An a-maize-ing theory?

As soil fertility is reduced by deforestation, the number of species present is also lowered. According to Santley et al. (1985), this resulted in a shift of concentration of weeds, insect infestations and plant diseases to the agricultural plots. Although this process is gradual and takes several years, it almost certainly would have helped reduce crop yields. This would have became a significant problem where fields were intensively cultivated and where monocropping was practiced, as the land became more susceptible to disease.

I found a paper that advocated the theory of the planthopper-borne virus (maize mosaic virus (MMV)) and its major role in the collapse of the lowland populations, predominantly focusing on the Peten area in northern Guatemala (Brewbacker, 1979). It argues the case fairly convincingly and draws on much more recent examples of how diseases to crops have caused agricultural abandonment, mass migration and population declines, such as the Irish Potato Famine. Even though I don’t think it was the primary cause of the collapse, it certainly seems that it caused food shortages large enough to affect population sizes. It also provides an explanation for the differential collapse, as the relative significance of pests and diseases was assessed and compared for various regions of the Maya area.

What is MMV and what are the effects?

• ·         A devastating virus that has a unique world distribution that just so happens to include the former Maya area.
• ·         It is transmitted by the corn planthopper, an insect specifically found in lowlands of the tropics.
• ·         Maize is one of two of its only definitive hosts thus it is only serious when maize is intensively cultivated throughout the year
• ·         It can reduce crops to less than 50cm in height and at its extreme, entire fields can be decimated
• ·         The Mv gene is the only known form of high level resistance (yet not immunity) of the virus. However, this gene has not been found to occur in maize grown in Central America, in contrast to other races of maize such as in the Caribbean.

It is hypothesised that the MMV originated in northern South America and was blown into the Peten across the Caribbean around the 8^th century. This facilitated the emergence of an epidemic among susceptible maize types (that lacked the Mv gene) grown by the Peten Maya.

It is suspected that this disease would not have affected the highlands in the south and west (where the corn planthoppers find it difficult to thrive), as well as areas that had longer dry seasons like in the northern Yucatan, as much as it did Peten.

This theory fits into the agricultural failure hypothesis that I discussed last week, since diseases like MMV can directly affect food production by decreasing yields and also indirectly, as it requires more labour to remove weeds and disinfest fields. The Maya agricultural system that was initially efficient, began to decline, as more people were required to work on the fields...you may think that this is not really a problem since they had an increasing population and therefore a larger workforce. Yet the amount of land suitable for crop cultivation was not unlimited- they soon began to run out.

However, it is important to consider a few points: firstly, the theory fails to explain how the northern Yucatan and the highlands also experienced abandonment and large population declines later on, especially since the MMV was not present in those areas. Secondly, it is not very clear on how exactly the MMV and the viruliferous leafhoppers were blown into the Peten, and whether it had also occurred previously, and if so, why was it only in the 9th century that the Maya civilisation collapsed. Such questions highlight the gaps of the hypothesis and so we must look to other suggestions of possible causes.