Neglected land. Ancient terraces (top) in the Lake Pátzcuaro Basin in Mexico collapsed (bottom) when native populations declined.

Too Few Hands Is Hard on Land

When populations begin to boom, it might seem natural for the land to be stressed. Deforestation can lead to the erosion of topsoil, for example. But a team led by archaeologist Christopher Fisher of Kent State University in Ohio has found that the opposite happened in ancient Mexico: the landscape fell to pieces when the population crashed.

The Tarascan people dominated western Mexico for centuries, up to the time of the arrival of the Spanish. The land was in a dire state then, but it wasn't exactly clear when the trouble began. Because water levels in Lake Pátzcuaro have dropped 10 meters since 1985, Fisher was able to investigate the history of erosion in the area over 2000 years. Between A.D. 120 and 775, white sediment was eroded as villages were built. It washed into the lake and built up at a rate of about 15 millimeters a year. Then, erosion dropped by as much as 90%, even though population in the area had risen from about 20 people per square kilometer to perhaps 334. Fisher thinks that the advent of terracing allowed the Tarascans to grow enough food without eroding the soil.

But the situation changed dramatically after the arrival of Europeans in the 1520s. Erosion rates went through the roof, the team reports online this week in the Proceedings of the National Academy of Sciences. The lakebed was covered in red sediment, and the mineralogy revealed that the topsoil had been washed away, followed by the volcanic bedrock. "The whole landscape let go," Fisher says. He believes that when smallpox and other Western diseases decimated the local population, there weren't enough people left to keep up the terraces: "If you don't maintain it, and the wall breaches, all the soil erodes away." Making matters worse over the next century was heavier rain and herds of introduced pigs.

The lesson, Fisher says, is that the Tarascans had a "success story for centuries" in feeding themselves while preventing erosion, but their method was still vulnerable because of the massive amount of labor required. "I think [the interpretation] is pretty convincing" and fits observations from other regions and other times, says University of Cincinnati anthropologist Vernon Scarborough. And by studying what did work for the Tarascans, Fisher says, one might be able to glean tips for how to manage modern landscapes.

Related site
Christopher Fisher's home page