Dirt: The Erosion of Civilizations - Kootenay Local Agricultural Society

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different fates. Tikopia developed into an idyllic island paradise, while
Mangaia and Easter Island descended into perpetual warfare. Recalling
that Tikopia’s utopian system was maintained at the cost of lives prevented
or eclipsed in the name of population control, we can justifiably ask which
was the higher price. Nonetheless, Tikopian society prospered for thousands
of years on a tiny isolated outpost.
An essential difference between the stories of these islands lies in their
soils. Deeply weathered soils in Mangaia’s sloping volcanic core are nutrient
poor. The sharp coral slopes of the uplifted reef hold no soil at all. In
contrast, Tikopia hosts young phosphorous-rich volcanic soils. The greater
natural resilience of Tikopia’s soils—because of rapid weathering of rocks
with high nutrient content—enabled Tikopians to sustain key soil nutrients,
using them at about the rate that they were replaced from the underlying
rocks through intensive, multistory gardening that protected topsoil.
After deciphering the environmental history of both Tikopia and Mangaia,
Patrick Kirch suspects that geographic scale also influenced the social
choices that shaped these island societies. Tikopia was small enough that
everyone knew everyone else. Kirch suggests that the fact that there were
no strangers on the island encouraged collective decision making. By contrast,
he suggests, Mangaia was just large enough to foster an us versus
them dynamic that fueled competition and warfare between people living
in neighboring valleys. Easter Island supported a larger and less cohesive
society, leading to even more disastrous results. If Kirch is right and larger
social systems encourage violent competition over collective compromise,
we need to take a sober look at our global prospects for managing our
island in space.
The story of dramatic soil loss following human colonization of islands
is not restricted to the South Pacific. Viking colonization of Iceland in ad
874 catalyzed an episode of catastrophic soil erosion that continues to consume
the country. At first the new colony prospered raising cattle and
growing wheat. The population rose to almost eighty thousand people by
ad 1100. Yet by the late eighteenth century the island’s population had
dwindled to half the medieval population. Cooling during the Little Ice
Age from about ad 1500 to 1900 certainly influenced the fortunes of the
Iceland colony. So did soil erosion.
Iceland had an extensive forest cover when first colonized. In compiling
the Íslendingabók in the late twelfth century, Ári the Wise described the
island as “forested from mountain to sea shore.” 2 Since human settlement,
more than half of Iceland’s vegetation cover has been removed. The native
islands in time