Deforestation: A Conclusion

So the intriguing question we can conclude from the last few posts is: Did the Easter Islanders completely destroy the island’s dense subtropical forest, or was Rapa Nui since the beginning of human colonization, a poor environment covered only with local spots of forest, and was it drought and the introduction of rats in combination with humans that finally triggered the extinction of the already rare plant species (Jubaea chilensis).


Due to problems and inaccuracies associated with dating human arrival, deforestation, climate variability and the impact of rats, it is hard to tell which was the dominant cause, or what combination of causes could have triggered and brought about the deforestation of Easter Island. Nonetheless, what we can be sure of, is the deforestation on Easter Island was a complex process. Many scholars researching the collapse of Easter advocate that deforestation played an important, or even central role in the collapse of Rapa Nui, for example Jared Diamond’s highly popularised ‘Ecocide’ theory that humans caused deforestation, which in turn caused their own collapse. However, there are numerous other theories that try to explain the collapse of Rapa Nui, for example overpopulation, soil erosion, warfare and conflict, and the arrival of the Europeans and slave trade. The subsequent posts will investigate these theories as a cause of collapse. 

Easter Island Ecocide: Deforestation due to Humans?

Easter Island underwent deforestation between the arrivals of the Polynesians c. AD 800,and the arrival of the Europeans 1722. The circumstances, causes and triggers of this environmental change have been partly discussed in the last few posts, now this post will attempt to uncover whether humans caused deforestation on Easter Island. This is perhaps the most researched, written about, and debated cause of deforestation, not only because of evidence and support from Jared Diamond’s much publicised view of ‘Ecocide’, but also because of the parable it has for today’s society.

In their paper ‘Humans, climate or introduced rats – which is to blame for the woodland destruction on prehistoric Rapa Nui (Easter Island)?’ Meith and Bork (2010) investigate the causes of deforestation on Easter Island, and concluded that it was humans who caused their own demise…thus it was ‘suicide’.

After settlement on Rapa Nui, population rapidly expanded to a peak of maximum 10,000 (Diamond 2007). The now extinct Jubaea chilensis palm provided numerous uses, including firewood and timber, the construction of canoes, possible transportation methods for the Moai, edible fruits and seeds, and palm leaves that could be used for thatching and rope. As the population expanded, not only did the need for the palm increase, but also more open space was essential for the increased agriculture and number of dwellings and gardens.

One of the best ways to investigate whether anthropogenic deforestation has occurred is to distinguish if charcoal appears in both the pollen diagrams and the soil profile, as this is a clear indicator of man’s influence on the forests. Soil profiles on Rapa Nui provide evidence of ‘slash and burn’ fires in the former palm woodland (as shown by Figure.1), where some of the charcoal layers (specifically on the Poike Peninsula and the southwest of Rapa Nui) contained burned nutshells of the Jubaea chilensis.

Figure 1. Two segments of soil profile in South West Poike. Summarized stratigraphy: (1) Weathered volcanic bedrock. (2) Cone of palm root molds. (3) Pre- clearing garden soil. (4) Accumulation of charcoal from a burned palm stump containing in situ aggregates of burned soil. Result of intentional burning by humans. (5) Charcoal layer. Charcoal layers are best preserved in concave down-slope sections. (6) Post-clearing garden soil with planting pits. (7) Fine layered sediments, resulting from post-clearing sheet erosion of unprotected soil. 

Carbon dating these burnt nutshells indicate that the oldest date of charcoal is around AD 1244-1254, and therefore supports the conclusion made by Meith and Bork (2010) that deforestation and ‘slash and burn’ on Rapa Nui started around the 13^th Century. These findings by Meith and Bork are supported by Hunt and Lipo (2006), where Figure 2 sums of Hunt and Lipo’s work by showing the chronological sequence of slash and burn for 12 sites of Rapa Nui. The suggestion by Meith and Bork (2010) that anthropogenic deforestation started in the 13^th Century is supported by this chronological sequence.

Figure 2. The chronological sequence of slash and burn for 12 sites on Rapa Nui.

From looking at Figure 2, and Figure 3 below, which provides probability distributions for 15 calibrated radiocarbon dates in context with woodland clearance from different sites around Rapa Nui, Meith and Bork (2010) have built up a picture as to what happened on Easter Island.

‘Clearance occurred in phases of deforestation on Poike Peninsula between approximately 1250 and 1510 AD, and at the slopes of Rano Kao roughly between 1400 and 1520 AD. All data demonstrate that the destruction of the palm woodland on Rapa Nui probably lasted no longer that 300-400 years….the main slash and burn activity occurred between 1200 AD and 1600AD.’

Figure 3. Probability distributions for 15 calibrated radiocarbon dates in context with woodland clearance from different locations on Rapa Nui. 

Meith and Bork (2010) thus conclude that:

‘Both the temporal placement (after the onset of human colonization) and the sequential chronology of woodland clearance in different parts of the island underscore that the deforestation was an act of humans’

 This paper also investigate the role that rats play in deforestation (see previous post), and from looking at the nutshells in the soil profile, less than 10% of these charred palm nuts actually contained teeth marks from rats, therefore they conclude that rats are NOT the cause of deforestation on Rapa Nui. They also use a several other lines of evidence to support this conclusion. Firstly, they found that the Jubaea chilensis palm in La Campana National Park, Central Chile, sustains a healthy population of 80,000 palms whilst coexisting with a similar type of rat that is suspected to have inhabited Rapa Nui. And secondly, these rats simply could not have felled mature trees.

Pollen proxies in the lake sediment (Flenley and King 1984) also verify a long existence of the palm woodland (around 35,000 years) on Rapa Nui before the arrival of humans, suggesting that the palms survived all climate variations from the late Pleistocene to the late Holocene.

‘What were the Easter Islanders thinking when they cut down the last tree’?

Once the last tree had gone, they no longer had firewood and timber, ropes, thatching, possible transportation of the Moai, and canoes (their only way off the Island). This then starts the ‘Ecocide’ theory that Jared Diamond suggests….

A quick note: The uses of the palm on Rapa Nui, and the anthropogenic deforestation theory is summed up very well in a paper by Rosalind L. Hunter Anderson (1998). I havn’t used it in my post here as some of the research is slightly out of date, however it does provide a perfect summary of what has been indicated in this post!

Climate Variability, An Afterthought

In relation to my last post on whether climate variability and change could have been a contributing factor to the deforestation of the Jubaea chilensis palm on Easter Island, i thought it was important to share with you a couple of graphs that support this hypothesis.

There is controversy over whether ENSO could have affected deforestation, whereby some studies have found no evident correlation between both variables. However, Stenseth and Voje (2009) show that there is an interesting 'coincidence' between the most intense ENSO (El Nino-Southern Oscillation) activity during the last millennia and the period of deforestation on Easter Island, as shown by the graph below. Could this be more that coincidence? 

It is exceptionally complicated to reconstruct past climatic changes and provide evidence for its relationship to deforestation due to the sheer complexities assocated with the climate system and its relations to bioloy, as shown specifially for Easter Island below.

Again, this reinforces the fact that further research is required on Easter Island before we can be making such claims, which could have significant consequences on the reasons as to why Easter Island was so severely deforested and why its society collapsed.

Easter Island Deforestation: At The Hands of Mother Nature?

As discussed in the previous post, within a matter of a few hundred years, the forested landscape that existed on Easter Island since the Pleistocene disappeared to the grassy stoney landscape that exists today. The collapse of Easter Island is widely percieved as murder or suicide by the settlers themselves…but what if it was murdered by mother nature?

This post aims to assess the theory proposed by Mann et al (2008) in the paper: ‘Drought, vegetation change and human history on Rapa Nui (Isla de Pascua, Easter Island)’, which suggests that climate variability and change caused deforestation on Easter Island.

Mann et al (2008) reports on sediment, charcoal and pollen stratigraphy in the Rano Raraku lake core on Easter Island, and use their results to help identify the timing of widespread forest clearance and help test the hypotheses about involvement of drought. Stratigraphic records from lake sediment cores and slope deposits on Rapa Nui indicate that the Rano Raraku lake basin dried out after 4090-4410 cal yr BP (BC 2140-2460). A depositional hiatus (in this case an angular unconformity whereby younger sediment lies on the eroded surface of older rocks) in the sediment record suggests that drought occurred between 1180-3990 cal yr BP (BC 2040 to AD 770) due to the fact that the soil structure originated from dessication-cracking of the dry lake bed. So what caused these droughts?

There are three meteorological phenomena that are important for controlling rainfall patterns in southeast pacific; Subtropical southeast Pacific high, tracks taken by cyclonic storms carried in the westerlies, and ENSO

1.    Subtropical southeast Pacific High: can block approach of storms carried by westerlies, therefore when it weakens, cyclonic storms track further north (equatorwards) it reduced rainfall and lake levels at Easter.

2.    Cyconlic Storms: Most of the rains come from these cylconic storms moving eastward across the Pacific. Mann et al (2008) argure that shifts in the latitude of storm tracks are probably an important trigger for drought on Rapa Nui, as they shift latitudinally depnding on the overall pole-equator temp gradient.

3.    ENSO: During phases of La Nino, Easter Island experience warmer sea surface temperatures and extended droughts.

Mann et al (2008) conclude that:

“latitudinal shifts in the subtropical storm track and resultant changes in the intensity and frequency of cyclonic storms provide feasible, though untested, climatological trigger for radical changes in moisture balance on Rapa Nui”

They also provide another line evidence based on the knowledge that the geography of the subtropical high pressure system, due to the fact we would expect synchronous droughts on Easter Island and mainland Chile. Based on the Rano Raraku sediment record, the refilling of the lake basin at AD 1180-1290 coincides with the end of a warm and dry climate episode over central Chile, which thus strengthens evidence for drough found in the Rano Raraku sediment record.

There is some literature to support the conclusions made in this paper, for example Stenseth and Voje (2009) present evidence that the ENSO causes reduced marine biomass and resources on Easter, and thus during extended La Nina phases, and other climate variability, reduced marine resources leads to more intensive agriculture on land, or the search for new fishing grounds, both of which require wood of some sort, consequently increasing deforestation.

However, even at a first glance, it is clear to see that many issues surround this theory. Firstly, the trees and shrubs on Easter Island survived climate variability due to ENSO, cyclonic storms and changes in the Subtropical southeast Pacific pressue belt for tens of thousands of years before (Flenley and Bahn 2007), therefore it seems very unlikely that the droughts proposed by Mann et al (2008) could have caused such extensive and widespread deforestation. A paper by Saez et al  (2009) that investigates the sedimentary record of the Rana Raraku lake basin on Easter Island also supports this point, whereby the sedimentary record shows intense drought periods occurred from the mid to late Holocene, yet trees and shrubs survived throughout this period. They also go on to suggest that the Medieval Warm Period aided population expansion, howver because population expansion and deforestation are supposed to go hand in hand, climate is not likely to have had any impact on deforestation.

Secondly, the depositional hiatus in the sediment record is assumed to be caused by drought, however because there is no sediment record, it is impossible to say what the cause of this hiatus is, as we simply have no evidence. As a result, detailed records and evidence of past climate variability on Easter Island remains (and accordingly conclusions as to the effect of climate on Easter’s deforestation) unfeasible to date.

So, did drought and climate variability cause deforestation? From looking at the evidence it is clear that before we can make any solid conclusions on the relationship between climate variability and deforestions, a more accurate sedimentary record is needed. For now though, it is safe to say that it seems likely that climate did not cause deforestation, BUT it could have exacerbated it.

Terry Hunt and Carl Lipo Podcast

Podcast: Terry Hunt and Carl Lipo discuss Easter Island...

Above is a link to listen to a podcast where Terry Hunt and Carl Lipo discuss Easter Island, and their theory that rats caused deforestation with New Scientist's Ivan Semeniuk. Their view on Easter Island, when it was settled and deforestation has caused much controversy recently, particularly with Flenley and Bahn, and Jared Diamond, the most prolific scholars associated with Easter Island.


Enjoy!

Deforestation: The Rat Theory!

The palaeoenvironmental record on Easter Island suggests that the landscape Roggevann arrived on in 1722 would have been very different if he had arrived at the same time as the first polynesian settlers, where the vegetation was once dominated by millions of Jubaea palms. The palaeoenvironmental record indicates that the Jubaea palms have existed on Easter since Pleistocene times, and have survived significant climate changes and numerous environmental perturbations. So what caused deforestation to occur on Easter Island since humans first settled in AD 800-900? The next few posts will investigate the different theories that try to explain this very phenomenon.

Firstly, a theory proposed by Terry Hunt (2007) in his paper, ‘Rethinking Easter Island’s Ecological Catastrophe’ suggests that rats brought over by the first Polynesian settlers were the cause of deforestation on Rapa Nui. In this paper, Hunt advocates that:

“Rats are remarkable fecund and given a nearly unlimited food supply such as the fruits and seeds of pristine native forest, they can irrupt into enormous, dense populations”

By investigating evidence of the relationship between rats and deforestation of palms on Little Barrier Island, New Zealand and Hawaiin Islands, both of these sites underwent extreme deforestation within 200 years due to rat population explosions, and he therefore suggests that recent extinction of palms in New Zealand and Hawaii due to rats can be used as modern analogs to Easter Island’s deforestation. The figure below shows some results from the research on Little Barrier Island, New Zealand, implying a stong correlation between rat population increase and deforestation.

From looking at the closest relatives of the extinct plants on Rapa Nui, the majority were highly vulnerable to impacts by rat predation. Most yielded relatively large edible fruits/seeds, lacked toxicity, and faced destruction rather than dispersal by rodent predation. By using this evidence, and evidence from the abundance of rat bones and skeletons in archaeological  

archaeological excavations and the discovery of many palm fruits with signs of gnawing and removal of the interior nuts, Hunt proposes that rats were the cause of deforestation on Rapa Nui.. He also tentatively suggests that the pollen record shows that the decline of forest pollen (i.e. trees and shrubs) started to decline BEFORE charcoal and charred remains were found in the pollen record (indicators of burning caused by humans), however the pollen-charcoal evidence from Rapa Nui must be understood more fully before he can make such conclusions.

However, Hunt’s theory that rats caused deforestation on Rapa Nui has been contradicted by numerous lines of evidence proposed by Diamond (2007), Flenley and Bahn (2007), Meith and Bork (2010) and Rolett (2008).

1.   Rats simply could not have felled the mature trees, which grew from between 15 and 30m high. The life span of some of the trees reached 2000 years old (Rolett 2008), and from looking at pollen evidence undertaken by Flenley and Bahn (2007), deforestation occurred quite rapidly (on some parts of the island within centuries of the first settlers arrival), and therefore rats could not have caused deforestation on such massive trees within this short time period.

2.   Research undertaken by Meith and Bork (2010) drew attention to the fact that healthy populations of a similar type of palm to the once grew on Easter Island, Jubaea Chiliensis, coexists with rats on mainland Chile.

3.   Among more than 200 completely preserve and charred nutshells that we discovered in the burned layer of a pollen record undertaken by Fleley and Bahn (2007), whereby  less than 10% of the nutshells had bite marks. Thus indicating that the extensive deforestation attributed to rats by Hunt may not have been extensive as he insinuates.

4.   There is a wealth of evidence to suggest causes other than rats were the major cause of deforestation (to be discussed in following posts), for example climatic variability and drought, and human impacts.

Despite the contradictions and controversy stemming from the theory that rats caused deforestation on Easter Island, both the proponents and opponents agree on Hunt’s concluding remarks in his paper that firstly,

“Additional research will be essential to disentangle the contributing factors…the environmental catastrophe of Rapa Nui is likely a complex history”

And secondly,

“As biological invasions and extinctions continue at unprecedented rates, deciphering ecological histories and the consequences of biological invasions has gained urgent significance.”

References

Flenley, J. and Bahn, P.G. (2007) The Enigmas of Easter Island: An Island on the Edge Oxford: Oxford University Press

The Moai Statues

Before exploring the causes of collapse on Easter Island, it is important to introduce the famous Moai statues, as some scholars believe that they are part of the cause for Easter’s demise. Traditional Easter Island society was structured by territories (or clans), whereby each territory had its own chiefs, priests and commoners. Each of these territories had its own major ceremonial platforms known as ahu, which supported the famous moai statues. Some suggest that each of these territories/clans competed peacefully by attempting to outdo each other by building these statues and platforms (Diamond 2005). It is thought that the cultural and religious significance of these statues descended from Polynesian culture, particularly East Polynesia.

There is an estimated 887 moai stone statues on Easter, with the average statue measuring 13 feet tall and weighing 10 tons, however the biggest statue is 33 feet tall and 82 tons. Around 300 ahu stone platforms exist, weighing from 300 to 900 tons (Diamond 2005). One of the major mysteries on Easter Island is how these massive statues and platforms were transported from the single quarry where they were created to where they finally ended up in each territory, as far as 6 miles away. Two main theories have been put forward to explain how the statues were transported:


     1. The statues were dragged using pure human power and ropes
     2. The statues were laid down and rolled on tree trunks deforested from the native palm forest (Rainbird 2002)

Although the latter is widely considered the dominant theory (Rainbird 2002), which explains how the statues were transported, ropes needed to drag the statues also require deforestation of the native palm, since rope is fashioned out of the bark from the trees. The construction and transportation of the moai and ahu further reinforces the understanding that Easter’s society was complex and highly organised as they were able to bring together the resources from around the island, and transport the statues up to 6 miles away. Their construction also suggests the presence of a large population, where moai construction peaked from 1100 to 1400 AD (Wolcott and Conrad 2011), thus indicating peak population numbers around this time.

Forest-clearing for the moai and ahu construction, agriculture and logging for firewood and canoes all contributed to deforestation on Easter Island (Rainbird 2002), and thus leads onto the next post which discusses the role of deforestation on Easter Island.

UCL and Easter Island

For any who are interested in UCL’s involvement in research on Easter Island, members within the archaeology department of UCL received funding earlier this year to research the landscape and statues on Rapa Nui, but specifically how the statues weighing up to 30 tonnes were moved. In 2010, researchers from UCL discovered a complex network of roads up to 800 years old. This has significant consequences for Easter Island, as prior theories suggested that palm tree trunks were used to roll the statues, and thus contributed to deforestation on the island, which has been proposed by some as the main cause of Easter’s collapse (to be discussed in following posts).

New funding from the AHRC award (http://www.ucl.ac.uk/archaeology/calendar/articles/20110204) is allowing UCL researchers to investigate this further, particularly the spatial organisation of the moai, the quarries and tribe settlement, and the cultural and religious reasons for the moai erection.

Watch this space in the future!