Chapter 15 Human Ecology • MHR 517

A second method has involved fitting logistic
curves to historical data on human population
growth and extrapolating into the future, to
determine at what population growth could be
expected to level off. In 1924, researchers using
this method estimated that the maximum size of
the human population would be two billion. The
number of people on Earth reached this around
1930 and kept increasing. In 1936, another group
estimated that the maximum would be 2.65 billion
and that this would not be achieved for another
150 years. In fact, this number was surpassed less
than 20 years later. Clearly, it is difficult to fit
curves to the pattern of human population growth,
perhaps because for awhile the human population
grew faster than any model would predict.

Limiting Factors

Many studies of the human carrying capacity have
looked at the impact of a single constraining factor
on growth. Food has been commonly thought to be
the single most important limiting factor. Thus, a
widely used formula for determining the maximum
population size is:

Although this formula seems objective, results
obtained by using it can vary depending on estimates
of the amount of food available on Earth and
estimates of the amount of food required by each
person. The food supply varies with the choice of
crops and the variety of each crop planted; these
will change as weather and soil conditions change
or as advancements in technology produce new
varieties. The supply will also depend on the land
area available for planting and amount of watering
that can be done. Year-to-year variation in food
availability will occur as a result of loss due to
pests, weather fluctuations, and waste due to
spoiling if food must be transported long distances.
The denominator of this equation can also vary,
depending on the trophic level of the humans
involved (whether they are primary or secondary
consumers) and on the amount of energy (measured
in calories or kilojoules) that they are assumed
to require.
Some ecologists have identified variables other
than food as being the most important for setting a
limit on population size. These other factors have
included things like energy, biologically accessible
nitrogen or phosphorus, fresh water, light, and
space for waste disposal. Regardless of the factor
believed to be most important, all the researchers

who have used an equation similar to the one

above have failed to examine whether some other

variable or variables might take effect even before

the factor they consider to be the most important

has begun to have an impact.

Multiple Factors

In contrast, some studies have considered

population size to be limited by the action of

multiple factors acting independently. These

are combined into equations in a variety of ways.

For example, if both water and food are considered

to act as constraints on population growth, then:

A problem with even these more complex

equations is that they do not take into account that

different parts of the human population have

different food and water requirements. Also, these

requirements may fluctuate over time. For example,

the water requirements of people in tropical

countries may increase during the dry season,

while the food requirements of people in temperate

countries may be higher in winter. Therefore, using

such equations to estimate the carrying capacity for

Earth will likely produce inaccuracies.

Chance Events

All of the methods described above are more or

less static. That is, they do not take into account

chance events such as volcanic eruptions, epidemics,

El Niño effects, and genetic changes in viruses and

bacteria, among other things. Any of these chance

events can produce random fluctuations in

populations on a local or regional scale. Although

some very complex models have been developed

to estimate carrying capacity while taking such

random events into account, they are in the earliest

stages of development and have not been well

tested. Further study of such models is important.

They could help us determine at what level the

human population could be maintained most of the

time (perhaps 95 years out every 100), given a certain

anticipated amount of random variation. The

concept organizer shown in Figure 15.13 on the

following page provides a summary of some of the

factors influencing the carrying capacity of Earth

for humans.

In the Thinking Lab on page 519, you can use

your own food intake for one day as an estimate of

the requirements of a typical person. Extrapolating

Population

that can

be fed and

watered

=

minimum

of

,

food supply

individual food

requirement

individual water

requirement

water supply

()

Population that

can be fed

=

food supply

individual food requirement