Dirt: The Erosion of Civilizations - Kootenay Local Agricultural Society

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washed off the slopes now bury the ahus’ foundations. So the erosion that
stripped topsoil from the slopes happened after the ahus were built.
Radiocarbon dating of the slope-wash deposits and soil profiles exposed
by erosion, in road cuts, or in hand-dug soil pits record that the top of the
island’s original soil eroded off between about ad 1200 and 1650. Apparently,
vegetation clearing for agriculture triggered widespread erosion of
the A horizon upon which soil fertility depended. Easter Island’s society
faded soon after its topsoil disappeared, less than a century before Admiral
Roggeveen’s unplanned visit.
A detailed study of the soils on the Poike Peninsula revealed a direct link
between changing agricultural practices and soil erosion on Easter Island.
Remnants of the original soil still standing on a few tiny hills, flat-topped
scraps of the original ground surface, attest to widespread erosion of the
native topsoil. Downhill from these remnant soil pedestals, hundreds of
thin layers of dirt, each less than half an inch thick, were deposited on top
of a cultivated soil studded with the roots of the endemic palm tree. A halfinch
thick layer of charcoal immediately above the buried soil attests to
extensive forest clearing after a long history of cultivating plots interspersed
among the palm trees.
Initial agricultural plots in planting pits dug between the trees protected
the ground from strong winds and heavy rainfall, and shielded crops from
the tropical sun. Radiocarbon dating of the charcoal layer and material
obtained from the overlying layers of sediment indicate that the soil eroded
off the upper slopes, and buried the lower slopes, between ad 1280 and
1400. The numerous individual layers of sediment deposited on the lower
slopes show that the soil was stripped off storm-by-storm a fraction of an
inch at a time. These observations tell the story of how after centuries of
little erosion from fields tucked beneath a forest canopy, the forest of the
Poike Peninsula was burned and cleared for more intensive agriculture that
exposed the soil to accelerated erosion. Agriculture ceased before ad 1500,
after just a century or two in which the soil slowly disappeared as runoff
from each storm removed just a little more dirt.
The island’s birds disappeared too. More than twenty species of seabirds
inhabited Easter Island when Polynesians arrived. Just two species survived
until historic times. Nesting in the island’s closed canopy native forest,
these birds fertilized the soil with their guano, bringing marine nutrients
ashore to enrich naturally poor volcanic soils. Wiping out the island’s
native birds eliminated a key source of soil fertility, contributing to the
decline of the soil and perhaps even the failure of the forest to regenerate.
islands in time