Te Pito Te Henua: Botanical Investigations from the navel of the world

The collapse of the civilisation on Easter Island, or Rapa Nui in the native language, became very popular with the film "Rapa Nui" (1994) and the book of the American biologist Jared Diamond "Collapse - How societies choose to fail or survive" (2005). After the proposed reconstruction in both film and book, human population growth and overexploitation of natural resources, especially clearing of forest, caused soil degradation and resulted in diminished agriculture production, not able any more to feed the population. In the resulting famine and civil wars one of the highest developed cultures in the Pacific Ocean collapsed.

Fig.2. Map of Easter Island, or Rapa Nui / Te Pito Te Henua (Navel of the world) with the mentioned localities (after THOMSON 1891).

This scenario is based primarily on the discovery during an archaeology expedition prior to 1961 of unknown palm-like pollen in sediments, on an island today lacking completely any native tree species. In subsequent years further cores were analysed for pollen, also root imprints in soil and subfossil nuts found in caves supported the claim that the island was covered by a forest in past times.

Fig.3. Rongorongo, the unique "scripture" of Rapa Nui is still a mystery. The displayed symbol is interpreted to represent or be inspired by a palm tree (after THOMSON 1891).

Today Rapa Nui is covered by meadows (90%), planted Eucalyptus trees (5%), shrublands (4%) dominated by invasive plant species and pioneer and urban vegetation (1%).

Until today 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 it´s forests.
According to recent palynological studies the island experienced a cold and dry climate until the end of the last glacial maximum ca. 12.000 years BP, then during the moister climate of the Holocene the forest expanded and persisted until the arrival of Humans AD 800 to 1200. Deforestation then presumably took place between these ages and the arrival of Europeans in 1800 AD.

There exist two major categories of hypothesis to explain the massive destruction of plant live and lose of species diversity on the island. Some hypothesis, summarized in the book "Collapse", impute deforestation to direct and indirect human behaviour, clearing of the forest until the last tree and/or introducing invasive plant or animal species, that concurring with native species caused their extinction. Other hypothesis, more speculative, deal with a possible massive impact of past climate changes.

Fig.4. Pictures of the three volcanic craters with continuous cores used for pollen analysis. A) Rano Aroi, B) Rano Kao and C) Rano Raraku (photos by V. Rull from RULL et al. 2010).

Sediment records of the island past can be obtain from the swamps and lakes situated in the larger craters of the volcanic island, because larger sediment traps are more likely to hold thicker and undisturbed sediments. 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 lacustrine sediments.

The count of pollen grains in sediment recovered from a core of the lake Ranu Raraku shows a replacement of palm-dominated by grass-dominated pollen assemblages in the sedimentary record as of 1200 AD.

Fig.5. Percentage pollen diagram from Rano Raraku compared to charcoal concentration in the analyzed sediment. The calibrated C14 ages show a major gap in sedimentation between 800 and 4.000BP. The decline of palm pollen and increase in charcoal (a proxy for anthropogenic induced fires) happens shortly after this presumed gap (from MANN et al. 2008).

Unfortunately interpretation of pollen diagrams can be very tricky.
Pollen sum curves do reflect a relative change in pollen production, which not necessarily reflect the absolute number of palm trees in the surrounding. Depending on the tree species, and if it is pollinated by animals or by wind, different species can produce very different quantities of pollen. To correct this error a calibration factor must be known.

The tree species that produced the pollen on Rapa Nui is unknown. Pollen-morphological similarities exist to widespread species on pacific islands like Pritchardia, Cocos (coconut) and Jubaea chilensis (wine palm). But sparse macroremains, like the damaged nuts, seems to discard all mentioned species (assuming that all known remains like roots, pollen and nuts represent only one species), and are more similar to the nuts of Juania australis, an endemic palm species on the Juan Fernández Islands.
Based on these remains finally the extinct species Paschalococos disperta, with dubious systematic affinities to recent tree species, was established. So the pollen calibration factor for the extinct species can not more obtained.

The pollen signal conserved in the bog also depends strongly from the location of trees in the catchment. Few trees very near the shore of a lake can give stronger signals that many trees located in great distance.

On Easter Island the sedimentological records shows a prominent gap in the last centuries and millennia, maybe as result of a major drought and dry out of the studied lakes, unfortunately just in the time where the strongest human impact is postulated.
In the end pollen records can give only approximately ages of vegetation changes occurring between 1900 and 600 years BP, but not the extent and cause of such changes.

The number of former palm trees on the island was estimated by density of root imprints to 16 million, covering up to 70% of the surface. But these numbers are in contrast to the small amount of charcoal or wood fragments recovered until now on the island., It is however possible that the missing wood-debris and charcoal was eroded, transported and deposited in the surrounding ocean.

Considering the mentioned problems arising from the reconstruction of the former vegetation by means of pollenanalysis, the observed pollen data can also be explained by the presence of sparse forest patches or small numbers of trees growing near the shores of the studied lakes or on steep slopes, until finally the arrival of Europeans and their cattle and goats finished off the last survivors.

The intriguing questions remains, did the former inhabitants lumber completely the island's dense subtropical forest to still their megalomaniac hunger for bigger and bigger moai, and finally caused their own demise, or was Rapa Nui since the beginning of human colonization a sensible and tree poor ecosystem, and climatic causes like a drought combined with human impact caused an inexorable collapse?
The available botanical data still don't allow an exclusion of one or the other scenario.

References:

THOMSON, W.J. (1891): Te Pito Te Henua, or Easter Island. Report of the National Museum 1888-89, Smithonian Institution. Washington
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 doi:10.1016/j.earscirev.2010.02.003
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 doi:10.1016/j.yqres.2007.10.009 Fulltext (pdf)

Introductory Picture from Wikipedia