Let's now move away from Rull et al, 2013's commentary, who has faced criticism by other academics. Mieth and Bork, 2010 concluded from their investigation of Easter Island that any deforestation was done by humans. They found burnt palm stumps and areas of burnt soil indicating that anthropogenic activity was the cause of deforestation, rather than by rats. Similarly they found patches where palm trees had begun to regenerate, which they assert would not happen if rats had prevented regrowth by eating palm seeds.
More recently, other criticism has come from Larsen and Simpson, 2014 who argue that Rull et al relies too heavily on paleo-data. They argue that past environmental change provides limited insight into societal changes or collapse because every society has some capacity to adapt. They are also critical of Rull et al for failing to consider the potential role of natural disasters such as tsunamis, floods, earthquakes or volcanic eruptions. Any one of these events may have proven too much for a small, isolated population. However there does not seem to be much evidence for any of these events around the time or Rapa Nui decline. Larsen and Simpson also comment on the absence of rats as a potential influence in Rull et al’s discussion.
More recently, other criticism has come from Larsen and Simpson, 2014 who argue that Rull et al relies too heavily on paleo-data. They argue that past environmental change provides limited insight into societal changes or collapse because every society has some capacity to adapt. They are also critical of Rull et al for failing to consider the potential role of natural disasters such as tsunamis, floods, earthquakes or volcanic eruptions. Any one of these events may have proven too much for a small, isolated population. However there does not seem to be much evidence for any of these events around the time or Rapa Nui decline. Larsen and Simpson also comment on the absence of rats as a potential influence in Rull et al’s discussion.
The role of rats was first investigated by Hunt, 2006 and has since been developed by Hunt and Lipo, 2012. When the
Polynesian settlers reached Easter Island, they brought chickens and rats with
them, shown by the sudden appearance of bones from these species in the
sediment record of Easter Island. The pristine forest conditions of Easter
Island (pre-human disturbance) provided ideal conditions for rats due
to the almost infinite food available – from seeds, fruits and large insects. It
is thought that rats ate seeds from trees which slowed the regeneration rates
of the forests.
Rat gnawed seed cases - they can penetrate hard, thick seed cases that birds cannot.
Source: Hunt, 2006
At first this theory might seem a bit far-fetched. However Easter Island is fairly small (164km2) and in ideal conditions, rat populations can reproduce at rates doubling every 47 days. Over a short amount of time Easter Island would have become inundated with rats, especially seeing as, being an introduced species, the rats had very few predators.
The impacts of rats are well studied in other locations such
as Hawaiian Islands and New Zealand. Therefore the presence of rats may have
had devastating impacts, especially at a time of human resource exploitation. In
conjunction with this, at the time of rate population growth, bird extinctions
occur demonstrating a double-edged sword – this would have further influenced
seed dispersal and potentially a food source of the Rapa Nui.
This theory has since been developed and refined (2012) and still stands. It just goes to show – don’t underestimate rats.
Till next time!
Source: Radiolab
Really interesting Celia! Do rats continue to survive on the island or have they too died out? If they are still on the island does it question how humans were unable to adapt to an environment of less food if the rats could?
ReplyDeleteThanks! Yes - there are still rats on Easter Island. That's an interesting point - maybe rats might become the next super race. But rats require much less food and energy to reproduce (especially as they are generalist feeders and will eat almost anything) and so are able to sustain fast population growth despite low food resources. In the case of Easter Island, the effects felt by humans were much greater than those felt by the rats.
ReplyDelete