1
The Space and Strategy of Demographic Growth

1.1 Humans and Animals

Throughout human history, population has been synonymous with prosperity, stability, and security. A valley or plain teeming with houses, farms, and villages has always been a sign of well-being. Traveling from Verona to Vicenza, Goethe remarked with pleasure: "One sees a continuous range of foothills . . . dotted with villages, castles and isolated houses . . . we drove on a wide, straight and well-kept road through fertile fields . . . The road is much used and by every sort of person."1 The effects of a long history of good government were evident, much as in the ordered Sienese fourteenth-century landscapes of the Lorenzetti brothers. Similarly, Cortés was unable to restrain his enthusiasm when he gazed over the valley of Mexico and saw the lagoons bordered by villages and trafficked by canoes, the great city, and the market (in a square more than double the size of the entire city of Salamanca) that "accommodated every day more than sixty thousand individuals who bought and sold every imaginable sort of merchandise."2

This should come as no surprise. A densely populated region is implicit proof of a stable social order, of non-precarious human relations, and of well-utilized natural resources. Only a large population can mobilize the human resources necessary to build houses, cities, roads, bridges, ports, and canals. If anything, it is abandonment and desertion population rather than abundant population that has historically dismayed the traveler.

Population, then, might be seen as a crude index of prosperity. The million inhabitants of the Paleolithic Age, the 10 million inhabitants of the Neolithic Age, the 100 million inhabitants of the Bronze Age, the billion inhabitants of the Industrial Revolution, or the 10 billion inhabitants that we may attain by mid-twenty-first century certainly represent more than simple demographic growth. Even these few figures tell us that demographic growth has not been uniform over time. Periods of expansion have alternated with others of stagnation and even decline; and the interpretation of these, even for relatively recent historical periods, is not an easy task. We must answer questions that are as straightforward in appearance as they are complex in substance: Why are we 8 billion today and not more or less, say 100 billion or 100 million? Why has demographic growth, from prehistoric times to the present, followed a particular path rather than any of numerous other possibilities? These questions are difficult but worth considering, since the numerical progress of population has been, if not dictated, at least constrained by many forces and obstacles that have determined the general direction of that path. To begin with, we can categorize these forces and obstacles as biological and environmental. The former is linked to the laws of mortality and reproduction that determine the rate of demographic growth, and the latter determines the resistance that these laws encounter and further regulates the rate of growth. Moreover, biological and environmental factors affect each other reciprocally and so are not independent of each other.

Every living collectivity develops particular strategies of survival and reproduction, which translate into potential and effective growth rates of varying velocity. A brief analysis of these strategies will serve as the best introduction to consideration of the specific case of the human species. Biologists have identified two large categories of vital strategies, called r and K, which actually represent simplifications of a continuum.3 Insects, fish, and some small mammals practice an r strategy: these organisms live in generally unstable environments and take advantage of favorable periods (annually or seasonally) to reproduce prolifically, even though the probability of offspring survival is small. It is just because of this environmental instability, however, that they must depend upon large numbers because "life is a lottery and it makes sense simply to buy many tickets."4 r strategy organisms go through many violent cycles with phases of rapid increase and decrease.

A much different strategy is practiced by K-type organisms -mammals, particularly medium and large ones, and some birds - that colonize relatively stable environments, albeit populated with competitors, predators, and parasites. K strategy organisms are forced by selective and environmental pressure to compete for survival, which in turn requires considerable investment of time and energy for raising offspring. This investment is possible only if the number of offspring is small.

r and K strategies characterize two well-differentiated groups of organisms (Figure 1.1). r strategies are suited to small animals having a short life span, minimal intervals between generations, brief gestation periods, short intervals between births, and large litters. K strategies, on the contrary, are associated with larger animals, long life spans, long intervals between generations and between births, and single births.

Figure 1.2 records the relation between body size (length) and the interval between successive generations for a wide array of living organisms: as the first increases, so does the second. It can also be demonstrated that the rate of growth of various species (limiting ourselves to mammals) varies more or less inversely with the length of generation and so with body size.5 At an admittedly macroscopic level of generalization, the lower demographic growth potential of larger animals can be linked to their reduced vulnerability to environmental fluctuations, and this, too, is related to their larger body size. Because their life is not a lottery and their chances of survival are better, the larger animals do not need to entrust the perpetuation of the species to high levels of reproduction. The latter, in fact, would detract from those investments of protection and care required to ensure the offspring's reduced vulnerability and keep mortality low.

These ideas have been well known at least since the time of Darwin and Wallace, founders of the theory of natural selection. Nonetheless, they provide a useful introduction to the discussion of the factors of human increase. Our species obviously practices a K strategy, in that it has successfully controlled the fluctuating environment and invests heavily in the raising of its young.

Figure 1.1 r strategy and K strategy.

Two principles will be particularly helpful for confronting the arguments in the following pages. The first principle concerns the relation between population and environment; this should be understood broadly to include all the factors - physical environment, climate, availability of food, and so on - that determine survival. The second principle concerns the relation between reproduction and mortality insofar as the latter is a function of parental investment, which in turn relates inversely to reproductive intensity.

Figure 1.2 The length of an organism at the time of reproduction in relation to the generation time, plotted on a logarithmic scale.

Source: J. T. Bonner (1965) / with permission of Princeton University Press.

1.2 Divide and Multiply

Many animal species are subject to rapid and violent cycles that increase or decrease their numbers by factors of 100, 1,000, 10,000, or even more in a brief period. The four-year cycle of the Scandinavian lemming is well known, as are those of the Canadian predators (10 years) and many infesting insects of temperate woods and forests (4-12 years). In Australia, "in certain years the introduced domestic mouse multiplies enormously. The mice swarm in crops and haystacks, and literal bucketfuls can be caught in a single night. Hawks, owls and cats flourish at their expense . . . but all these enemies have little effect in reducing the numbers. As a rule the plague ends rather suddenly. A few dead mice are found on the ground and the numbers dwindle rapidly to, or below, normal."6 Other species maintain equilibrium. Two centuries ago, Gilbert White observed that eight pairs of swallows flew around the belfry of the church in the village of Selborne, just as is the case today.7 There are, then, both populations in rapid growth or decline and populations that are more or less stable.

The human species varies relatively slowly over time. Nonetheless, as we shall see in Section 1.7, long cycles of growth do alternate with others of decline, and the latter have even led to extinction for certain groups. For example, the population of Mesoamerica was reduced to a fraction of its original size during the century that followed the Spanish conquest (initiated at the beginning of the sixteenth century), while that of the conquering Spaniards grew by half. Other populations have disappeared entirely or almost entirely - the population of Hispaniola (modern Haiti and Dominican Republic) after the landing of Columbus or that of Tasmania following contact with the first explorers and settlers - while, at the same time, others nearby have continued to increase and prosper. In more recent times, the population of England and Wales multiplied sixfold between 1750 and 1900, while that of France increased by barely 50 percent in...