Casefile: Rats! All the trees are gone...

Greetings readers! Now while the title may confuse you don't worry this post is all about the collapse that occurred in Easter Island - one you are all familiar with thanks to the last seminar and so here's just a brief overview of what may have happened.

Background:
Rapa Nui (as its is also known) is located in between the coast of Chile and the Polynesian Pitcairn Islands and is a small island covering an area of 171km2 (Fig. 1). Polynesians are thought to have settled in Fiji and Hawaiian islands before a community developed on Rapa Nui at a late date of around 1200 A.D.. Evidence for this data came from various sources including: the radiocarbon dates of seeds eaten by Rattus exulans (a Poynesian rat that is associated with humans), by a change in vegetation after this period and by study the only dune on the island - Anakena which provided a chronological record of the island. Anakena is thought to have been the first site of occupation by the early settlers as there was no evidence of a settlement before 1200 A.D. after which cultural objects indicative of humans appeared. We also know that the occupants the land and changed both the near-shore and terrestrial surfaces of their island through farming and the erection of statues and it is thought they had reached a population of at least 10,000 before the decline. At 1650 A.D. the society collapsed and wars ensued as populations dwindled and the society experienced cultural shifts. Rapa Nui was then discovered by Dutch sailors on Easter Sunday and renamed the Easter Islands by Jacob Roggeveen who saw the island in 1722. And when visited by James Cook in 1744 the island only had a population of 1000 and had no trees. 

A number of theories have been put forth as the reason for the drastic decline in population and degradation that occurred in Rapa Nui but much of this is still heavily debated making this examples an interesting but certainly one of the most controversial casestudy on collapse out there.

Fig. 1 - Map of Rapa Nui (Source:Welt-Atlas)

The Theories:

Rats! - Terry Hunt (2007) suggested that Rattus exulans were to blame as they caused widespread deforestation on the fragile island ecosystem. Islands are unique in that there isolation creates endemic species with few large predators and simple ecosystem thus the introduction of the Polynesian rat meant the species thrived as it faced very few limiting factors (no predators, no seasons and lots of food). Since the Pleistocene Jubaea palms have been present on Rapa Nui and this is indicative that the deforestation experienced on the island could not be explained by changes to the climate. Thus the rats, thriving on the abundance of food (seabirds and Jubaea nuts) and lack of predator, were able to quickly reach a high population (45 rats per acre meaning a population of at least 1.9 million probably more!) which could cause significant degradation. 
Evidence for the negative impact of rats comes from palynology which studies pollen and seeds which could be used to reconstruct the islands vegetation and which had evidence of gnawing by rats. Faunal remains were also studied and showed the species composition and abundance of rats on the islands.

However, Meith and Bork (2010) argue that rats couldn't have caused the devastation on Rapa Nui as less than 10% of nut had evidence of teeth mark, the age and presence of stumps suggested humans had cut them, carbon remains suggest fires occurred, there was still evidence of young palm trees and trees do have some resistance to predation. Nonetheless, Hunt states that the rats along with logging and the use of fire by humans would have caused the devastating impact seen the land. He also states the role of rats have been understated as they can cause major damage as seen in 'Ewa Plain of O'ahu'  where rodents obliterated a palm forest. As opposed to humans simply felling the last trees he offers the opinion that rat may have eaten the seed and this illustrates the negative impact an alien species can have on fragile ecosystems.

'Rat Attack!' (Source:Radiolab)

Europeans! - We all know of the damage that colonists do to a country and it has been proposed the same happened to Rapa Nui. The European colonisers brought about the destruction of the island through introducing new diseases, reducing the population by using them in the slave-trade or genocide and by taking resources from the island further depleting the islands natural stores. However whilst the data does show a sharp collapse in population after Europeans first arrive, the environmental destruction and initial population decline cannot be solely attributed to them.


Ecological Destruction (or 'ecocide' for the dramatic)! - One of the most prevalent theories but also controversial as it depicts the islanders as irresponsible. This theory is known as 'ecocide' as it is believed humans caused widespread devastation to their local environment (tell us something new right?). The trees were likely felled to be used for building materials, boats and for the construction of the moai statues. The theory goes that the Polynesians cut down trees faster than they were growing back this along with cut and burn practices left the vegetation with little chance of recovery. This evidence was found among trees stumps which were clearly felled, the presence of the iconic Easter Island statues, pollen data which showed the prevalent presence of palm trees until the arrival of humans and charcoal and carbon found in sediment records. J. Diamond even suggests that the population resulted to cannibalism as overpopulation and a degraded environment had weakened the ecosystem the society relied on.


Drought! - Lake sediment records found that the Rano Raraku lake dried out once after 4410 B.P. before returning in 1180-1290 A.D. After this drought the Polynesians started felling trees and burning the soil leading to serious erosion. The islands climate is heavily controlled and affected by ENSO, shifting storm tracks and sub-tropical high pressure system. Whilst there is not much evidence from the island itself, proxy data from Patagonia on moisture levels (tree-rings and lake sediment records) illustrate a shifting pattern of cool-moist to dry climates between 900 to 1270 AD.. Therefore, it is expected that the island did experience a droughts at the same time as in Chile. Thus the climate may have caused the collapse as moisture levels changed and affected aridity.

'They found neither trees nor chocolates' (Source:New Yorker/TheAwl)

Whilst we may never know what caused the collapse in Rapa Nui and blaming the islanders due to their unsustainable practices seems like an environmental determinism point of view, this casestudy is interesting in the addressing the problems societies face even today. Many island nations are already starting to feel the pressure of climate change and rising sea levels but this example also resonates with societies who continue to over-exploit resources quicker than their replacement rate. What's more it introduces the possibility that invasive species or even degradation of fragile habitats can have devastating effects on humanity...