Harvesting the Biosphere: What We Have Taken from Nature

8. Crops and Animals

The most important force driving the evolution from foraging to crop cultivation and the domestication of animals is clear: gathering and hunting cannot support population densities higher than about one person per square kilometer, even in benign environments with abundant standing biomass. As the numbers of early Holocene humans began to increase, they gradually turned to more intensive ways of food procurement, to what are—in comparison with later traditional agricultures, and even more so with modern farming—still rather extensive ways of growing crops and rearing animals for meat and milk. Given the enormous variety of environments and dominant food sources, there could be no typical population density for foraging: the actual rates ranged over nearly two orders of magnitude, from just one person per 100 km2 (or 0.01/km2 ) in both densely wooded rainy tropics and on cold boreal plains to 0.1/km2 for hunters on tropical grasslands, all the way to more than 1/km2 in coastal societies relying on a rich supply (permanent or seasonal) of marine foods (Smil 2008). The largest forager data set, presented by Marlowe (2005), allows the following generalizations. As expected, population densities increase with primary biomass but level off when the phytomass reaches about 30 kg/m2 : Marlowe’s mean for the entire sample of 340 societies is 0.25 people/km2 , with a median of 0.11 people/km2 (table 8.1). The highest rates (in excess of 1 person/km2 ) were among the foragers along the northwestern Pacific coast of North America, but their exceptional densities reflected the seasonal abundance of anadromous salmon rather than high habitat richness. Food shares contributed by gathering, hunting, and fishing correlate significantly with latitude: the latter two activities were dominant in high northern latitudes and important in their southern counterparts, while gathering was the most important contribution in the tropics and subtropics. In contrast, the population densities of pastoralists were almost always above 1 person/km2 even in arid grasslands, and shifting cultivation commonly supported 8 Crops and Animals 104 Chapter 8 more than 10 people/km2 , easily 100 times as many as could be supported in the same environments with foraging. This transition to more intensive modes of food procurement was a matter of complex evolution, and any notion of agricultural revolution (Childe 1951) is an indefensible intellectual construct. For example, wild cereals were collected and processed and the resulting flour was used in baking in Israel during the Upper Paleolithic (19,500 BCE), at least 12,000 years before the domestication of cereals (Piperno et al. 2004). In contrast, Mexico’s Tehuacán Valley had no permanent settlements but thousands of years of crop cultivation (Bray 1977), and many settled agricultural societies retained significant elements of foraging for millennia. Clearly, there was no orderly lock-step progression of cultivation and sedentism, no sharp divide between foraging and incipient cultivation. The cumulative impact of animal domestication and shifting and permanent crop cultivation began to transform ecosystems not only on local or small regional scales but eventually across large areas of some key biomes. As already noted, Ruddiman (2005) has argued that humans actually took control of the climate For centuries, rice has been the world’s most important staple grain crop. This photograph shows the mechanized harvesting and drying of sheaves on wooden supports in a field near Kyōtō in the early 1990s. Photograph by V. Smil. Crops and Animals 105 Table 8.1 Density of Human Populations Population Density (people/km2 ) Live Weight (kg/ha) Foraging 0.01–>1 0.005–0.5 Pastoralism 1–2 0.5–1 Shifting cultivation 20–30 9–14 Traditional farming Predynastic Egypt 100–110 45–50 Medieval England 150 75 Global mean in 1900 200 100 Chinese mean in 1900 400 180 Modern agriculture Global mean in 2000 400 200 Chinese mean in 2000 900 410 Jiangsu province in 2000 1,400 630 Note: All densities for foragers and pastoralists are per unit of exploited of land; all densities for traditional and modern agriculture are per unit of arable land. through these actions. That conclusion is debatable, but there can be no doubt about the large-scale and often lasting environmental transformations brought about by pastoralism and cropping. And these changes affected not only fertile river valleys, coastal lowlands, or forests near growing population centers but even Amazonia, a region that was considered until recently the very epitome of...