Dirt: The Erosion of Civilizations - Kootenay Local Agricultural Society

I doubt the Easter Islanders had any idea that eating all the birds could
undermine their ability to grow sweet potatoes.
The story of Easter Island is by no means unique. Catastrophic erosion
followed forest clearing by Polynesian farmers on many other—but by no
means all—Pacific Islands. Among the last places colonized on earth,
South Pacific islands provide relatively simple settings to study the evolution
of human societies because they had no land vertebrates before people
imported their own fauna of chickens, pigs, dogs, and rats.
The islands of Mangaia and Tikopia provide stark contrasts in human
adaptation to the realities of a finite resource base. Sharing many common
traits and similar environmental histories until well after people arrived,
these societies addressed declining resource abundance in very different
ways. As worked out by UC Berkeley anthropologist Patrick Kirch, their
stories show how transgenerational trends shaped the fate of entire societies.
Mangaia occupies just twenty square miles—a small dot of land in the
South Pacific twenty-one and a half degrees south of the equator. Visited
by Captain James Cook in 1777, Mangaia looks like a medieval walled
fortress rising from the sea. The deeply weathered basaltic hills of the
island’s interior climb more than five hundred feet above sea level, surrounded
by a gray coral reef lifted out of the ocean. A hundred thousand
years ago, growth of the nearby volcanic island of Rarotonga warped
Earth’s crust enough to pop Mangaia and its fringing reef up out of the sea.
Streams flowing off the island’s core run into this half-mile-wide wall of
razor-sharp coral that rises half the island’s height. There they drop their
sediment load and sink into caves running down to the island’s narrow
beach. Radiocarbon-dated sediment cores recovered from the base of the
island’s interior cliffs tell the story of Mangaia’s last seven thousand years.
Covered by forest for five thousand years before Polynesians arrived
about 500 bc, Mangaia eroded slowly enough to build up a thick soil in
the island’s volcanic core. Kirch’s sediment cores record sweeping changes
between 400 bc and ad 400, when a rapid increase in the abundance of
microscopic charcoal particles records the expansion of slash-and-burn
agriculture. Charcoal is virtually absent from sediment older than 2,400
years; dirt deposited less than 2,000 years ago contains millions of tiny carbon
fragments per cubic inch of dirt. In the sediment cores, sharp increases
in the abundance of iron and aluminum oxides, along with decreased
phosphorus content, show that erosion of a thin, nutrient-rich layer of
topsoil rapidly exposed nutrient-poor subsoil. The native forest depended
on recycling nutrients that the weathered bedrock could not readily resup-
islands in time 221