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,

Table 1.2 2003 human population data from the Population Reference Bureau (Anonymous 1981-2004).

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

Table 1.3 World human demographic trends since 1981. All data from the Population Reference Bureau (Anonymous 1981-2004).

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].

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