not replacing themselves in the population. Average family size is four or less.
A country at this stage can grow substantially only through immigration and
may experience shortages in the labor supply, particularly in lower-paying jobs.
A number of countries may be considered to have reached this stage of the model,
including many of the countries of Western Europe and Japan. The population
pyramid representing this stage is not a pyramid at all, but resembles a column,
due to the nearly equal numbers of people in most age categories, except for those
at the very limits of the human life span.
The Demographic Transition Model has many detractors, and indeed the model
has shortcomings when applied to individual countries or regions. For example,
critics of the model argue that it fails to take into sufficient account the complexity
of factors influencing birth and fertility rates in developing countries, and also
point out that the model is based exclusively on patterns observed in the western-
ized, developed world, and therefore is plagued byethnocentrism. Nevertheless,
the model remains widely employed and debated among population geographers.
Desert
The term derives from the Latin worddesertumfor “abandoned.” Ironically, the
Sahara Desert and the Gobi Desert are redundant names because “Sahara” and
“Gobi” both translate as “desert” in their original languages. During the settlement
of the trans-Mississippi west of North America in the 1800s, the term “Great
American Desert” was commonly applied to semiarid areas thought not capable
of supporting agriculture and significant habitation. Occasionally, current writers
will refer to “polar deserts” because polar climates are some of the driest on the
planet. For instance, places in the interior of Antarctica receive less than 25 mm
ofprecipitationa year; moreover, the precipitated water is in frozen form not
directly usable by life.
How many deserts does the world contain? Although there are various ways to
define desert, the widely used Ko ̈ppen classification system defines desert climates
as those averaging at least twice as much potential evapotranspiration as precipita-
tion during the year. This definition fits large stretches of the planet with adjacent
land considered to be “semiarid” with substantial problems for their use. The larg-
est desert is the Sahara, which occupies 9 million sq km of northern Africa. The
aridregionof which it is a part extends through southwestern and central Asia.
All continents save Antarctica contain dry, hot deserts. Australia is usually reck-
oned to be the driest continent in that somewhat over 40 percent of its surface is
desert with much other adjacent semiarid land. The long, narrow strip of the South
Desert 95