Population Crises And Population Cycles

Over very large areas of the wet tropics, in rain forest or adjacent regions lacking such advantages as volcanic soil or river-borne silt, soils tend to have plant nutrients washed out of the topsoil, becoming acid and infertile: this is true, for instance, of 78% of soils in the Amazon basin. Nevertheless, an excellent form of farming has evolved in such regions all over the world. An area of forest is cleared, the resulting litter spread over it, dried and burned, and a variety of crops planted. After a few years yields decline, and the area is allowed to revert to forest fallow for a much longer period (Table 1). This admirable system, called swidden farming (from the Old English word for a burned clearing), uses no agricultural chemicals and can yield food energy up to twenty times the energy invested.

However, because so little of the land is producing food at any given time (Table 1), the system demands much land per family, and a correspondingly low density of population. In different regions, with better or worse soils, terrains etc, different critical densities of population have been calculated (Table 2). If the critical density in a district is exceeded, there has to be more land under crops, and this can only be done by shortening the fallow period in each cycle. With a shorter fallow, the forest cannot fully restore fertility, yields decline, and yet more land is needed, in a vicious cycle. Eventually the forest cannot recover at all. The now unprotected soil is invaded by tough grass weeds, or eroded away altogether by wind or water. Some tropical soils, for instance in the Amazon basin, are lateritic, that is with virtually no elements in the topsoil except iron and aluminium. Exposed to the sun without plant cover, such soils may bake into a hard substance that can never again grow crops: the Khmers of Cambodia (see the eighth paper) used laterite as a building material. In all these ways land goes right out of crop production, reducing the population carrying capacity of the district: by this time land hunger will have led to serious violence (Table 2). Population crises thus take a rather special form in swidden farming regions.

With such low critical population densities, swidden farmers live in either very small or widely separated villages, in ‘a pattern of dispersed settlement and scattered population’ (Harris, 1972). This does not normally permit the appearance of cities and civilisation. But one great civilisation was based on swidden farming - the Maya of what are now Guatemala, Belize, Honduras and Southern Mexico. In its heartland alone, across the base of the Yucatan peninsula, this civilisation numbered some 80 cities and over 200 lesser centres.

During their heyday (AD300-600), ‘the Maya ... reached intellectual and artistic heights which no others in the New World, and few in the Old, could match at that time’ (Coe, 1971). In mathematics, they evolved place value and the zero symbol independently of, and probably earlier than, the corresponding invention in India. In astronomy, their calculations for the motions of Venus were accurate to an error of one day in 6000 years. Their calendar was capable of greater accuracy than the Gregorian calendar introduced in Europe in AD1582 and still in use today. Besides splendid buildings, sculptures and paintings, they regularly set up dated monuments inscribed with historical records and astronomical data.

These achievements were based on a pattern of settlement quite unlike the urban concentrations usual among civilised societies. Tikal, perhaps the greatest Mayan city, had a very modest population of about 10,000, scattered over 16 square kilometres: contrast the Sumerian city of Uruk, with 100,000 people within the 4.5 square kilometres enclosed by its ramparts. The Mayan city was the only kind that could be based on the ‘dispersed settlement and scattered population’ characteristic of swidden farmers.

The Maya practised swidden farming with maize and beans as the main crops. They sometimes grew manioc on ridges in periodically flooded swamps. But there is no evidence that they practised artificial irrigation or the large-scale cultivation of permanently cleared plots. Their spread-out cities and city-states were a cultural adaptation to the low land productivity of swidden farming. But even this much concentration was only attained at the cost of exceeding the critical population density for the region.

During the Classic period (AD 300-600), the population of the Central Mayan area grew steadily. At Barton Ramie in the Belize Valley, there was a four-fold increase in house occupation. At Uaxactun, Tikal and Dos Aguadas, population densities probably reached 386, 625 and 888 per square kilometre, respectively. These are comparable to the disastrous density of South-East Nigeria in the 1960s (Table 2).

To feed their growing populations, the Classic Maya must have shortened their fallows with the inevitable results. During the Late Classic Period (AD 600-900), all the signs of overpopulation crisis make their appearance. Burials at Tikal show a sharp drop in stature, suggesting malnutrition due to food shortage. The people of Tikal constructed a massive defensive embankment and ditch, ten kilometres long, across the main route to the neighbouring city of Uaxactun. This defensive work was 4.5 kilometres from the city centre, and was clearly intended to defend agricultural land. Warfare, hitherto very infrequent, was becoming common and intense. At Bonampak, wall paintings depict prisoners being tortured. Elsewhere there are signs of peasant revolt: at Piedras Negras, the dais of the rulers was deliberately smashed. In the course of the eighth and ninth centuries, the cities ceased to raise their dated monuments. The cities were being abandoned. A few illiterate peasants still camped in abandoned city centres: at Piedras Negras, they re-erected an inscribed monument upside-down. But the civilisation had collapsed, and aerial surveys and excavations show a drastic fall in population during the ninth and tenth centuries. ‘Since the end of the tenth century AD, most of the area has been a green wilderness’ (Coe, 1966). Much of the Peten region is visited only by gatherers of chicle, the raw material for chewing-gum.

The present depopulation of the region (cf. Table 2) is mainly due to the lasting damage done to the land when the Maya swidden farmers exceeded their critical population density. Lateritic areas are denuded of trees, and ‘the insect life in the humus of the present forests in Peten is far from as abundant as in virgin forests, which is a further proof that the forests of this district have been entirely cleared away and have had great difficulty in recovering’ (Gourou, 1966). The clay filling various lakes in this region probably came from eroded soil. Both grassland formation and the danger of soil erosion have been reported in swidden farming districts with rising populations in modern Guatemala and Mexico.