The popular image of a desert is a hot, dry area devoid of vegetation, but this
stereotype is not uniformly true. Net primary productivities are lower than any
land regions but for those covered by ice. Both plants and animals are scarcer
and less diverse than in other realms but they do exist. Plant life is adapted to
extreme dryness through the development of deep or wide root systems or forms
that lessen transpiration such as leaves modified into thorns. In short, desert plants
are very conservative of water. Most desert animals are nocturnal and so not
obvious to the casual observer.
An oasis is a desert locale in which there is verdant life clustered around
springs, lakes, ponds, or streams emanating fromgroundwatersources. The water
table is close to the surface and artesian water emerges under its own pressure. The
vegetation around oases provides food for limited numbers of grazing animals so
for thousands of years nomadic peoples have traveled from oasis to oasis to allow
their animals to eat and drink.
Deserts make up about a third of Earth’s lands and, so, have shaped the habita-
tion patterns of the planet. Perhaps 5 percent of humans live in deserts. There
are large cities (e.g., Phoenix and Cairo) in some deserts, but these places are able
to obtain water not dependent on local rainfall. In the case of Phoenix, it uses
groundwater and an aqueduct that brings water from the Colorado River almost
300 km away. Cairo is situated on the Nile River, which flows without significant
tributaries, bringing water from wet tropics near the equator thousands of kilometers
distant.Desertification
Much of the world’s land surface is arid or semiarid (seeDeserts). First used by
the French scientist Aubreville in 1949, the concept of desertification has main-
tained its perceived importance in an increasingly populated world. Desertification
is land degradation in arid and semiarid areas and has a negative connotation for
sustaining humanpopulations. It is a transformation that includes the lessening
of water resources, decreased fertility of soil, and disappearance of most of the
biomass. This is the result of interrelated processes that are difficult to sort. Some
desertification is natural and some a complicated interplay of natural and human
causes. Desertified land has a larger yearly moisture deficit (precipitationminus
evapotranspiration), more modesthumidity,andhighertemperaturesthan land
that has not undergone desertification.
Desertification can be identified using land resources satellites (seeRemote
sensing). The assessment of biomass status is now common and has monitoring98 Desertification