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affect population size by decreasing food supplies. For
example, in parts of East Africa, a form of sleeping sick- TERMS TO KNOW . . .
ness known as nagana attacks livestock. That disease is population density: The average number of people per
almost always fatal to cattle but not to humans. The peo- unit of land area
ple in this part of East Africa depend heavily on cattle, arithmetic (crude) density: The average number of
which provide food, represent wealth, and serve a reli- people per unit of land area (usually per square mile or
gious function in some tribes. The spread of this disease kilometer)
by tsetse flies has caused entire tribes to migrate away
from infected areas, leaving those areas unpopulated.
Arithmetic or Crude Density

7-3 Wh a t ar e the thr ee methods f or Often used interchangeably with population density,
7-3 What are the three methods for
calculating population density?
calculating population density? arithmetic (crude) density is the average number of people
Methods for Calculating per unit of land area (usually per square mile or kilome-
ter). It is the most commonly used measure of population
Population Density density and can be calculated by dividing the total pop-
ulation of a country (or place) by its corresponding land
Identifying population distribution patterns is useful, area. The higher the result, the more crowded a country
but it does not tell us how crowded a place is and how pop- is. In mid-2018, the world population was 7621 million,
ulation size compares with available natural resources, and Earth’s total land surface is 57.51 million square
particularly land. To better understand the relationship miles (148.94 million square kilometers). We calculate
between population and environment, geographers use the arithmetic density for the whole world as follows:
the concept of population density, or the average number 7621 ÷ 57.51 = 133 persons per square mile
of people per unit of land area (Figure 7.10). Three popu-
lation density indicators measure the ratio of population 7621 ÷ 148.94 = 51 persons per square kilometer
size to land area: arithmetic or crude density, physiolog- The very uneven population distribution on Earth
ical density, and agricultural density. has created huge disparities in density. Refer back to

(a) (b)

Figure 7.10 Comparison of densely populated and sparsely populated areas. Population density varies considerably on Earth’s
surface as seen in the (a) densely populated city of Hong Kong versus (b) a sparsely populated rural area in the United States.
(a) ymgerman/iStock/Getty Images (b) ClassicStock/Getty Images

Module 7 Population Distribution and Its Consequences 15
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