Biology 12

Chapter 15 Human Ecology • MHR 505

500 million people on Earth. By 1850, many of the
world’s nations were conducting actual censuses
of their citizens and the world’s population
was estimated to be about one billion. Human
population growth has been very rapid since then;
additional billions are being added in much less
time than it took for the first billion to be reached.
Figure 15.2 shows the time required to reach each
additional billion.

Figure 15.2Since the development of agriculture, the rate
of human population growth has dramatically increased. It
took much longer for the population to reach one billion
than it took for each additional billion to be added.

Demographic Transition

Recall from Chapter 14 that the growth rate of a
population (r) depends on the difference between
the birth and death rates of that population (that is,
r=b−d). From studying many human populations
and how they changed after the introduction of
technology, demographers have developed a model
of how shifts in birth and death rates alter patterns
of population growth. The model suggests that
increases and decreases in roccur as a population
goes through a series of stages known as a
demographic transition.
In all early human populations and in some
modern societies, death rates (especially of infants
and children) were and are high. As a result, these
populations developed customs that encouraged
high birth rates so that the birth and death rates
came close to balancing. This corresponds to the
first stage of the demographic transition, in which
the overall growth rate of a population is more or
less stable. This is illustrated in Figure 15.3.
Demographers have estimated that until 1700,
mortality rates were constant or declined very
slowly and were just about matched by birth rates.
Temporary increases in mortality occurred as a
result of wars or epidemics, including that of the
black death (bubonic plague) that killed millions of

humans in Europe and Asia during the fourteenth

century. Although it sometimes took years for

a local population to recover from these periods

of high mortality, high birth rates eventually

compensated and the world population continued

to grow slowly.

Figure 15.3Diagram (A) shows how birth and death rates

change as a population passes through the stages of

a demographic transition. Diagram (B) shows the

corresponding changes in the rate of population growth.

In the early years of the eighteenth century in

England, and slightly later in Europe and North

America, the Industrial Revolutiontook place.

During this time, people shifted from working in

traditional agricultural jobs and making goods by

hand to working in factories and making mass-

produced goods. As industries expanded over the

next 150 years, the standard of living for many

people increased. At about this time, scientists

such as Louis Pasteur and Joseph Lister provided

convincing evidence that bacteria and other small

organisms caused many of the diseases that had

previously ravaged populations. The resulting

improvements in sanitation and medical care

lowered the death rate in some countries, as did

improvements in nutrition. The invention of the

steam engine made it possible for a single farmer

with a tractor to do the work of dozens, thus

increasing food production. Trains and ships could

distribute food quickly and reduce the impact of

local famines. Later, near the middle of the twentieth

century, the widespread use of antibiotics,

vaccines, and other medicines lowered the

death rate even more.

Since it generally takes several generations for

cultural values and customs to shift in response

to lowered death rates, birth rates in these

Stage 1 Stage 2 Stage 3 Stage 4

birth rate

death rate

(mortality)

population

increase

population stable

population stable

Time

population

increasing

B

A

first billion

second

third

fourth

fifth

sixth

Number of years to add each billion

30 years (1960)

130 years (1930)

(1800)

14 years (1974)

13 years (1987)

12 years (1999)

roughly 100 000 years (nearly all of human history)