Ecology. Michael Begon

Figure 5.16 The global urban population has overtaken its rural counterpart and will probably run away from it. The sizes of the total, rural and urban populations of the world from 1950 to 2010, and projections from the United Nations up to 2050.

      Source: After UNEP (2014).

      population growth up to the present

With exponential growth (Section 5.6), the population as a whole grows at an accelerating rate simply because the growth rate is a product of a constant individual rate of increase and the accelerating number of individuals. For thousands of years, the growth of the global human population appears indeed to have been exponential (Figure 5.17). But the growth rate was slow, despite a jump around 10 000 years ago at the dawn of agriculture. More recently, however, with growing urbanisation and industrialisation, growth, far from slowing, accelerated to become faster than exponential for several centuries because the per capita rate itself increased. Only very recently has the rate slowed again.

Figure 5.17 The global human population grew slowly for millennia but has recently shown faster than exponential growth. The estimated size of the global human population over the past 30 000 years and projected into the future.

      Source: After Population Reference Bureau (2006).

      Are these modest indications of a slow‐down a sign that competition is intensifying? If so, this is far from being the whole story. We humans already appropriate a high proportion of the global plant production for our own uses (discussed further in Application 20.2), and average food consumption per person has not been falling, as it would with intensifying competition, but rising. It has increased steadily over the past 50 years, from 2360 calories per day in the mid‐1960s to 2940 calories today (WHO, 2013). Both figures exceed the 2250 calories per day estimated by the US National Institutes of Health to be sufficient for a moderately active adult. Of course, hunger and malnutrition remain major problems in many areas, with perhaps one billion people receiving insufficient food. Yet even in developing countries, average consumption has increased from 2054 calories per day in the 1960s to 2850 today. Hunger results not from inadequate global food production but from unequal distribution.

      demographic transitions

In fact, the slow‐down in population growth seems to have less to do with a direct effect of resource shortages than with a change in individuals’ social conditions and decision‐making. In particular, we have seen in human populations in many parts of the world a demographic transition– a switch from a combination of high birth and death rates to one of low birth and death rates. Indeed, we can distinguish three categories of human population: those that passed through this demographic transition before 1945 (‘early’) (Figure 5.18), those that have passed through one since 1945 (‘late’), and those that have not yet passed through the transition. The pattern is as follows. Initially, both the birth rate and the death rate are high, but the former is only slightly greater than the latter, so the overall rate of population increase is only moderate or small. As we saw in Figure 5.17, this, broadly, was the case for the global human population until around 300 years ago. Next, the death rate declines while the birth rate remains high, so the population growth rate increases, giving us the more‐than‐exponential rate we also saw in Figure 5.17. Next, however, the birth rate also declines until it is similar to or perhaps even lower than the death rate. Hence, the population growth rate eventually declines again and may even become negative, though with a far larger population than before the transition began.

Figure 5.18 The birth and death rates in Europe since 1850. The annual net rate of population growth is given by the gap between the two. Death rates declined in the late 19th century, followed decades later by a decline in birth rates, leading ultimately to a narrowing of the gap between the two.

      Source: After Cohen (1995).

The generally accepted explanation, though probably not the whole story, is that the transition is an inevitable consequence of industrialisation, education, and general modernisation, leading first, through medical advances, to the drop in death rates, and then, through the choices people make (such as delaying having children) to the drop in birth rates. Certainly, when we consider the populations of the different regions of the world together, there has been a dramatic decline from the peak population growth rate of about 2.1% per year in 1965–70 to around 1.1–1.2% per year today (Figure 5.19a).

Figure 5.19 What happens to the global human population size depends on future fertility patterns. (a) The average annual percentage rate of change of the world population observed from 1950 to 2010, and projected forward to 2100 on the basis of various assumptions about future fertility rates. (b) The estimated size of the world’s population from 1950 and 2010 and projected forward to 2100 on the basis of various assumptions regarding fertility rates. (c) The estimated size of the populations of the world’s main regions from 1950 and 2010 and projected forward to 2100 assuming ‘medium’ fertility rates.

      Source: After United Nations (2011).

      a global carrying capacity?

      It seems clear, then, that the rate of human population growth is slowing not simply as a result of intraspecific competition, but as a result of the choices people make. Nonetheless, if current trends continue, we might hope that the size of the global human population could level off and approach what, in terms of intraspecific competition, we would call a global carrying capacity. This in turn raises the question of what a reasonable global carrying capacity would be. Estimates have been proposed over the last 300 years or so. They vary to an astonishing degree. Even those suggested since 1970 span three orders of magnitude – from 1 to 1000 billion. To illustrate the difficulty of arriving at a good