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
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