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Climate, Overpopulation & Environment – The Rapa Nui debate

The views expressed are those of the author and are not necessarily those of Scientific American.


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The story of the “ecocide” and collapse of the civilization on Easter Island, or Rapa Nui in the native language, became very popular with the film “Rapa Nui” (1994) and the book by American biologist Jared Diamond “Collapse – How societies choose to fail or survive” (2005). After the proposed scenario shown in both film and book, the human population grow too large and a fierce overexploitation of the limited natural resources of the island started. Especially wood was needed for the construction of the moai – large statues that impersonate the forefathers and became a symbol of power and prestige on the small island. However after clearing completely the forest of large grown palm trees, the soil was quickly eroded by the heavy rain falls that periodically occur. The barren volcanic rocks could no longer sustain agriculture production and soon the farmers were no longer able to feed the population – without timber no one was able to build boats to escape the impending doom. In the resulting famine, chaos and civil wars one of the highest developed cultures in the Pacific Ocean, with an own scripture and astonishing construction skills, rapidly collapsed.

This scenario is based primarily on the discovery during an archaeology expedition prior to 1961 of unknown palm-like pollen in sediments. The layer with pollen was found in various cores recovered from swamps, also root imprints in fossil soils and subfossil nuts, found in the lava caves, prove that Easter Island once supported large grown palm trees. Today the landscape of Rapa Nui is dominated by meadows, which cover 90% of the island; the rest is shrublands and planted forests of Eucalyptus trees, which host almost no native species.

Obviously the society of Easter Island, ignoring the destruction of their environment and valuing symbols of status over common sense and sustainability, was finally doomed to extinction by its own greed – a tale that reminds us that our earth is like Easter Island – nothing more than a dot in the vastness of space and we too in the end will lack the possibility to escape from an environment that today must sustain 7 Billion People.

Fig.1 & 2. In 1786 the “La Pérouse” expedition (1785-1788) visited Easter Island, the artist Duché de Vancy produced the first map and the first drawings of the locals (somehow idealized) and the moai-statues (images in public domain). Note that the moai are shown still standing upright – so apparently not all statues were toppled down in a prehistoric war as proposed by some authors.

However in contrast to what the “based on true facts” Hollywood-production proposes, this grim fable and the catastrophic scenario on Rapa Nui were and are still disputed.
The number of studied cores to reconstruct the paleoecology of the island is limited and most were analyzed with a very coarse resolution and present mayor sedimentary gaps, so many doubts remain how fast and when Rapa Nui lost entirely its native forests. According to the most recent palynological studies the island experienced a cold and dry climate until the end of the last glacial maximum some 12.000 years B.P. During the moister climate of the Holocene the forests expanded and persisted until the arrival of humans sometime between 300 and 800 A.D. Deforestation then presumably took place between these ages and the arrival of Europeans in 1722.

Based on this limited information two main sets of hypotheses to explain the massive loss of plant live and species diversity on the island were proposed. One set of hypotheses, summarized in the book “Collapse“, impute deforestation to direct and indirect human behavior. Humans cleared actively the entire forest and hunted the local fauna until the brink of extinction. In a modified version of the human-impact hypothesis the colonists were not the main and only culprits of the environmental collapse, but invasive plant or animal species brought by them on the isolated island, which in fierce concurrence with native species caused their rapid decline and extinction.

A second set of hypotheses deal with a possible massive impact of past climate changes, like prolonged droughts, on an already sensible and instable island environment and society.

Fig.3. Simplified history of the environmental phenomena on Rapa Nui.

Good sediment cores and therefore records of the past of Rapa Nui can be obtained from the swamps and lakes situated in the three main craters of the volcanic island, because larger sediment traps are more likely to hold thicker and undisturbed deposits. Rano Aroi crater holds a bog with an outflow and connections to the groundwater table. Rano Raraku and Rano Kao craters hold permanent lakes without outflows and are disconnected from the main groundwater bodies by impermeable lake sediments.

Counting the pollen grains in sediments recovered from Lake Ranu Raraku showed a replacement of palm-dominated by grass-dominated pollen assemblages in the sedimentary record beginning with the year 1200, as the supposed result of the almost complete clearing of the dense palm-tree forest. Unfortunately interpretation of pollen diagrams can be very tricky.
Pollen sum curves do reflect a relative change in pollen production, which not necessarily reflects the absolute number of palm trees in the surrounding area of the sample site. Depending on the tree species and how this species is pollinated (a species pollinated by wind will produce much more pollen grains that a species that can rely on more trustworthy vectors like animals) different species can produce very different quantities of pollen. To reconstruct the true vegetation cover from a pollen assemblage we must know the calibration factor of the studied plant species.
It is however not clear what tree species produced the pollen on Rapa Nui. Pollen-morphological similarities exist to widespread species on pacific islands of the genus Pritchardia (the Pritchard palm), Cocos (the coconut palm) and the species Jubaea chilensis (the wine palm). Some authors assume that Easter Island was dominated by a forest of wine palms. The sparse macroremains found however do not match all the mentioned species. The incomplete fossil nuts show most similarities to the nuts of Juania australis, an endemic palm species found today only on the Juan Fernández Islands. Assuming that all the remains – the root-casts, the pollen grains and the nuts – came from just one plant, it was also suggested that the palm of Rapa Nui was an endemic – and today extinct – species: Paschalococos disperta, with dubious systematic affinities to recent palm species and unknown “pollen-conversion-factor”.

Even when exactly knowing the conversion-factors, the pollen signal conserved in bogs and swamps also depends strongly from the location of the trees in the catchment area. Few trees very near the shore of the sample site can give stronger signals that many trees or even an entire forest located in great distance.

Not only botany, also geology causes problems when studying the past of Easter Island.

All the studied cores show erosion and a prominent gap in the sediments until the year 800, maybe as result of a major drought – unfortunately just in the time period when the first human impact is postulated.  The peak of land use and cutting/burning of the forest occurred probably some time later in the years 1300 to 1600.
The sediment and pollen records therefore can give only approximately ages of vegetation changes occurring in this interval and also not the exact extent and cause of such changes.

16 million palm trees, covering almost 70% of the surface, were estimated to grow once on Rapa Nui. Were all these palms really consumed by humans and for what purpose ? Construction of moai or as source of drinkable plant sap? But these numbers are in strong contrast to the relative thin charcoal layers and wood fragments discovered until now on the entire island. It is however possible that the missing wood-debris and charcoal was eroded, transported and deposited in the surrounding ocean.

However a third scenario is possible considering also the archaeological remains from Rapa Nui. In the archaeological record there is almost no evidence for an increased rate of conflicts or violence on the island in response to overpopulation; stone artefacts thought to be spearheads were simple tools to cut and scrap and there are no fortifications to be found. Hundreds of studied skeletons showed no particular signs of war or violence and cannibalism remains unproven for Rapa Nui. According to this scenario the deforestation was a “natural” process (maybe helped by introduced rats) and not intentionally forced by humans. Also the loss of the forests had no disastrous effects on the population, society and the quality of living of the Rapanui.

In the end the most intriguing questions remain still unanswered: Did the former inhabitants destroy completely the island’s dense subtropical forest, causing their own demise? Was Rapa Nui since the beginnings of human colonization a poor environment, covered only with local spots of forest and was it a drought, maybe in combination with human impact, that finally triggered the extinction of the already rare plant species? Did the natives realize the impending change – did they even care?

What seems sure is that the deforestation of Rapa Nui was a complex process. Blaming climate change alone seems inappropriate. The plants and animals had already survived harsh climatic changes in the last 10.000 years on the former uninhabited island and human presence in the last thousand years would have added further pressure on the environment. However humans coexisted for centuries with the forest, for example by using planting pits sheltered in the forests by the palm trees.

The “end” of the moai-culture was also more probably a slow process (not concluded at the arrival of the first Europeans in 1722) than a sudden traumatic Hollywood-catastrophe. Also the very first written reports describe the locals not necessarily as desperate survivors and the island as a post-apocalyptic wasteland. The Rapanui replaced the trees with shrubs and prevented soil erosion, they grow plenty of food using effectively millions of rocks as stone mulch. Much damage and soil erosion observable today on the island was done after the European colonization, especially with the introduction of large livestock that increased significantly soil erosion in the 20th century.

The lesson from Easter Island remains nevertheless important; even if the demise of the ecosystem was not exclusively the fault of the people or a doomsday-scenario and the impoverished environment continued to sustain a society – it was however a society deprived of many future possibilities. It demonstrates that a society depends on the environment – so in the end it is in only in our own interest to care about it.

Bibliography:

HUNT, T.L. (2007): Rethinking Easter Island’s ecological catastrophe. Journal of Archaeological Science 34: 485-502
HUNT, T.L. & LIPO, C.P. (2007): Chronology, deforestation, and “collapse:” Evidence vs. faith in Rapa Nui prehistory. Rapa Nui Journal 21(2): 85-97
HUNT, T.L. & LIPO, C.P. (2009): Revisiting Rapa Nui (Easter Island) “Ecocide”. Pacific Science 63(4): 601-616
HUNT, T. & LIPO, C. (2011): The Statues that Walked: Unraveling the Mystery of Easter Island. Counterpoint: 256
THOMSON, W.J. (1891): Te Pito Te Henua, or Easter Island. Report of the National Museum 1888-89, Smithsonian Institution, Washington: 552
RULL, V.; CANELLAS-BOLTA, N.; SAEZ, A.; GIRALT, S.; PLA, S. & MARGALEF, O. (2010): Paleoecology of Easter Island: Evidence and uncertainties. Earth-Science Reviews 99:50-60
MANN, D.; EDWARDS, J.; CHASE, J.; BECK, W.; REANIER, R.; MASS, M.; FINNEY, B. & LORET, J. (2008): Drought, vegetation change, and human history on Rapa Nui (Isla de Pascua, Easter Island). Quaternary Research 69:16-28
MIETH, A. & BORK, H.-R. (2010): Humans, climate or introduced rats – which is to blame for the woodland destruction on prehistoric Rapa Nui (Easter Island)? Journal of Archaeological Science 37: 417-426
MIETH, A. & BORK, H.-R. (2005): History, origin and extent of soil erosion on Easter Island (Rapa Nui). Catena 63: 244-260

David Bressan About the Author: Freelance geologist dealing with quaternary outcrops interested in the history and the development of geological concepts through time. Follow on Twitter @David_Bressan.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. Acoyauh2 4:01 pm 11/6/2012

    Typo? “without timber no one was able to build boots to escape the impending doom.”
    You can’t escape an island with boots, silly Rapa Nui!

    Link to this

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