A desert is a place that has few,
or sometimes even no, life forms. Sometimes life forms adapt to living in deserts,
but conditions tend to be extreme, and survival is challenging.
Some deserts can be visited but not lived in. Some deserts
are so inhospitable that life as we know it cannot survive in them at all.
In terms of rainfall, areas that receive less than ten inches
of rain a year are considered to be deserts.
Some deserts receive only three or four inches of rain a year. A
few places do not receive any rain at all.
When we think about deserts, we think about limiting factors.
These factors include:
on earth, liquid water is necessary for life. Some life forms survive
periods when water is not available by becoming spores or seeds, or by becoming dormant
(hibernation or estivation). Some plants can survive for many years as seeds. Insects
and unicellular life forms can also wait out drought. Sooner or later, however, liquid
water is necessary. Survival is essential, but it is not all of life. Without growth
and reproduction, life is on hold, not progressing.
Salinity can also interfere with an organism's use of water. Fresh
water fish cannot live in the ocean, and land plants watered with sea water will
die. The excess salt in briny water pulls water out of the organism and dehydrates
it. If you put a salt water fish in fresh water it will die, too, because the organism
will retain too much water in its cells.
is the ultimate source of most of the energy used by living things on earth. Plants
use sunlight for photosynthesis. Lack of light in caves and under deep water make
these environments unsuitable for photosynthesizing plants.
Hydrogen Sulfide: a small set of life forms live around deep sea volcanic vents, using
a process called chemosynthesis to extract energy from the mineral rich hot water.
Without these chemicals, these cold, dark areas are almost lifeless.
Essential Elements: include nitrogen, calcium, and phosphorus. These are elements are
captured by plants and circulated through food chains. Much of the open ocean is
actually a desert because it lacks these nutrients and cannot support life forms.
In places where cold water is upwelling from the ocean depths, some of these nutrients
are carried up from the ocean bottom, and provide food for life forms at the surface.
provides energy (fuel) and also amino acids and vitamins necessary for life. Animals
need both fuel and vitamins. Without them, they do not thrive.
Life processes are chemical processes. Life processes operate
most efficiently within a narrow temperature range. Think of how we put food in the
freezer to keep it from spoiling: freezing temperatures stop those processes. Too
much heat also kills: fevers of around 108 degrees Fahrenheit kill humans, because
the proteins of which we are formed begin to break down at those temperatures. Both
plants and animals have ways of protecting themselves from intermittent extremes
of temperature, but need to stay within certain temperature ranges in their environments
in order to survive.
Life forms are adapted to the atmospheric or aquatic
pressures at which they live. People die at high altitudes because they cannot extract
enough oxygen from the air. Life forms brought up from the deep ocean will explode
as the gasses dissolved in their tissues expand.
We don't have radiation deserts on earth
(yet), but we know that living things need protection from ultra-violet radiation
(protection currently provided by the ozone layer), radioactive radiation (high around
Chernobyl), and gamma radiation out in space.
On our climate charts, a desert is a
region that receives less than 10 inches of rain a year. We tend to think of deserts
as being like the sand dunes in the movie "Lawrence of Arabia", but dunes
are only a small part of desert landscapes, which may be hot or cold. Go on to these
pages to learn more about them.
Photographs at top of page are
from Corel CD-ROM s: for viewing only, not for downloading.
Glacier Picture courtesy of Dr Thomas Lowellat the University of Cincinnati
Copyright © 1999. Elizabeth Anne Viau and her licensors. All
rights reserved. This material may be used by individuals for instructional purposes
but not sold. Please inform the author if you use it at firstname.lastname@example.org