According to Collapse, why did changes on Mangareva Island post-1450 affect Pitcairn and Henderson Islands disastrously? Why is this relevant today?

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Deforestation of Mangareva led to soil erosion, making it difficult to grow enough food for the entire population. The loss of trees also led to a loss in the number of canoes, needed for fishing and trade. The fall of Pitcairn and Henderson was due to the lack of trade partners after Mangareva's deforestation. Diamond believes that our dependence on foreign trade is similar, and thus we need to be aware that we may face similar problems in the future.

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Mangareva, Pitcairn, and Henderson are the only habitable islands in the remote region of the South Pacific known as Southeast Polynesia.

There is ample evidence that these islands traded extensively with each other. In particular, Mangareva supplied the two smaller islands with oyster shells and basalt stone suitable for making...

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a range of different utensils. Mangareva quite likely also provided the two other islands with a variety of crops and with pigs. Of vital importance, due to the small populations of Pitcairn and Henderson, Mangareva would have been a source of marriage partners for them.

Diamond lists nine different factors that can contribute to deforestation on islands in the Pacific. Mangareva exhibited most of these including a location at a relatively colder high latitude and lesser fallout from atmospheric dust to supplement the soil. As a result, when Mangareva's inhabitants eventually felled the forest in the interior of the island, it led to permanent deforestation. Soil erosion followed, limiting the area available for their gardening.

Critically, deforestation also removed the larger trees needed to construct canoes for fishing. Existence on the island descended into a nightmare of hunger and war, and trading with Pitcairn and Henderson ceased. The population of the two smaller islands, deprived of goods and partners from Mangareva, dwindled to nothing.

Diamond points out that our time's increasing globalization and dependence on foreign trade mirrors on a vastly larger scale the situation of the islands in Southeast Polynesia. He states that we are affected by "many economically important but ecologically fragile areas" just as Henderson and Pitcairn were.

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The changes on Mangareva were centered around the island's deforestation. The problem with this action was far more widespread than the damage it caused the island nation itself. The reason behind the deforestation was that the island sold and traded its lumber for other goods and services with nearby islands. Being a small island, when trade became large enough, the foresting industry quickly became unsustainable, destroying the forests and killing off the majority of the fauna on the island.

Unfortunately for nearby islands of Pitcairn and Henderson, they were all in mutually beneficial trading arrangements. None of these islands was large enough to sustain a growing population well on its own. In order to keep up with the demand and strain from an increasing population on each island, they specialized in areas of production and traded with one another. When Mangareva was overwhelmed from deforestation, they crumpled, collapsing into Civil War.

This effectively removed the island as a viable trading partner with Pitcairn and Henderson. Without their aid, these other islands were unable to sustain their growing population and began to suffer from the stress of caring for the people and producing enough goods for them. This led to the collapse of multiple islands.

This is a telling story for the modern age because of the ecological collapse humans are creating throughout different regions. As we destroy large swathes of nature, other regions are impacted by these actions. Since we live in an increasingly connected society, collapse in one area means a total shutdown of the global cycle of trade and support. This story is a strong warning.

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Diamond explains in chapter 3 how Mangareva, Pitcairn, and Henderson islands were connected through trade. Mangareva was the largest island and had the most natural resources but crucially lacked stone that was suitable for making good tools. Henderson was the most marginal of the three islands but had a wealth of shellfish and turtles. All three islands were able to support permanent populations for time because of this trade.

This balance was destroyed when the population on Mangareva and Pitcairn became too great. Forests were destroyed to increase land available for gardens, which started a cycle of environmental degradation and mass extinctions. Eventually, with the decimation of the forests, it became impossible for islanders to make the ocean-going canoes necessary for sustained trade. Cut off from the outside world, the Polynesian inhabitants of Pitcairn and Henderson eventually died off, while the people of Mangareva fell victim to violence, starvation, or even cannibalism.

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The answer to this can be found in Chapter 3 of Collapse.  There, Diamond tells us that the people of Mangareva deforested their island and caused an ecological catastrophe there.  When that happened, the people of Pitcairn and Henderson were deprived of a very important trading partner and were no longer able to survive on their islands.

In this book, one factor in Diamond’s “framework” of things that can cause a society to collapse is a change in the circumstances of a friendly neighbor that trades with your country.  If you depend on a friendly neighbor for something very important, you are in a lot of trouble if that neighbor stops being willing and able to trade with you.  That is what happened to the people of Pitcairn and Henderson.

Both of these islands had severe limitations in terms of their ability to sustain human populations.  But the human populations on those islands overcame those limitations, in part, by trading with Mangareva.  In particular, Pitcairn and Henderson needed oyster shells from Mangareva to use for making tools.  When Mangareva’s population outgrew its carrying capacity, Mangareva dropped into civil war and famine.  This prevented it from trading as much as it had and made Pitcairn and Henderson essentially uninhabitable.

Diamond says this is important for us because it shows how interconnected we are and how much trouble one society can get into if another society collapses.  Because of our globalized economy, the fates of other societies can strongly affect the fate of our own.

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