Collapse_ How Societies Choose to Fail or Succeed - Jared Diamond [54]
That evidence for agricultural intensification is of several types. One type consists of stone-lined pits 5 to 8 feet in diameter and up to 4 feet deep that were used as composting pits in which to grow crops, and possibly also as vegetable fermentation pits. Another type of evidence is a pair of stone dams built across the bed of the intermittent stream draining the southeastern slope of Mt. Terevaka, in order to divert water onto broad stone platforms. That water diversion system resembles systems for irrigated taro production elsewhere in Polynesia. Still further evidence for agricultural intensification is numerous stone chicken houses (called hare moa), mostly up to 20 feet long (plus a few 70-foot monsters), 10 feet wide, and 6 feet high, with a small entrance near the ground for chickens to run in and out, and with an adjacent yard ringed by a stone wall to prevent the precious chickens from running away or being stolen. If it were not for the fact that Easter’s abundant big stone hare moa are overshadowed by its even bigger stone platforms and statues, tourists would remember Easter as the island of stone chicken houses. They dominate much of the landscape near the coast, because today the prehistoric stone chicken houses—all 1,233 of them—are much more conspicuous than the prehistoric human houses, which had only stone foundations or patios and no stone walls.
But the most widespread method adopted to increase agricultural output involved various uses of lava rocks studied by archaeologist Chris Stevenson. Large boulders were stacked as windbreaks to protect plants from being dried out by Easter’s frequent strong winds. Smaller boulders were piled to create protected aboveground or sunken gardens, for growing bananas and also for starting seedlings to be transplanted after they had grown larger. Extensive areas of ground were partly covered by rocks placed at close intervals on the surface, such that plants could come up between the rocks. Other large areas were modified by so-called “lithic mulches,” which means partly filling the soil with rocks down to a depth of a foot, either by carrying rocks from nearby outcrops or else by digging down to and breaking up bedrock. Depressions for planting taro were excavated into natural gravel fields. All of these rock windbreaks and gardens involved a huge effort to construct, because they required moving millions or even billions of rocks. As archaeologist Barry Rolett, who has worked in other parts of Polynesia, commented to me when he and I made our first visit to Easter together, “I have never been to a Polynesian island where people were so desperate, as they were on Easter, that they piled small stones together in a circle to plant a few lousy small taro and protect them against the wind! On the Cook Islands, where they have irrigated taro, people will never stoop to that effort!”
Indeed, why did farmers go to all that effort on Easter? On farms in the northeastern U.S. where I spent my boyhood summers, farmers exerted themselves to carry stones out of fields, and would have been horrified at the thought of intentionally bringing stones into the fields. What good does it do to have a rocky field?
The answer has to do with Easter’s windy, dry, cool climate that I already described. Rock garden or lithic mulch agriculture was invented independently by farmers in many other dry parts of the world, such as Israel’s Negev desert, southwestern U.S. deserts, and dry parts of Peru, China, Roman Italy, and Maori New Zealand. Rocks make the soil moister by covering it, reducing evaporative