According to the researchers, these results provide strong evidence that the early inhabitants of Kohala had discovered a "sweet spot" of high soil fertility a swath of land, rich in phosphorus and bases, which received enough precipitation to harvest vast quantities of sweet potatoes.
That rare combination of rainfall and fertile soil did not exist on Kauai, Oahu and Molokai, however. According to the research team, the soils on those ancient islands had lost their nutrients through weathering and other factors thus precluding the development of large-scale sweet potato farming.
The research team also dug beneath the earthen walls, which the Hawaiians used as windbreaks, and collected samples of virgin soil that had never been farmed. "We found higher phosphorus levels under the walls than in the abandoned fields," Vitousek said. "This suggests that the amount of phosphorus originally in the soil may have been depleted by agriculture and/or subsequent ranching."
"Clearly, the Hawaiians were pushing agriculture to its limits," said Patrick V. Kirch, a professor of anthropology at the University of California-Berkeley and co-author of the Science study. "We can see that the fields on Hawai'i were getting smaller and smaller, and that there was no place for them to expand geographically."
According to Vitousek, the shortage of arable land probably played a role in the rise of aggressive chiefdoms on Maui and Hawaii in the 18th century.
"The Hawaiian Islands had a true class system led by chiefs who enjoyed elite privileges," Kirch noted. "To maintain the social order at the level they were accustomed to, the chiefs had to go into a mode of aggressive action. It's interesting that the really aggressive chiefdoms came from the highly intensified dryland systems on Maui and Hawai
Contact: Mark Shwartz