Death and Population Growth Statistics

Exponential growth 

This is the most basic model of population growth, shown in equation, differential equation and graph form below. When the population is small, it grows at a smaller rate, and when the population is larger it grows at a larger rate because of the larger number of reproducing individuals.

The constant r represents the per capita rate of growth. For example, in the United States the population was recently growing at about 9.4 people per thousand and globally the growth rate was 17.0 per thousand. The term dN/dt is the change in number of individuals per unit time, the result of multiplying r and the N, the current number of individuals. In the integrated equation, A represents the population at time zero, and N is the population at time t.

Carrying capacity 

Usually, a given area, because of food or available land, can only support a limited number of individuals, and so the ideal exponential growth graph starts to flatten out. Populations may even crash if they pass this limit. This maximum number of possible individuals is known as the carrying capacity. Below is a simple model of capacity-limited growth. Although actual growth is still more complex, such models at least provide a method of assessing theories about population and the impact of the environment.

Here, K is the carrying capacity, the average number of individuals that can be supported by conditions. In practice, the population will oscillate around this number. This growth is known as logistic or sigmoid because its S-shaped graph resembles that of the Greek letter sigma. You can better understand it by using the model of cows competing for grass, available here. Notice how the exponential graph evolves into a sigmoid.

The human population 

The growth of human populations is a function of a variety of biological factors and their interplay with societal developments. During the beginnning of humanity, a low birth rate resulted from the inability to conceive until 19 or 20 years of age and the inability of women to ovulate during breast-feeding, which lasted up to four years. A pressure for long intervals between children may have also resulted from the hassle of having children while being nomadic. However, the development of agriculture probably brought about changes in these various factors, increasing the birth rate. The slowing of population growth likely resulted from a higher death rate, that in turn may be attributed to the susceptibility to disease when living close together, child mortality and taboos related to newborn children. The development of science has lowered death rates and increased population growth, and social traditions in various third world nations keep birth rates high. Scientists disagree about what the future holds for human population. In China, the government has even resorted to legally limiting the number of children a couple can have.

time	population	developments
25,000 years ago	3 million	Estimated beginnings of mankind
10,000 years ago	5 million	Agriculture developed
5,000 years ago	100 million	Growth slows
350 years ago	1 billion	Technology, science, industrialization
68 years ago	2 billion	Growth continues
Est. year 2000	6.3 billion	Growth continues