The erosion of tropical soils on hill-sides where trees have been cut down is a common sight today. Now, scientists are finding that people have been causing such damage for several thousand years, and the affected land has never recovered.
In the highlands of central Mexico there have been several episodes of devastating soil erosion since people began to cultivate maize on a wide scale 3500 years ago. At times, soil erosion has been so severe that it has forced people to abandon their settlements.
Evidence for such prehistoric degradation comes primarily from lake sediments. Alayne Street-Perrott and Alan Perrott, of the University of Oxford, and Douglas Harkness of the Natural Environmental Research Council's Radiocarbon Laboratory in Glasgow, have been studying the sediment around Lake Patzcuaro to the west of Mexico City. They have discovered two devastating episodes of soil erosion in prehistoric times. One began about 2300 years ago and the other around 1000 years ago (American Antiquity vol 51, p 759).
The effects of this destruction can be seen today on the steep-gullied hillsides that surround the lake and in the fans of red soil redeposited around the lake shore. Where once there was abundant pine forest, now there is only impoverished shrub. Only small pockets of forest have regenerated. The local fisheries are also threatened because the lake is becoming eutrophic--enriched with nutrients from the soil washed into the lake.
The two major phases of soil erosion were preceded by a minor event about 3500 years ago, in the early days of maize cultivation. Sarah O'Hara at the University of Oxford has found signs of this erosion "blip" in cores of lake sediment. Although people cleared only a limited area of trees, the soil was washed into the lake, starting the process that would make it eutrophic.
The first serious ecological disaster to hit the Patzcuaro Basin began about 2300 years ago. By this time, say Street-Perrott and her colleagues, people were living in villages, and had built small ceremonial centres. Farmers cultivated only areas of the lake shore and the lower slopes. As a result, there was considerable overcultivation.
When the farmers cleared and burnt the pine forests on the steep hillsides around the lake, they caused soo much devastation that huge amounts of red topsoil were washed down into the lake. In exposed sediments on the lakeside, Street-Perrott and her colleagues found a layer of eroded forest soil. The layer is rich in tiny fragments of pine charcoal from the burning. So severe was the erosion at the northern end of the lake basin, say the researchers, that the area became completely depopulated.
A second, more intense phase of erosion in the Lake Patzcuaro area also shows up in the lake sediments. This probably began between 900 and 1000 years ago, with the arrival of the Purepecha, a warlike people who were similar to the Aztecs.
This period of extensive erosion continued into the late 16th and 17th centuries. People burnt the forest for a variety of reasons.
Increasingly, there is evidence that the cycles of soil erosion around the lake were repeated around other centres of population in the region. Street-Perrott says: "Soil erosion has to be taken very seriously as a cause of the collapse of city states and the shifting of power from centre to centre in Mesoamerica in pre-Hispanic times."