Dirt: The Erosion of Civilizations - Kootenay Local Agricultural Society

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ply. So topsoil loss retarded forest regeneration. Well adapted to grow on
the nutrient-poor subsoil, ferns and scrub vegetation useless for human
subsistence now cover more than a quarter of the island.
By about ad 1200 the pattern of shifting slash-and-burn agriculture had
stripped so much topsoil from cultivated slopes that Mangaian agriculture
shifted to reliance on labor-intensive irrigation of taro fields in the alluvial
valley bottoms. Occupying just a few percent of the island’s surface area,
these fertile bottomlands became strategic objectives in perpetual intertribal
warfare. Control of the last fertile soil defined political and military
power on the island as population centers grew around these productive
oases.
Polynesian colonization changed the ecological makeup of the island,
and not only in terms of the soil. Between ad 1000 and 1650 guanoproducing
fruit bats vanished as the islanders killed off more than half the
native bird species. Historical accounts and changes in the abundance and
variety of bones in prehistoric deposits indicate that by the time of Cook’s
visit Mangaians had eaten all their pigs and dogs, and probably all their
chickens too. The Mangaian diet began to change radically—and not for
the better.
After most protein sources were gone, charred rat bones became prevalent
in deposits excavated from prehistoric rock shelters. Early nineteenthcentury
missionary John Williams wrote that rats were a favorite staple on
Mangaia. “The natives said they were exceedingly ‘sweet and good’; indeed
a common expression with them, when speaking of any thing delicious,
was, ‘It is as sweet as a rat.’” 1 Charred, fractured, and gnawed human
bones appear in excavated rock shelter deposits around ad 1500, attesting
to intense competition for resources just a few hundred years before European
contact. Chronic warfare, rule by force, and a culture of terror characterized
the end state of precontact Mangaian society.
Reconstructions of Mangaia’s human population mirror those of Easter
Island, albeit on a smaller scale. Starting with perhaps a few dozen colonizers
around 500 bc, the island’s population grew steadily to about five
thousand people by ad 1500. The population fell dramatically over the
next two centuries, hitting a low soon after European contact and then
rebounding to a modern population of several thousand.
The environmental and cultural history of Tikopia, a British protectorate
in the Solomon Islands, provides a striking contrast to Mangaia
despite very similar backgrounds. With a total area of less than two square
miles, Tikopia is smaller than Mangaia. Even so, the two islands supported
islands in time