleo-climate proxies that supports this theory and this argument has become a key explanation for the collapse of the Khmer empire. Although only the abstract is available at the moment, Afrifa et al, 2014 demonstrate the ongoing development of this theory. Fang et al, 2013 explain how drought, caused by a reduced Asian summer monsoon and dampened Westerlies, was a key factor in decline.

Angkor Wat, surrounded by a major water work. Source: Angkor Guide

In one case, radiocarbon dates taken from sediment found in a canal showed that it was filled in during the 14^th century. This shows that the canal was filled by flood debris from the time of Angkor and was never cleared out. This specific canal was one of the primary links between Angkor and Lake Tonle Sap which supplied water to the empire and Buckley et al conclude that this is evidence of the devastating impacts climate change had on the empire.

On top of devastating impacts of climate on water supply, other economic, social and geopolitical factors were also at play. Buckley et al acknowledge these, which include growing conflict with the Siamese Kingdom of Ayuddhaya (which is often blamed as the cause of collapse) and the increasing importance of maritime trade which caused the Khmer empire to turn away from inland agriculture and increased integration with regional trade.

I will look into these other factors in my next few posts.

The Khmer Empire

The Khmer Empire was established in the early 9^th century (800 C.E.) and had collapsed by 1430s. It was once the largest empire in South East Asia and existed in what is now Cambodia.

The Khmer empire was an agriculturalist society and relied heavily on rice cultivation. The extensive rice irrigation network allowed rice surpluses which helped support a large population. The Khmer empire had a sophisticated irrigation and water storage system as well as stable trade connections.

A temple once used for worship now covered by trees, Angkor. 

Source: WanderingMee

Historical records become very sparse after the 1300s, however we know that no large temples or monuments were erected after this period. Widespread abandonment of the Khmer Empire occurred roughly around this time. Some records attribute this decline to an invasion of the Thai army. Check out Plubins, 2013 for the Ancient History Encyclopedia. This is a good article for background knowledge, however I think it relies too heavily on the Thai army as an explanation.

It’s thought that one line of kinds remained in Angkor, the centre of the Khmer empire, while other kings moved to Phnom Penh. Gradually the economy was transferred to the new kingdom in Phnom Penh and therefore so did political power. This is thought to have led to the collapse of the Khmer empire in combination with conflicts within the royal family and Phnom Penh becoming the new centre of trade (New World Encyclopaedia).

However, recently other theories have emerged. The first to be looked at will be the role of climatic change which I’ll cover in my next post.

Things are not always as they seem

So we've looked at the main theories behind the collapse of the Rapa Nui of Easter Island. I think it's fair to say that there is little consensus as to what actually led to the decline of this culture.

Rather than hypothesize about possible triggers behind their decline, Bower (2014) has developed all of these ideas and takes an alternative approach. Bower argues that maybe the case of Easter Island shouldn't even be considered as an example of collapse.

Source: Harris, Cartoonstock

He stresses that unlike other articles, such as Mann et al (2008) from my previous post, deforestation does not equal societal collapse and correlations must not be confused for causation. He points out that, contrary to popular belief, the Rapa Nui civilization did not implode and disappear before European colonization in 1722. In fact the Rapa Nui society persisted at a constant level until 1774 when the culture declined. This decline is thought to have been caused by diseases introduced to the Island by European colonizers. Agriculturalists adapted to deforestation and the decrease in palm trees by turning to yam and sweet potato cultivation. Clever methods to reduce wind damage to small plants and reduce rain-water evaporation were developed, such as enclosing agricultural areas with rocks. This also deterred weed growth and improved soil nutrients.

Bower states that "the social collapse of the Rapa Nui has long been assumed, but there is so scientific evidence to support this". This demonstrates that the finer details need to be looked at when it comes to civilization collapse - things are not always as they seem!

Source: WorldlessTech                                                           Source: The Daily Omnivore

Something else that isn't what it seems - the Moai aren't just statues on the surface but have large bodies that are buried deep in the ground!

Next time I'll be introducing my final case study.

Merry Christmas!!

Burn baby burn

In a 2008 study, Mann et al use macrofossils and pollen data from 2 sediment cores to investigate drought, vegetation changes and human settlement on Easter Island.

Interestingly they assert that they found no evidence of a rat outbreak that could have caused collapse. As a rat outbreak would affect tree abundances and species composition, any such event would be reflected in the pollen record. However no evidence was found. However they do point out that the paleo record of Easter Island is still very poorly understood and the stratigraphic record may be too coarse to reflect this event. Therefore if a higher resolution record can be obtained, it may be possible to accredit or disprove Hunt (2006)’s rat hypothesis.

Source: SkyMe

From their sediment cores, they are able to infer ecological changes and drought events that occurred on Easter Island. They found that the beginning of agriculture on the island occurred straight after a severe drought. Marked soil erosion was found to have occurred at 1200AD which they attribute to human deforestation.

They also investigate the role of fire on the island. Before human settlement, there is little evidence of fire in the stratigraphic record (i.e. very little charcoal in the record). This shows that natural fire on the island is very uncommon and resultantly the ecosystem is ‘fire-naïve’ or in other words is not used to fire, has a low tolerance and cannot recover well from fire events. After human settlement, human induced fire shown by increased charcoal in the sediment record provides evidence to suggest that human degradation of the environment was a primary cause in societal collapse.

What Easter Island may have looked like pre-human settlement. 

Source: Science-history-lover

But, it is very difficult to determine environmental changes on Easter Island. As Mann et al point out, the remoteness and isolation of the island makes it difficult to compare environmental changes with surrounding areas, as the nearest land is very far away and quite probably has a different environmental regime. However some links can be drawn. Mann et al explain that there is evidence of correlative drought events in central Chile that agree with the climate record on Easter Island. Therefore they suggest that central Chile may provide a comparison site and might demonstrate synchronous drought events. Rainfall changes in Chile and tree ring events from Patagonia and Tasmania correlate with a known drought event on Easter Island between 900-1100AD. Perhaps with a better understanding of the Easter Island paleo record we might be able to infer more from climate events in Chile and Patagonia.

However this evidence has since been developed by Mulrooney, 2013 who points out that we must not confuse correlation with causation: deforestation may not have caused societal collapse. The radiocarbon dates of this study contradict other studies that claim the island underwent widespread abandonment. Mulrooney found that inland areas were not abandoned as previously thought and the Rapa Nui utilized most of the island up until European colonization in the late 1600s. Similarly the archaeological evidence from the island does not fit a societal collapse scenario model. Radiocarbon dates show continued habitation of island throughout the previously assumed collapse period. 

Till next time!

The curious incident of the rats in the night time

Let's now move away from Rull et al, 2013's commentary, who has faced criticism by other academics. Mieth and Bork, 2010 concluded from their investigation of Easter Island that any deforestation was done by humans. They found burnt palm stumps and areas of burnt soil indicating that anthropogenic activity was the cause of deforestation, rather than by rats. Similarly they found patches where palm trees had begun to regenerate, which they assert would not happen if rats had prevented regrowth by eating palm seeds.

More recently, other criticism has come from Larsen and Simpson, 2014 who argue that Rull et al relies too heavily on paleo-data. They argue that past environmental change provides limited insight into societal changes or collapse because every society has some capacity to adapt. They are also critical of Rull et al for failing to consider the potential role of natural disasters such as tsunamis, floods, earthquakes or volcanic eruptions. Any one of these events may have proven too much for a small, isolated population. However there does not seem to be much evidence for any of these events around the time or Rapa Nui decline. Larsen and Simpson also comment on the absence of rats as a potential influence in Rull et al’s discussion.

The role of rats was first investigated by Hunt, 2006 and has since been developed by Hunt and Lipo, 2012. When the Polynesian settlers reached Easter Island, they brought chickens and rats with them, shown by the sudden appearance of bones from these species in the sediment record of Easter Island. The pristine forest conditions of Easter Island (pre-human disturbance) provided ideal conditions for rats due to the almost infinite food available – from seeds, fruits and large insects. It is thought that rats ate seeds from trees which slowed the regeneration rates of the forests. 

Rat gnawed seed cases - they can penetrate hard, thick seed cases that birds cannot. 

Source: Hunt, 2006

At first this theory might seem a bit far-fetched. However Easter Island is fairly small (164km2) and in ideal conditions, rat populations can reproduce at rates doubling every 47 days. Over a short amount of time Easter Island would have become inundated with rats, especially seeing as, being an introduced species, the rats had very few predators.

The impacts of rats are well studied in other locations such as Hawaiian Islands and New Zealand. Therefore the presence of rats may have had devastating impacts, especially at a time of human resource exploitation. In conjunction with this, at the time of rate population growth, bird extinctions occur demonstrating a double-edged sword – this would have further influenced seed dispersal and potentially a food source of the Rapa Nui. 

This theory has since been developed and refined (2012) and still stands. It just goes to show – don’t underestimate rats.

Till next time!

Source: Radiolab

Let's not blame it all on the Rapa Nui

Generally speaking there has been a move away from the original ecocide theory. As more and more is learnt about past climate changes, how these changes relate to vegetation and society, more complex theories develop.

Firstly I just want to draw your attention to the importance of proxies in enhancing our understanding of past events. Rull et al 2013 (and an updated 2014 version) explore evidence showing that human presence on Easter Island was quite probably 1500 years earlier than previously thought. The introduction of new plants and increases in charcoal at around 450 AD suggest human populations reached the island long before we previously thought.

The Moai at night. Source: National Geographic

Rull et al demonstrate this in their meta-study, which looks at emerging evidence supporting the move away from a simple ecocide hypothesis. The Ecocide theory dismisses any causational factor that isn’t human activity and implies that the change from palm forests to open grassland (which dominates Easter Island today) was abrupt and sudden.

However paleoecological and archaeological evidence suggests the island actually underwent a more gradual change, spanning over 2000 years. In addition, results from the authors’ previous studies show no empirical evidence of any abrupt changes. Diatom assemblage changes and sediment from Lake Raraku (Easter Island) illustrate a very slow change towards slightly wetter conditions. With the onset of wetter climates, an increase in palm demise was found and an increase in charcoal, used as a proxy for fire.

Lake Raraku, Easter Island. Source: Flikr

Rull et al’s argument is that with greater water availability due to a wetter climate, populations on Easter Island would have been encouraged to increase agricultural land. This increasing demand, driven by a growing population in a more favourable climate, increased the use of resources and need for cultivable land. Rull et al do accept that currently this theory is highly hypothetical and more archaeological evidence is needed to increase its standing. However this is a very logical argument and follows observed trends in most human population growth.

Rull et al conclude that the history of Easter Island and therefore our understanding of what happened there needs rethinking and suggests a total revision may be necessary. I agree with this conclusion because too often an over-simplified explanation is accepted as fact. As the capabilities of looking at past climate and vegetation changes improves with paleolimnology and paleoclimatology we should revisit these cases of civilization collapse. I think we could probably learn a lot and may realise that more credit should be given to these ancient societies. Rull et al points out that new theories are always emerging, two most recent ideas focussing on climate changes relating to the Little Ice Age and ENSO. These may become more prominent hypotheses if evidence to support them is found.

Until next time!