# Applications to human populations

Few biological populations grow either geometrically or exponentially for long. As we will explore in the sections on intraspecific competition and logistic growth, as populations grow, resources become scarce. The resultant changes in birth and/or death rates slow growth. The human population of the world, however, has continued to grow since around 1650; it reached 6.0 billion by late 1999, and 6.3 billion by 2003 (Fig. 1.6a). Many scientists question how long this growth can be sustained. While most ecologists insist that human population growth must cease in the near future, some economists (Simon 1996) see no reason for limits to the human population. In the next section we will use data from the Population Reference Bureau (Anonymous 1981-2004) to illustrate how Equations 1.8 to 1.10 may be used in population projections.

Recall from Equation 1.9 that if we graph natural log of population growth versus time we can determine the intrinsic rate of increase by finding the slope of the graph. In Fig. 1.6b we have plotted the natural log of human population growth against time. The slope of this line, as determined by the statistical technique of linear regression and computed for us in an Excel™ spreadsheet, is 0.007. This is the best fit for the intrinsic rate of increase for the human population from 1650 to 2003.

If we examine Table 1.2, in which human populations in 2003 are broken down by continental regions, the strengths and weaknesses of this simple model become apparent. Most striking are the immense differences among populations. While the human population as a whole is growing twice as fast in 2003 as compared to the period of 1650 to the present (contemporary r = 0.013, historical r = 0.007), Europe has a negative r, while that of Africa is 0.024, almost twice the global growth rate. Secondly, over 60% of the human population resides in Asia.

Clearly, although human population growth is of global concern, it is a highly regional problem. From Table 1.2 you should be able to see that r is readily calculated as the difference between the birth and death rates. Secondly, you should try calculating projected doubling times based on Equation 1.10. You will find that the data published by the Population Reference Bureau differ slightly from your calculations. They are using more sophisticated models and are taking age distributions into account. Nevertheless, the differences in doubling times are remarkably minor. Finally, if you examine the last column you will also notice another great difference among these populations. The percentage of the population in the pre-reproductive years (15 years or younger) varies from 42% in Africa to a low of 17% in Europe.

Year

Figure 1.6 Human population growth since 1650: (a) world population, in billions; (b) natural log of population growth, in millions.

Year

Figure 1.6 Human population growth since 1650: (a) world population, in billions; (b) natural log of population growth, in millions.

In his book The Skeptical Environmentalist, Bjom Lomborg (2001) is rather sanguine about human population growth. He accepts the demographic transition model, which states that rapid growth has occurred because of a rapid drop in the death rate (due to modern methods of sanitation, improved food growth and distribution, better medical care, etc.) and that eventually, with improved standards of living and wealth, birth rates drop to match the low death rates. Indeed, in most European countries, human population growth has slowed, and even gone negative. In 2003, 20 countries out of 43 in Europe had a growth rate of zero or negative, including all 10 Eastern European countries. As noted above, the population growth rate (r-value) for Europe as a continent is negative. As for the future, Lomborg accepts a "medium variant forecast" from the UN. This prediction is zero population growth for the world by the year 2100. However, by then the world population is projected to be 11 billion. Consider that the world population was only one billion in 1850, two billion in 1950, and 6.3 billion in 2003. Lomborg is correct when he says that 60% of growth is from just 12 countries. Perhaps the world outside of Africa and Asia will not necessarily suffer a catastrophe from human population density,

 Region Population Birth rate Death rate Rate of Doubling Percent size (per (per increase per time under (millions) thousand) thousand) individual (r) (years) 15 years World 6314 22 9 0.013 53 30% Africa 861 38 14 0.024 29 42% North America* 323 14 8 0.005 139 21% Latin America+ 540 23 6 0.017 41 32% Asia 3830 20 7 0.013 53 30% Europe 727 10 12 -0.002 NA 17% Oceania* 32 18 7 0.011 63 25%

* North America = the United States and Canada.

+ Latin America includes Central and South America and the Caribbean Islands.

* Oceania includes Australia, New Zealand and the South Pacific Islands. Countries of the former USSR have been distributed between Asia and Europe.

but what will happen in China, India, Pakistan, Bangladesh, and Nigeria, for example, in the next 100 years? The 2003 data sheet from the Population Reference Bureau predicts that China's population will stabilize at about 1.4 billion (compared to its present estimated population of 1.289 billion) by 2050. By 2050, however, the PRB predicts a population for India of 1.6 billion (compared to present population of 1.069 billion). The question on the mind of the concerned biologist: Will there be any room for natural habitats on a planet with 11 billion or, worse yet, 15 billion people?

Examine Table 1.3, which describes overall human demographic trends since 1981. Lomborg (2001, p. 47) states that world population growth, in numbers per year, reached

 Year World Birth Death r per Projected Actual average population rate per rate per individual growth in growth per year estimate thousand thousand numbers during specified (billions) per year (millions) time period (millions) 1981 4.492 28 11 0.017 77.0 1985 4.845 27 11 0.016 78.1 1981-85: 88.3 1987 5.026 28 10 0.018 91.3 1985-87: 90.5 1989 5.234 28 10 0.018 95.1 1987-89: 104.0 1991 5.384 27 9 0.018 97.8 1989-91: 75.0 1995 5.702 24 9 0.015 86.2 1991-95: 79.5 2000 6.067 22 9 0.014 85.5 1995-2000: 73.0 2003 6.314 22 9 0.013 82.6 2000-03: 82.3

Table 1.4 Human demographic trends in North America since 1981. Data from the Population Reference Bureau (Anonymous 1981-2004].