Collapse due to soil erosion?

Ignoring the deforestation debate, Beach et al. (2006) concentrated solely on the levels of soil erosion across the temporal and spatial extents of the Mayan empire.

Their paper provided new data from two sites (Blue Creek and Cancue´n) and synthesised more than a decade of the authors’ research in Guatemala, Belize, and Mexico. These research projects analysed more than 100 excavations in upland and depression sites, cored lakes and wetland sediments, and studied sediments in the field and laboratory using radiocarbon dating, a battery of soil chemistry tests, stratigraphic analysis, magnetic susceptibility, elemental analyses, and artefact identification. Their objective was to date when sedimentation and soil erosion occurred, identify stable surfaces, and correlate them with the state of knowledge about past land use.

Beach et al.’s (2006) findings indicated three general epochs of accelerated soil erosion. These occurred in the Pre-classic period (c. 1000 BC to AD 250), the Late Classic (AD 550 to 900), and in the last several decades.

At some sites (the Petexbatun, the Three Rivers, and the Belize River) a higher than expected soil erosion in the Pre-classic period, due to the region’s first pioneer farmers, was found. These sites also showed less than expected soil erosion in the Late Classic when population peaked and land use was the most intensive. It is stated that this resulted from the wide diffusion of many types of terracing that may have conserved soils when ancient Maya populations were greatest, but may also be partly the result of sediment exhaustion, in which erosion may have already removed the readily erodible component of upland soils.

In other regions like Cancue´n, Guatemala, however, most soil erosion occurred during the Maya Late Classic (AD 550–830). Erosion here was intense but short-lived, whereby depressions record 1-3 m of aggradation in two centuries.

Beach et al. (2006) conclude that though there is some isolated evidence for a rise in soil erosion during the Classic Mayan period when population pressure was highest, in fact erosion had started before this time. It is apparent that there is also ample evidence for lower soil erosion at several sites during the periods of highest human populations and intensive land uses. Though the Mayans did cause widespread geomorphic change, both in terms of soil erosion and soil conservation, Beach et al. (2006) state that their impacts started near the beginning of their civilization, which persisted more than a thousand years beyond the start of the population decline.

It therefore seems unlikely that soil erosion could account for the devastating population decline that was experienced by the Maya at around 900 AD.