Population Crises and Population Cycles

Among lower animals, high fertility and competition prevail. Three great trends of evolutionary progress have been:-

1. reduction in fertility

2. development and extension of parental behaviour, and

3. sociability towards and adoption of individuals not closely genetically related.

These trends have culminated in cultural evolution in the co-operative societies of higher primates and cetacea (whales and dolphins), and ultimately in the achievements of man.

Despite their relatively low fertility, mammalian populations are still liable to outgrow their environmental resources (animal prey or plant food populations), and are in danger of irretrievably depleting them. To avert this, there has evolved a behavioural and physiological response to population crisis. When a mammalian population becomes dangerously dense, but before it can deplete its resources, the stimulus of overcrowding leads to a complete reversal or regression of behaviour. Co-operation and parental behaviour are replaced by competition, dominance and aggressive violence. The effect of crowding on aggression in wild rabbits, for instance, is shown in Table 1.1. Females and young, demographically most important, are most likely to be killed, and thus the population is reduced. In the elaborate societies of higher primates, the effects may be quite complex, involving the replacement of friendly leaders by aggressive bullies, lethal mob attacks on persecuted individuals, and war between bands. But the end result is the same - mortality of females and young, and a reduced population. The stress of crowding and of the resulting violence impairs both the immune and the reproductive systems. Hence epidemics complete the crash of the population, and reproduction is slowed for three or four generations, giving the resources ample time to recover. In some mammal species, crisis and crisis response recur in a regular fashion, leading to cycles of population growth and collapse, oscillating about a fixed mean, as shown for the snowshoe hare in Figure 1.1.

Population crisis response and population cycles have been equally prominent in the history of human societies, but with certain differences related to the unique character of our species. In man, thanks to our advancing technology, successive advances in food production have made possible growing populations, though with every such advance population soon outgrew the current level of resources. Hence human population cycles have generally been superimposed on a rising curve, producing a saw-tooth graph. Since the cycles in different societies have not been in phase until recently, the graph for a very large region may look smooth, but in small regions such as Spain, or large homogenous regions such as China, the saw-tooth effect is clear, as shown in Figure 1.2. Because advances in food production amounted to sudden disturbances in the relations between human populations and their environments, the crisis responses in man have not been able to achieve their evolutionary function in time, and hence each full-scale crash, when it came, has generally involved famine, and often resource damage, as well as massive violence and very high death-rates from disease, in just the manner described in the classical works of Robert Malthus. Finally, since the coming of settled agriculture and cities, human societies have obviously been far larger and more complex than those of any mammal, and the characteristics of crisis periods, and of the intervening relief periods when population crash brought the population down to a better balance with current resources, have been correspondingly complex, and can be described under various headings.

Fig. 1.2 Population cycles of China and Spain (after McEvedy and Jones, 1978)