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The root of many of the environmental problems facing us is a large and growing global human population. More people means more competition: for renewable resources like fish and forests, for food production from agriculture, but also for energy, and non‐renewable resources like oil and minerals. During the first half of the 20th century, the global population increased by 40%, from 1.8 to 2.5 billion people. But since then the population has almost tripled to over seven billion. What’s more, the percentage of people living in cities has also grown steadily (Figure 5.16). By 2010, the number of city dwellers had equalled the number in rural environments for the first time in history, and the United Nations predicts that this trend will continue, with two‐thirds of the global population living in cities by 2050. The human population is growing ever larger and getting disproportionately crowded. We have seen through this chapter that the normal consequence, when populations grow, is that competition eventually slows that rate of growth and ultimately stops it, and that the overall size of the population settles, if not at a fixed carrying capacity, then within some regulated band. Is this what we’ve seen with the human population?

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 estimate, we can look at a few examples (see Cohen, 1995 , 2005 for further details).

In 1679, van Leeuwenhoek estimated the inhabited area of the Earth as 13 385 times larger than his home nation of Holland, whose population then was about one million people. He assumed all this area could be populated as densely as Holland, yielding an upper limit of roughly 13.4 billion. In 1967, De Wit asked how many people could live on Earth if photosynthesis was the limiting factor (but neither water nor minerals were limiting) and suggested 1000 billion, though if people wanted to eat meat or have a reasonable amount of living space the estimate would be lower. By contrast, Hulett in 1970 assumed levels of affluence and consumption in the USA were optimal for the whole world, and he included requirements not only for food but also for renewable resources like wood and non‐renewable resources like steel and aluminium. He suggested a limit of no more than 1 billion. Kates and others made similar assumptions using global rather than US averages. They estimated a global carrying capacity of 5.9 billion people subsisting on a basic diet (principally vegetarian), 3.9 billion on an ‘improved’ diet (about 15% of calories from animal products), or 2.9 billion on a diet with 25% of calories from animal products.

As Cohen (2005) has pointed out, most estimates have relied heavily on a single dimension – biologically productive land area, water, energy, food and so on – when in reality the impact of one factor depends on the value of others. Thus, for example, if water is scarce and energy is abundant, water can be desalinated and transported to where it is in short supply, a solution that is not available if energy is expensive. And as the examples above make clear, there is a difference between the number the Earth can support (the concept of a carrying capacity we normally apply to other organisms) and the number it can support at an acceptable standard of living. It is unlikely that many of us would choose to live crushed up against an environmental ceiling or wish it on our descendants.

what is the ‘human population problem’?

Our difficulties in defining a global carrying capacity raise a deeper difficulty. What is ‘the human population problem’? It may be simply that the present size of the global human population is unsustainably high – greater than the (presently unknown) carrying capacity. Or it may be not the size of the population but its distribution over the Earth that is unsustainable. Crowding as much as population size is the problem. As we have seen, the fraction of the population concentrated in urban environments has risen from around 3% in 1800 to more than 50% today. Each agricultural worker today has to feed her‐ or himself plus one city dweller. By 2050 that will have risen to each worker feeding two urbanites (Cohen, 2005). Or perhaps it is not the size but the age distribution of the global population that is unsustainable. In developed regions, the percentage of the population over 65 rose from 7.6% in 1950 to 12.1% in 1990. This proportion is now increasing faster still, as the large cohort born after World War II passes 65. Or finally it may not be that resources are limited but that their uneven distribution is unsustainable. Competition may be unbearably intense for some, while for others density‐independence prevails. In 1992, the 830 million people of the world’s richest countries enjoyed an average income equivalent to US$22 000 per annum. The 2.6 billion people in the middle‐income countries received $1600. But the two billion in the poorest countries got just $400. These averages themselves hide other enormous inequalities.

Of course, the human population problem, just like the problem in any crowded population, is not simply one of intense intraspecific competition for limited resources. Individuals in poor condition may be more vulnerable to predation and parasitism, and the spread of parasites may itself be enhanced. We return to the ways in which the abundances of populations are determined by the combination of forces acting on them in Chapter 14.

inescapable momentum

Finally we can ask what would happen if it were possible to bring demographic transition to all countries of the world so birth rates equalled death rates and population growth was zero. Would the population problem be solved? The answer is no, for at least two important reasons. We saw in Chapter 4 that the net reproductive rate of a population is a reflection of age‐related patterns of survival and birth, but these patterns also give rise to different age structures within the population. If birth rates are high but survival rates low (‘pre‐transition’), there will be many young and relatively few old individuals in the population. But if birth rates are low and survival rates high – the ideal to which we might aspire post‐transition – relatively few young, productive individuals must support the many who are old, unproductive, and dependent: an aspect of the problem that we noted above.

In addition, even if our understanding was so sophisticated and our power so complete that we could establish equal birth and death rates tomorrow, would the human population stop growing? The answer, once again, is ‘No.’ Population growth has its own momentum, and even with birth rate matched to death rate, it would take many years to establish a stable age structure, while considerable growth continued in the meantime. According to projections by the United Nations, even with low fertility the world’s population will grow from slightly more than seven billion today to more than eight billion by 2050 (Figure 5.19b). There are many more babies in the world now than 25 years ago, so even if birth rate per capita drops considerably now, there will still be many more births in 25 years’ time than now, and these children, in turn, will continue the momentum before an approximately stable age structure is eventually established. As Figure 5.19c shows, it is the populations in the developing regions of the world, dominated by young individuals, that will provide most of the momentum for further population growth.